[flipper]

On Fri, 29 Jun 2012 03:59:51 -0700 (PDT), Patrick Turner
wrote:

On Jun 28, 1:03*pm, flipper wrote:
On Wed, 27 Jun 2012 17:30:35 -0700 (PDT), Patrick Turner

wrote:

Yeah, I was going to say the talk about 3 transistors was actually
about a Gilbert Cell. I couldn't spend another 3 months perfecting
what I did with 3 transistors because I always have other things to
do.

I thought you were trying to build a Gilbert cell, back then. That
takes more than 3 transistors.

Using a gilbert cell chip probably would have worked better then
me trying to fiddle with discretes. But could a Gilbert cell be made
using 3 triodes?

Well, no, because a Gilbert Cell isn't '3 transistors'.

For the '3 transistor' cell I think pentodes might be more appropriate
because the bipolar collector current vs collector-emitter voltage
looks just like a pentode. Although, I'm not sure what the effect of
screen current mixed in might be. Or a single 6ME8 because that's
essentially how it works with the deflector plates acting as the two
upper transistors and G1 the bottom.

That's why I used a 6ME8: it's 'conceptually pure' with equivalent
'ins and outs'. As used, however, one plate is simply tied to B+ so,
if screen current and plate current 'see saw' in a heptode, which the
curves indicate is the case, then it might work similarly. I.E. what
goes into the 'unused' second plate externally connected to B+ is
'internally attached' screen current in the heptode. At least it seems
to be, in theory, but I didn't know what 'subtleties' might crop up
and, especially when doing something 'new', like to cut down the
unknowns as much as possible.

Another tube with strong g3 action is the 7 pin 6DT6 commonly used for
quadrature FM detection. Another one I was intersted in was "gated
beam pentode" 6BN6, which some said never worked well as 6DT6. Maybe
these have apps at BC band F.
Patrick Turner.

Gee, I don't know about the 6BN6. That thing is a gated beam
discriminator made for FM demod and has a 'limiter' built in, which is
sort of the antithesis of AM. Of course, one doesn't have to use it
'as intended' but it's beyond me.-

Its awhile since I mucked around with a Gilner, or Turbert Cell or
whatever I tried to use as an am modulator with some simularity to
Gilbert, which does have more than 3 transistors, 6 as you say.
But looking at my notes and the collection of Gilbert cell schematics
I have, what I used was a 1/2 Gilbert cell with an LTP driving
balanced RF OPT, one side of LTP to 0V, other side to RF carrier F,
and tail is transistor collector with AF applied to base. Very simple,
but not as good as using tubes as I had already.

Well, I think the balanced RF OPT was overkill but the 'LTP' should
work fine, unless you were holding one side to 0V without bipolar
supplies because there has to be 'something' across the bottom
transistor.

The basic topology you describe is the one I 'almost built':
http://flipperhome.dyndns.org/AM%20Transmitter.htm
And, as previously mentioned, it is the solid stage version of Beamus.
I.E., compared to the 6ME8, the bottom transistor is G1 and the top
two are the deflection plates.


Maybe Gilbert himself
used 6 triodes to make his original Cell,

Barrie's original cell was bipolar.

so 3 x 12AT7 might work
fine, although with two LTPs using one 12AT7, Ik is twice Ia in one
tube, so the cathode drive might best be a 6DJ8 or a pentode. I've
never had time to findoutabout and do my own complete research and
development. But one reason behind good sounding stereo FM is due to
Gilbert cells. Paravicini tried to con the world with a design for an
"all tubed MPX decoder" over 10 years ago, but it was just a chip with
Gibert within, and buffer tubes on output. The guy didn't do a real
tubed MPX decoder with tubed Gilbert cell. Manufacturers avoided doing
it like the plague because the cost of 3 twin triodes was far higher
than the usual crappy diode matrix.

I recall Fisher used 6BN6. I never bothered to build anything using
the tube. But 6DT6 was used in very many TV sets to get the audio from
the FM audio signal and chief benefit was the large amount of audio
output compared to using Foster Seely discriminator or ratio detector.
The 6DT6 gave quite low enough THD, and its use could eliminate the
use of a preamp tube so hence it "paid for itself" The g3 has fairly
high gm, and in FM demod, the circuit connected to g3 is an oscillator
at Fm frequency, very easy to get running reliably. Anyway, maybe the
tube could work well as an Am modulator.

The 6DT6, yes, because it's simply a dual control pentode. Here's one
http://amradio.freeiz.com/transmitter/6bk76dt6.jpg using the
'conventional' audio to G3 approach.

The 6BN6 is a whole different thing: a gated beam discriminator and
limiter with the express purpose of rejecting AM.

I know you are hooked on using just one tube,

Well, no. I was only 'hooked' on using one tube back when I was
experimenting with using one tube. That was 'the point' of it: to see
what one could do with one tube.

But everything I've done since that experiment involves more than one
tube.

What would be a more accurate characterization is I like the challenge
of getting the most'est from the least'est.

and someone said stuff
about FM being created and maybe that's true, because the audio
changes the effective C across a coil that determines the F so the
tuning F changes. I have a separate triode oscillator,

So does Beamus. Well, except I used a pentode osc.

then CF buffer,
and audio cannot get near anything oscillating at RF. I have a pentode
RF amp tube, and fairly large Rk bypassed for RF, but not for AF, so
when AF is applied to g1 with the RF from resistance divider from CF
RF source, the tube current is changed fairly linearly because there
is local current FB at AF, so wave form wasn't so terribly bad as say
50% mod. But adding more tubes to get NFB applied to audio stage much
reduced THD in wave form, and got me closer to 100% mod. I found many
old radios make a real mess when they try to detect audio when
modulation exceeds 50%, and of course back in good old days AM was not
fully modulated. But when you look at radio station waves, there is
high % mod, and of course compression maybe. And many superhet radio
sets can't produce a clean 100% mod IF wave when there is a 100% mod
RF wave input. I'm not sure how much NFB you have but certainly it
does work to make AM better you have non linear tubes or feeble tubes
to deal with.

I'm not sure what 'feeble' is supposed to mean but I estimate NFB at
between 15 dB and 20 dB.

I've not tried to use plate modulation which ppl say is
best, one reason being that the modulator adds power to the output
signal, and % mod is more linear to AF input to modulator.
There must be many ways to make AM.
I googled "tube gilbert cell" and other like things but no tubed
Gilbert Cell came onto my screen when I clicked 'images' ....so all
trial and error needed.
Patrick T.

[Alex Pogossov]

"flipper" wrote in message
...
On Thu, 28 Jun 2012 20:31:40 +1000, "Alex Pogossov"

With carrier at 640 KHz frequency deviation from idle to twice idle is
roughly 70 Hz. Actual measurement was .64015 MHz (resolution limit) to
.64022 MHz.

It is not bad. I expected worse.

Were you thinking maybe a CF would be needed?

In a quality equipment -- yes, but not in your case when you knowingly allow
for some FM.


Probably 6ME8 has less space charge
coupling than a regular heptode.

I suspect so and it's one of the things I think are 'different'
because of the beam deflection.

In the deflection tube the electron beam is far away (relatively) from the
deflection plates, not brushing them even when the deflection plates sit at
positive bias. In a heptode the space charge is right in the vicinity of the
G3, enveloping it and nearly touching it. Therefore the deflection tube
space charge effects are smaller.

But... the deflection tube requires +/-30V for full beam cutoff/switchover,
while a heptode probably would cutoff at -5V. Six times less. Now if you
arrange a tap on the oscillator running at 30V at 1/6-th, and feed it to a
heptode G3, the effect will be 6*6=36 times smaller. So you end up with the
same FM normalised to an available oscillator voltage. A hetode will give
low FM if coupled to a tap of the oscillator tank.

Because of huge deflection drive requirement, such tubes never gained
popularity as generic mixers.


Still 70Hz is not acceptable for listening on a synchronous or SSB
detector.
In these cases parasitic FM index shall not exceed about 0.25, so with the
lowest audio frequency of 50Hz, FM deviation shall not exceed 15Hz.

But who would deliberately listening to your transmitter on an SSB radio?
No
one.

Right. What I'd rather know is how much deviation is 'acceptable' for
the BCB.

Up to 1kHz, I would guess.


So, well done anyway!

Thanks. I'm getting the impression it might not be worth even trying a
heptode or DC pentode.

[Patrick Turner]

Its awhile since I mucked around with a Gilner, or Turbert Cell or
whatever I tried to use as an am modulator with some simularity to
Gilbert, which does have more than 3 transistors, 6 as you say.
But looking at my notes and the collection of Gilbert cell schematics
I have, what I used was a 1/2 Gilbert cell with an LTP driving
balanced RF OPT, one side of LTP to 0V, other side to RF carrier F,
and tail is transistor collector with AF applied to base. Very simple,
but not as good as using tubes as I had already.

Well, I think the balanced RF OPT was overkill but the 'LTP' should
work fine, unless you were holding one side to 0V without bipolar
supplies because there has to be 'something' across the bottom
transistor.

The basic topology you describe is the one I 'almost built':http://flipperhome.dyndns.org/AM%20Transmitter.htm
And, as previously mentioned, it is the solid stage version of Beamus.
I.E., compared to the 6ME8, the bottom transistor is G1 and the top
two are the deflection plates.

Maybe Gilbert himself
used 6 triodes to make his original Cell,

Barrie's original cell was bipolar.

I suspected it was, nothing tubed on Google, but whatever you can do
with bjts, you can also do with tubes, well, nearly whatever anyway,
until you need a pnp tube, which can't exist, and tubes are all npn,
and I mean figuratively, not literally..

The 6DT6, yes, because it's simply a dual control pentode. Here's one http://amradio.freeiz.com/transmitter/6bk76dt6.jpg using the
'conventional' audio to G3 approach.

Gee, that looks really nice and simple! Makes me wanna build one, I
might have a few 6DT6 laying around.



The 6BN6 is a whole different thing: a gated beam discriminator and
limiter with the express purpose of rejecting AM.

I know you are hooked on using just one tube,

Well, no. I was only 'hooked' on using one tube back when I was
experimenting with using one tube. That was 'the point' of it: to see
what one could do with one tube.

And now you know, and not bad considering you chose a tube which can
wear so many hats and does so many things, and which hardly anyone
knows anything about.

But everything I've done since that experiment involves more than one
tube.

What would be a more accurate characterization is I like the challenge
of getting the most'est from the least'est.

OK, but to me the 6ME6 is a "leastest with mostest" type of tube.

and someone said stuff
about FM being created and maybe that's true, because the audio
changes the effective C across a coil that determines the F so the
tuning F changes. I have a separate triode oscillator,

So does Beamus. Well, except I used a pentode osc.





then CF buffer,
and audio cannot get near anything oscillating at RF. I have a pentode
RF amp tube, and fairly large Rk bypassed for RF, but not for AF, so
when AF is applied to g1 with the RF from resistance divider from CF
RF source, the tube current is changed fairly linearly because there
is local current FB at AF, so wave form wasn't so terribly bad as say
50% mod. But adding more tubes to get NFB applied to audio stage much
reduced THD in wave form, and got me closer to 100% mod. I found many
old radios make a real mess when they try to detect audio when
modulation exceeds 50%, and of course back in good old days AM was not
fully modulated. But when you look at radio station waves, there is
high % mod, and of course compression maybe. And many superhet radio
sets can't produce a clean 100% mod IF wave when there is a 100% mod
RF wave input. I'm not sure how much NFB you have but certainly it
does work to make AM better you have non linear tubes or feeble tubes
to deal with.

I'm not sure what 'feeble' is supposed to mean but I estimate NFB at
between 15 dB and 20 dB.

Feeble means low Ia and low gm, and thast usually means low "figure of
merit". For example, EF86 is a gutless wonder of a tube which ws used
in thousands of amps in 1955 where one wanted gain between 80 and 150
and with only one nine pin socket and 0.7mA of anode current. It
replaced the use of a 12AU7 with 2 halves cascaded to get similar
gain, but with more total Ia and more R&C parts. I'd rather have the
12AU7. Look at Quad-II with EF86. Bloody awful! To get the gain needed
Quad used 180k RLa resistors and following bias R for KT66 were 680k
which often began to develop a high positive biasing voltage when
tubes age a bit. One shouldn't have Rg more than 120k max, ever, but
that weighs down the gain of feeble EF86. Later, in 1995, Chinese who
bought Quad used Andy Grove's revised Quad-II-Forty circuit with 6SH7
in pentode, almost identical in operation to early Quad-II. 6SH7 is an
octal tube with potentially far higher gm than EF86, or the 6SJ and
6SJ7 family of pentodes. Why Grove went with the original Quad-II
schematic is a mystery when it would have been much better to use 6SH7
in triode followed by 6SN7 as an LTP with CCS tail is beyond me. I
have repaired and re-wired a few Quad-II-Forty, because once you take
a careful look beyond the nice looking Chinese paint, its a POS amp,
and they have 6SH7 operating with Ia way below where its figure of
merit would be high, and with 470k biasing for the KT88 instead of the
KT66. Instead of gutless wonder pentodes with Ia at 1mA, I like more
linear triodes with 4 or 5 mA at least.

Enough about feeble tubes. In RF work, I like to see healthy Ia
because capacitances anywhere tend to have low Z paths at RF.

15-20dB FB is good, enough to do something worth doing, assuming the
gear makes maybe 7% Dn at 90% mod without NFB.

But I found that with NFB applied, the modulation bandwidth with
undistorted wave at say 95% mod was reduced, and to be expected of
course. In other words, modulation can be 95% OK with AF from say 15Hz
to 5kHz, with envelope wave having THD 1%, and enough to test most
AM radios, but above 5kHz, the envelope THD increases. OK though at
50% mod, good from 15Hz to 20kHz, less than maybe 0.5%. Its hard to
measure because one needs a linear detector, But then with dual trace
CRO one can compare input AF with recovered AF at detector output of
receiver, and if that detector is my double triode type with two CF
triodes then any visible distortion is likely to be in receiver mixer
and IF stage, not in sig gene, not in detector.
The AM transmissions here from radio stations are pretty good, and
probably with lower THD + IMD than in 1955, so recievers are the weak
link. My own AM set has cascode vari-mu twin triode input, 6AN7 mixer,
then 6BX6 IF amp with unbypassed Rk for some local current FB and no
AGC voltage is applied to it. 6BX6 or EF80 is a sharp cut off pentode
with strong gm normally used for 4.5Mhz TV IF stages. Its operating
with fixed bias. AGC voltage is generated by the detector CF stage,
and it works on mixer just a bit, and on RF input tube. I have
variable spacing between IFT1 like Hellicrafter used on their
communications radios with 3 IFTs to vary the Q for the IF band. But I
vary between flat top pass band and twin peak, so with two IFTs, and
IFT2 with normal 9kHz pass band, I get 18kHz pass band and 9kHz of AF
bandwidth. Some HF emphasis in R&C network boosts the AF BW to 10kHz.
I hate to seem immodest, but my AM radio outperforms ALL other AM
radios I have ever placed beside it to compare them, including Quad's
AM tuner which I think is next best. My point is that OK, one makes a
nice trick thing like a decent AM "mini-transmitter" and you can put
on a CD in the garage and fool ppl into believing there's this nice
new radio station without adds and some BS artist, but what about the
radio receiver perfomance?
Most AM radios are **** boxes, fair dunkum. The worst are the 3
transistor types added to an AM/FM receiver-tuner, 1.5kHz of audio BW
if you are lucky and very high THD IMD. Tubed types vary between
passable and "it needs total re-engineering". Passable means 4kHz BW,
typical THD/IMD 4%. Good means 7kHz BW, THD/IMD 2%, and Excellent
means 10kHz and Dn 1%.
There's no use going for more BW because AM transmissions are limited
to less than 10kHz, usually. Only 4.5kHz, where stations are close
together.

Patrick Turner.

[John Byrns]

In article ,
Patrick Turner wrote:

The AM transmissions here from radio stations are pretty good, and
probably with lower THD + IMD than in 1955, so recievers are the weak
link. My own AM set has cascode vari-mu twin triode input, 6AN7 mixer,
then 6BX6 IF amp with unbypassed Rk for some local current FB and no
AGC voltage is applied to it. 6BX6 or EF80 is a sharp cut off pentode
with strong gm normally used for 4.5Mhz TV IF stages. Its operating
with fixed bias. AGC voltage is generated by the detector CF stage,
and it works on mixer just a bit, and on RF input tube. I have
variable spacing between IFT1 like Hellicrafter used on their
communications radios with 3 IFTs to vary the Q for the IF band. But I
vary between flat top pass band and twin peak, so with two IFTs, and
IFT2 with normal 9kHz pass band, I get 18kHz pass band and 9kHz of AF
bandwidth. Some HF emphasis in R&C network boosts the AF BW to 10kHz.
I hate to seem immodest, but my AM radio outperforms ALL other AM
radios I have ever placed beside it to compare them, including Quad's
AM tuner which I think is next best.

What is it about QUAD'a AM tuner that places it in the much coveted second place
behind the Turner tuner? At first glance QUAD's AM design doesn't seem much
different than dozens of other AM tuner designs, exactly what is it that gives
it the edge over the other pretenders?

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/

[Patrick Turner]

On Jul 1, 2:37*am, John Byrns wrote:
In article ,
*Patrick Turner wrote:





The AM transmissions here from radio stations are pretty good, and
probably with lower THD + IMD than in 1955, so recievers are the weak
link. My own AM set has cascode vari-mu twin triode input, 6AN7 mixer,
then 6BX6 IF amp with unbypassed Rk for some local current FB and no
AGC voltage is applied to it. 6BX6 or EF80 is a sharp cut off pentode
with strong gm normally used for 4.5Mhz TV IF stages. Its operating
with fixed bias. AGC voltage is generated by the detector CF stage,
and it works on mixer just a bit, and on RF input tube. I have
variable spacing between IFT1 like Hellicrafter used on their
communications radios with 3 IFTs to vary the Q for the IF band. But I
vary between flat top pass band and twin peak, so with two IFTs, and
IFT2 with normal 9kHz pass band, I get 18kHz pass band and 9kHz of AF
bandwidth. Some HF emphasis in R&C network boosts the AF BW to 10kHz.
I hate to seem immodest, but my AM radio outperforms ALL other AM
radios I have ever placed beside it to compare them, including Quad's
AM tuner which I think is next best.

What is it about QUAD'a AM tuner that places it in the much coveted second place
behind the Turner tuner? *At first glance QUAD's AM design doesn't seem much
different than dozens of other AM tuner designs, exactly what is it that gives
it the edge over the other pretenders?

Well, Quad managed to get the selectivity variation just right with
switched tertiary on IFT 1. Most other radio makers steadfastly
avoided such delights and fed the buying public with half the audio BW
of Quad. Quad also got it right with traditional diode detector AF
detector, not too much Dn. But nearly all these old AM radios and
tuners need a ferrite rod antenna with short leads to pick up the
magnetic wave more than the electrostatic wave which comes to most old
sets to a high Z coil loosely magneticly coupled to tuned RF input
coil ahed of an RF amp or mixer.
Today's household wull be filled with fluroresant lamps each with tiny
SMPS plus all the other SMPS crap, plus linear PSU crap which all
manages to modulate station signals which arrive at the house with
100Hz wave, so the receivers pick up some radio stations which have
100Hz added modulation, and this hum can't be avoided when tuning.

So in a good AM tuner for best fidelity local pick up, you need a
ferrite rod antenna, a switched IFT 1 for wider AF BW, and so Quad had
some way to go to equal what I have. But in 1999 when I made my radio
from spare junk-box parts there was little radio interference and I
had two cascaded RF input coils with stagger tuning at the low end of
the band for better AF bw on stations which I like. But I had to
change later to a ferrite rod because of noise. The Quad also has
beautifully made tuning controls and various bands, where just have
one band. Quad's detector could have been better, but they'd have had
to use something similar to my cathode followers with ge diodes etc.
If someone were to spend a few days altering a Quad AM tuner to make
it better, I'm sure they could.
Because AM radio stations have crystal controlled oscillators set for
very specific frequencies, it would be nice to add a digital press
button frequency generator in receiver with setable F to tune instead
of having any oscillator. But you'd still have to tune the RF coil, so
as they are will do. I must put a tuning meter and better dial into my
set, and I have a suitable old FM tuner module which sounds well which
I could install in my set, because I often switch from AM to kitchen
to get the least boring program or best music. Then all AM and FM can
be in the one box which really needs its timber front altered and a
new longer slide type dial put in. Another fix-up job to attend to
during my retirement beginning on August 1 in a month.

Its all not really needed now because we have Digital Audio
Broadcasting, and a digital radio is cheap because all such gadgetty
stuff is made in Asia for 20c each. But speakers and audio amps in
little digital radios are awful, so one can focus on tube audio amps
and decent speakers. I might switch to stereo for the kitchen, so
speakers would be small, but like my old VAF bookshelfs for my TV set,
bokkshelfs can be good enough. The 1995 Metz TV I was given had The
World's Worst audio system which I removed entirely. Audio comes to
old solid state pioneer amp and VAF speakers from the back of the set-
top box. Good enough for TV sound which is often poor to begin with.
Maybe I get a 105cm LCD screen soon, but still have to do better with
sound. Like a lot of things, I don't have the time or inclination to
stay put mentally in 1955, or 1999, when other things have come along.
Well done AM is cabable of very good reception, but it all got dumbed
down with many stations put too close to allow wider AF. I've often
thought of using IF at 2MHz, and coils would be easier, and probably 3
IFTs would be needed. Even with Q = 100, pass band is 20kHz, so 10kHz
of audio.

In 1999 I tried first to make a DG Tucker style tubed synchrodyne, but
that proved much more difficult while offering poor performance
compared to a good superhet. The coils needed were beyound my time
allowances. There were no data sheets or how-to-do anywhere in 1999,
and I didn't go online till 2000.

The Gilbert Cell IC chips can make a nice synchrodyne, ie, direct
conversion receiver. I just NEVER get time for all this going back to
finish 1955 stuff which was interrupted by having to grow up, face the
world, get a job, raise kids, etc, whatever. Just because one cannot
take up where one left off at 1955 does not mean there isn't good
stuff to do now. Hell, I spend 3 days a week on a bicycle, and I don't
miss the soldrin' iron then. I'd rather have a resting HR of 48, keep
my docs happy, not take any pills, and enjoy the great outdoors and
speed and zing of cycling around than being couped up in the shed.
As for DAB, don't ask me about it because I haven't a clue exactly
what the transmitted wave looks like, have no clue how the transmitted
wave is constructed, and have no idea what happens to it when is
picked up by somebody's set. The boffins want us to buy, plug in, play
it, and not understand it, let alone add tubey stuff anywhere to make
it go better, or to build a digital radio with tubes. Probably that
would be as bad as tryna make a CD player using tubes only. You'd end
up with a thousand tubes, and because random failure rates are high
for tubes, the darn thing would fail regularly, besides, if each tube
consumes 2 Watts, then you have a room heater - not wanted in summer.
Tubed AM/FM tuners are not always reliable, like old tubed TV sets.
Patrick Turner.


--
Regards,

John Byrns

Surf my web pages at, *http://fmamradios.com/- Hide quoted text -

- Show quoted text -

[John Byrns]

In article ,
Patrick Turner wrote:

On Jul 1, 2:37*am, John Byrns wrote:

What is it about QUAD'a AM tuner that places it in the much coveted second
place
behind the Turner tuner? *At first glance QUAD's AM design doesn't seem
much
different than dozens of other AM tuner designs, exactly what is it that
gives
it the edge over the other pretenders?

Well, Quad managed to get the selectivity variation just right with
switched tertiary on IFT 1. Most other radio makers steadfastly
avoided such delights and fed the buying public with half the audio BW
of Quad. Quad also got it right with traditional diode detector AF
detector, not too much Dn.

It's not clear to me what was unique/better about QUAD's "selectivity variation"
circuit, in the day this same type of circuit was virtually universal in Hi-Fi
AM tuners! Is your point simply that QUAD's bandwidth choices were better than
the choices made by other manufacturers, if so this choice would seem to be
largely a matter of individual taste and reception conditions?

The detector issue is probably a little more complicated, some manufacturers
clearly screwed up, while others got it right like QUAD. I don't know what
might have accounted for this, but I suspect that it may be partly the result of
AM tuner specs being measured at 30% modulation. As a result IIRC signal
generators of the day only provided low distortion up to the 30% modulation
figure and generator distortion increased radically between there and 100%
modulation, if the generator would even do anywhere near 100% modulation. As a
result distortion measurements above the required 30% modulation figure were
likely as much a measure of the signal generator's distortion as they were of
the detectors distortion, so the designers may not have been able to tell if
their detector designs were working as they expected.

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/

[John L Stewart]

Originally Posted by Patrick Turner

On Jul 1, 2:37*am, John Byrns wrote:
In article ,
*Patrick Turner wrote:





The AM transmissions here from radio stations are pretty good, and
probably with lower THD + IMD than in 1955, so recievers are the weak
link. My own AM set has cascode vari-mu twin triode input, 6AN7 mixer,
then 6BX6 IF amp with unbypassed Rk for some local current FB and no
AGC voltage is applied to it. 6BX6 or EF80 is a sharp cut off pentode
with strong gm normally used for 4.5Mhz TV IF stages. Its operating
with fixed bias. AGC voltage is generated by the detector CF stage,
and it works on mixer just a bit, and on RF input tube. I have
variable spacing between IFT1 like Hellicrafter used on their
communications radios with 3 IFTs to vary the Q for the IF band. But I
vary between flat top pass band and twin peak, so with two IFTs, and
IFT2 with normal 9kHz pass band, I get 18kHz pass band and 9kHz of AF
bandwidth. Some HF emphasis in R&C network boosts the AF BW to 10kHz.
I hate to seem immodest, but my AM radio outperforms ALL other AM
radios I have ever placed beside it to compare them, including Quad's
AM tuner which I think is next best.

What is it about QUAD'a AM tuner that places it in the much coveted second place
behind the Turner tuner? *At first glance QUAD's AM design doesn't seem much
different than dozens of other AM tuner designs, exactly what is it that gives
it the edge over the other pretenders?

Well, Quad managed to get the selectivity variation just right with
switched tertiary on IFT 1. Most other radio makers steadfastly
avoided such delights and fed the buying public with half the audio BW
of Quad. Quad also got it right with traditional diode detector AF
detector, not too much Dn. But nearly all these old AM radios and
tuners need a ferrite rod antenna with short leads to pick up the
magnetic wave more than the electrostatic wave which comes to most old
sets to a high Z coil loosely magneticly coupled to tuned RF input
coil ahed of an RF amp or mixer.
Today's household wull be filled with fluroresant lamps each with tiny
SMPS plus all the other SMPS crap, plus linear PSU crap which all
manages to modulate station signals which arrive at the house with
100Hz wave, so the receivers pick up some radio stations which have
100Hz added modulation, and this hum can't be avoided when tuning.

So in a good AM tuner for best fidelity local pick up, you need a
ferrite rod antenna, a switched IFT 1 for wider AF BW, and so Quad had
some way to go to equal what I have. But in 1999 when I made my radio
from spare junk-box parts there was little radio interference and I
had two cascaded RF input coils with stagger tuning at the low end of
the band for better AF bw on stations which I like. But I had to
change later to a ferrite rod because of noise. The Quad also has
beautifully made tuning controls and various bands, where just have
one band. Quad's detector could have been better, but they'd have had
to use something similar to my cathode followers with ge diodes etc.
If someone were to spend a few days altering a Quad AM tuner to make
it better, I'm sure they could.
Because AM radio stations have crystal controlled oscillators set for
very specific frequencies, it would be nice to add a digital press
button frequency generator in receiver with setable F to tune instead
of having any oscillator. But you'd still have to tune the RF coil, so
as they are will do. I must put a tuning meter and better dial into my
set, and I have a suitable old FM tuner module which sounds well which
I could install in my set, because I often switch from AM to kitchen
to get the least boring program or best music. Then all AM and FM can
be in the one box which really needs its timber front altered and a
new longer slide type dial put in. Another fix-up job to attend to
during my retirement beginning on August 1 in a month.

Its all not really needed now because we have Digital Audio
Broadcasting, and a digital radio is cheap because all such gadgetty
stuff is made in Asia for 20c each. But speakers and audio amps in
little digital radios are awful, so one can focus on tube audio amps
and decent speakers. I might switch to stereo for the kitchen, so
speakers would be small, but like my old VAF bookshelfs for my TV set,
bokkshelfs can be good enough. The 1995 Metz TV I was given had The
World's Worst audio system which I removed entirely. Audio comes to
old solid state pioneer amp and VAF speakers from the back of the set-
top box. Good enough for TV sound which is often poor to begin with.
Maybe I get a 105cm LCD screen soon, but still have to do better with
sound. Like a lot of things, I don't have the time or inclination to
stay put mentally in 1955, or 1999, when other things have come along.
Well done AM is cabable of very good reception, but it all got dumbed
down with many stations put too close to allow wider AF. I've often
thought of using IF at 2MHz, and coils would be easier, and probably 3
IFTs would be needed. Even with Q = 100, pass band is 20kHz, so 10kHz
of audio.

In 1999 I tried first to make a DG Tucker style tubed synchrodyne, but
that proved much more difficult while offering poor performance
compared to a good superhet. The coils needed were beyound my time
allowances. There were no data sheets or how-to-do anywhere in 1999,
and I didn't go online till 2000.

The Gilbert Cell IC chips can make a nice synchrodyne, ie, direct
conversion receiver. I just NEVER get time for all this going back to
finish 1955 stuff which was interrupted by having to grow up, face the
world, get a job, raise kids, etc, whatever. Just because one cannot
take up where one left off at 1955 does not mean there isn't good
stuff to do now. Hell, I spend 3 days a week on a bicycle, and I don't
miss the soldrin' iron then. I'd rather have a resting HR of 48, keep
my docs happy, not take any pills, and enjoy the great outdoors and
speed and zing of cycling around than being couped up in the shed.
As for DAB, don't ask me about it because I haven't a clue exactly
what the transmitted wave looks like, have no clue how the transmitted
wave is constructed, and have no idea what happens to it when is
picked up by somebody's set. The boffins want us to buy, plug in, play
it, and not understand it, let alone add tubey stuff anywhere to make
it go better, or to build a digital radio with tubes. Probably that
would be as bad as tryna make a CD player using tubes only. You'd end
up with a thousand tubes, and because random failure rates are high
for tubes, the darn thing would fail regularly, besides, if each tube
consumes 2 Watts, then you have a room heater - not wanted in summer.
Tubed AM/FM tuners are not always reliable, like old tubed TV sets.
Patrick Turner.


--
Regards,

John Byrns

Surf my web pages at, *http://fmamradios.com/- Hide quoted text -

- Show quoted text -

The EICO HFT-94 has both IF1 & IF2 with selective BW. They claim overall selectivity of 7 & 14 KHz. Audio spec is 20-9000 Hz WB & 20-5000 Hz NB at 3 db down. Also using a CK885 Ge diode as the detector followed by a 10KHz whistle filter, then a FB pair with a 12AU7 to audio output.

The RF selectivity is good since it has an RF amp ahead of the convertor.

I built two of these from the kits around 1960. One still sounds excellant & runs most days while I'm in the workshop. The other is on the shelf for now.

Hey Patrick, is your DAB (Digital Audio) by Ubiquity as here on the regular FM band or is the transmission on the 'L' Band. Just curious. I sometimes think I hear interference on one of the locals broadcasting both analogue & digital. The tuner in that case is a Sansui TU-717

Google IBOC (In Band On Channel) for info on the Ibiquity System. One of the companies I sold for, Aeroflex has a good AN on the system.

Cheers to all, John

[John L Stewart]

This link will take you to the App Note on IBOC I refered to earlier-

http://noisecom.com/~/media/Noisecom...iosignals.ashx

The Spec A in the article was developed by Will'Tek in Germany. In the US it was sometimes sold under the Boonton name.
That part of the company was sold off to Aeroflex about two years ago.

A very nice portable SA to work with. I managed to sell quite a few.

Here is a link to iBiquity for the curious-

http://www.ibiquity.com/i/pdfs/Conve...quirements.pdf

Cheers, John

[flipper]

On Sat, 30 Jun 2012 15:45:37 +1000, "Alex Pogossov"
wrote:


"flipper" wrote in message
.. .
On Thu, 28 Jun 2012 20:31:40 +1000, "Alex Pogossov"

With carrier at 640 KHz frequency deviation from idle to twice idle is
roughly 70 Hz. Actual measurement was .64015 MHz (resolution limit) to
.64022 MHz.

It is not bad. I expected worse.

Were you thinking maybe a CF would be needed?

In a quality equipment -- yes, but not in your case when you knowingly allow
for some FM.

Ahem. "Quality" is a subjective thing. Maybe change that to
"Professional Broadcast" equipment because I think its pretty dern
good 'quality' for 2 one buck tubes

Probably 6ME8 has less space charge
coupling than a regular heptode.

I suspect so and it's one of the things I think are 'different'
because of the beam deflection.

In the deflection tube the electron beam is far away (relatively) from the
deflection plates, not brushing them even when the deflection plates sit at
positive bias. In a heptode the space charge is right in the vicinity of the
G3, enveloping it and nearly touching it. Therefore the deflection tube
space charge effects are smaller.

Right. In short, there's little space charge coupling to the
deflection plates.

But... the deflection tube requires +/-30V for full beam cutoff/switchover,

Not to be picky but, as biased, its running +-21V (15Vrms).

while a heptode probably would cutoff at -5V. Six times less. Now if you
arrange a tap on the oscillator running at 30V at 1/6-th, and feed it to a
heptode G3, the effect will be 6*6=36 times smaller. So you end up with the
same FM normalised to an available oscillator voltage. A hetode will give
low FM if coupled to a tap of the oscillator tank.

What's missing there is 'how much' more the closer "right in the
vicinity" heptode's space charge coupling is. I presume you're
implying it should be '6 times' as much based on the drive required
but are were sure that holds true when the mechanism, beam deflection,
is different? I mean, "same FM?"

Frankly, I have no idea how one calculates 'effective capacitance'
chance from space charge coupling.

Still, the point is taken. The existing grid takeoff was strictly for
amplitude and if I were to try using a heptode I'd likely be using the
coil cathode tap. Which, btw, if 5V would be enough for a heptode,
that is exactly what's at the coil tap right now.

Because of huge deflection drive requirement, such tubes never gained
popularity as generic mixers.


Still 70Hz is not acceptable for listening on a synchronous or SSB
detector.
In these cases parasitic FM index shall not exceed about 0.25, so with the
lowest audio frequency of 50Hz, FM deviation shall not exceed 15Hz.

But who would deliberately listening to your transmitter on an SSB radio?
No
one.

Right. What I'd rather know is how much deviation is 'acceptable' for
the BCB.

Up to 1kHz, I would guess.

Thanks. That's a good thing to keep in mind for future reference.

So, well done anyway!

Thanks. I'm getting the impression it might not be worth even trying a
heptode or DC pentode.

[flipper]

On Fri, 29 Jun 2012 23:49:00 -0700 (PDT), Patrick Turner
wrote:

Its awhile since I mucked around with a Gilner, or Turbert Cell or
whatever I tried to use as an am modulator with some simularity to
Gilbert, which does have more than 3 transistors, 6 as you say.
But looking at my notes and the collection of Gilbert cell schematics
I have, what I used was a 1/2 Gilbert cell with an LTP driving
balanced RF OPT, one side of LTP to 0V, other side to RF carrier F,
and tail is transistor collector with AF applied to base. Very simple,
but not as good as using tubes as I had already.

Well, I think the balanced RF OPT was overkill but the 'LTP' should
work fine, unless you were holding one side to 0V without bipolar
supplies because there has to be 'something' across the bottom
transistor.

The basic topology you describe is the one I 'almost built':http://flipperhome.dyndns.org/AM%20Transmitter.htm
And, as previously mentioned, it is the solid stage version of Beamus.
I.E., compared to the 6ME8, the bottom transistor is G1 and the top
two are the deflection plates.

Maybe Gilbert himself
used 6 triodes to make his original Cell,

Barrie's original cell was bipolar.

I suspected it was, nothing tubed on Google, but whatever you can do
with bjts, you can also do with tubes, well, nearly whatever anyway,
until you need a pnp tube, which can't exist, and tubes are all npn,
and I mean figuratively, not literally..


I think you'd have a much worse time trying to balance up a triode
Gilbert Cell than you did with the discrete BJTs.

Except there's not much reason to do so since that's what beam
deflection tubes are for.


The 6DT6, yes, because it's simply a dual control pentode. Here's one http://amradio.freeiz.com/transmitter/6bk76dt6.jpg using the
'conventional' audio to G3 approach.

Gee, that looks really nice and simple! Makes me wanna build one, I
might have a few 6DT6 laying around.

There's a version of it for just about every dual control pentode made
and I think most are just tweaked derivations of Norm's 6888 version.
Linearity depends on the G3 curve.

If you like that one you might also get a kick out of this one
(completely different approach by Robert Weaver )
http://www.radiomuseum.org/forum/a_o...ansmitter.html

He gets by with self excite because it's crystal controlled.


The 6BN6 is a whole different thing: a gated beam discriminator and
limiter with the express purpose of rejecting AM.

I know you are hooked on using just one tube,

Well, no. I was only 'hooked' on using one tube back when I was
experimenting with using one tube. That was 'the point' of it: to see
what one could do with one tube.

And now you know, and not bad considering you chose a tube which can
wear so many hats and does so many things, and which hardly anyone
knows anything about.

No, this one has two tubes with three active devices.

The 'one tube' attempt was 3 years ago using a 6SC6 'phono oscillator'
topology.

But everything I've done since that experiment involves more than one
tube.

What would be a more accurate characterization is I like the challenge
of getting the most'est from the least'est.

OK, but to me the 6ME6 is a "leastest with mostest" type of tube.

"leastest with mostest" *is* getting the mostest from the leastest.

and someone said stuff
about FM being created and maybe that's true, because the audio
changes the effective C across a coil that determines the F so the
tuning F changes. I have a separate triode oscillator,

So does Beamus. Well, except I used a pentode osc.


then CF buffer,
and audio cannot get near anything oscillating at RF. I have a pentode
RF amp tube, and fairly large Rk bypassed for RF, but not for AF, so
when AF is applied to g1 with the RF from resistance divider from CF
RF source, the tube current is changed fairly linearly because there
is local current FB at AF, so wave form wasn't so terribly bad as say
50% mod. But adding more tubes to get NFB applied to audio stage much
reduced THD in wave form, and got me closer to 100% mod. I found many
old radios make a real mess when they try to detect audio when
modulation exceeds 50%, and of course back in good old days AM was not
fully modulated. But when you look at radio station waves, there is
high % mod, and of course compression maybe. And many superhet radio
sets can't produce a clean 100% mod IF wave when there is a 100% mod
RF wave input. I'm not sure how much NFB you have but certainly it
does work to make AM better you have non linear tubes or feeble tubes
to deal with.

I'm not sure what 'feeble' is supposed to mean but I estimate NFB at
between 15 dB and 20 dB.

Feeble means low Ia and low gm, and thast usually means low "figure of
merit". For example, EF86 is a gutless wonder of a tube which ws used
in thousands of amps in 1955 where one wanted gain between 80 and 150
and with only one nine pin socket and 0.7mA of anode current. It
replaced the use of a 12AU7 with 2 halves cascaded to get similar
gain, but with more total Ia and more R&C parts. I'd rather have the
12AU7. Look at Quad-II with EF86. Bloody awful! To get the gain needed
Quad used 180k RLa resistors and following bias R for KT66 were 680k
which often began to develop a high positive biasing voltage when
tubes age a bit. One shouldn't have Rg more than 120k max, ever, but
that weighs down the gain of feeble EF86. Later, in 1995, Chinese who
bought Quad used Andy Grove's revised Quad-II-Forty circuit with 6SH7
in pentode, almost identical in operation to early Quad-II. 6SH7 is an
octal tube with potentially far higher gm than EF86, or the 6SJ and
6SJ7 family of pentodes. Why Grove went with the original Quad-II
schematic is a mystery when it would have been much better to use 6SH7
in triode followed by 6SN7 as an LTP with CCS tail is beyond me. I
have repaired and re-wired a few Quad-II-Forty, because once you take
a careful look beyond the nice looking Chinese paint, its a POS amp,
and they have 6SH7 operating with Ia way below where its figure of
merit would be high, and with 470k biasing for the KT88 instead of the
KT66. Instead of gutless wonder pentodes with Ia at 1mA, I like more
linear triodes with 4 or 5 mA at least.

Maybe you should look up the 6ME8 specs before speaking of 'feeble',
then, because its gm is 4400.


Enough about feeble tubes. In RF work, I like to see healthy Ia
because capacitances anywhere tend to have low Z paths at RF.

The 'low Ia' is to be Part 15 100mW compliant and not a 'limitation'
of the tube.


15-20dB FB is good, enough to do something worth doing, assuming the
gear makes maybe 7% Dn at 90% mod without NFB.

But I found that with NFB applied, the modulation bandwidth with
undistorted wave at say 95% mod was reduced, and to be expected of
course.

Why is that to be expected?

I'd only 'expect' that if the audio loop were bandwidth or slew rate
limited and neither are the case here.

In other words, modulation can be 95% OK with AF from say 15Hz
to 5kHz, with envelope wave having THD 1%, and enough to test most
AM radios, but above 5kHz, the envelope THD increases. OK though at
50% mod, good from 15Hz to 20kHz, less than maybe 0.5%. Its hard to
measure because one needs a linear detector, But then with dual trace
CRO one can compare input AF with recovered AF at detector output of
receiver, and if that detector is my double triode type with two CF
triodes then any visible distortion is likely to be in receiver mixer
and IF stage, not in sig gene, not in detector.
The AM transmissions here from radio stations are pretty good, and
probably with lower THD + IMD than in 1955, so recievers are the weak
link. My own AM set has cascode vari-mu twin triode input, 6AN7 mixer,
then 6BX6 IF amp with unbypassed Rk for some local current FB and no
AGC voltage is applied to it. 6BX6 or EF80 is a sharp cut off pentode
with strong gm normally used for 4.5Mhz TV IF stages. Its operating
with fixed bias. AGC voltage is generated by the detector CF stage,
and it works on mixer just a bit, and on RF input tube. I have
variable spacing between IFT1 like Hellicrafter used on their
communications radios with 3 IFTs to vary the Q for the IF band. But I
vary between flat top pass band and twin peak, so with two IFTs, and
IFT2 with normal 9kHz pass band, I get 18kHz pass band and 9kHz of AF
bandwidth. Some HF emphasis in R&C network boosts the AF BW to 10kHz.
I hate to seem immodest, but my AM radio outperforms ALL other AM
radios I have ever placed beside it to compare them, including Quad's
AM tuner which I think is next best. My point is that OK, one makes a
nice trick thing like a decent AM "mini-transmitter" and you can put
on a CD in the garage and fool ppl into believing there's this nice
new radio station without adds and some BS artist, but what about the
radio receiver perfomance?
Most AM radios are **** boxes, fair dunkum. The worst are the 3
transistor types added to an AM/FM receiver-tuner, 1.5kHz of audio BW
if you are lucky and very high THD IMD. Tubed types vary between
passable and "it needs total re-engineering". Passable means 4kHz BW,
typical THD/IMD 4%. Good means 7kHz BW, THD/IMD 2%, and Excellent
means 10kHz and Dn 1%.
There's no use going for more BW because AM transmissions are limited
to less than 10kHz, usually. Only 4.5kHz, where stations are close
together.

Patrick Turner.

[flipper]

On Fri, 29 Jun 2012 04:16:31 -0700 (PDT), Patrick Turner
wrote:

On Jun 28, 1:59*pm, flipper wrote:
On Wed, 27 Jun 2012 22:06:46 -0500, John Byrns
wrote:

In article ,
flipper wrote:

For this application I tend to think of them as almost the same thing
but am beginning to realize that the 'extra' G4 screen is probably
what gives a heptode the higher plate impedance. Is that why, despite
the 'accidental' mentioning of dual control pentode, you seem to be
adamant about heptodes, or is there another reason?

The reason I am "adamant about heptodes", if that is what you want to call it,
is not because of the plate resistance, it is simply because I have no
familiarity at all with DC pentodes and their characteristics.

Oh, okay. Well, just saying you're not familiar with them was enough


I don't even know any DC Pentode type numbers and am not sophisticated enough to
know that the plate resistance of a DC pentode is lower than a heptode.
Assuming the plate resistance of a DC Pentode is lower than a heptode, is that a
bad thing?

Well, it affects plate tank Q.

6AS6, 6DT6, 6GY6, and 6888 are some of the more 'popular' ones for
broadcasters.

I am not "adamant about heptodes", in fact I don't particularly care for grid
modulation, preferring plate modulation instead, however it is hard to conceive
of how to build a plate modulated transmitter using only one single section
tube, leaving the heptode, or possibly the DC Pentode, as the only easy choice
for a single tube transmitter. *I do have a design for a single tube transmitter
using your 6ME8 that meets my design brief, however it would require me to build
my own well balanced push-pull RF antenna coupling transformer.

Yeah, a PP transformer is in some of my designs too but I'm skittish
about trying to wind one.

PP RF tranny is easy.

Yeah? Then which core material should I be using? I'd prefer a pot
core so I don't have to screw with the misery of winding a gazillion
turns on a toroid

You can have just one winding with CT to B+, and
double tuning gang with frame and moving plates at 0V with 0.01uF caps
from fixed plated to coil ends.

Yes, I know how 'simple' a PP winding looks on paper.

Dual gang tuning the plates looks simple enough till you then have to
add antenna tuning off the secondary.

the thing is to allow the tubes to drive a nice high ohm RLa-a load,
and have a secondary wound over the middle part of the winding, ie,
the earthy part, and have this winding about 1/5 of the total P turns,
so you get a 25:1 impedance match so antena C has hardly any effect on
coil tuning. if the RLa-a in parallel with tube Ra was say 25k, then
Zout from sec is 1k0, quite low enough to get over effects of C across
OP terminals etc. probably an air cored coil is OK using 40mm PVC pipe
former with cat 5 strands of solid wire for coil. But maybe best if
you drill many holes as possible in PVC to reduce diaelectric losses.
Layout and keeping coil away from other metal and keeping leads tidy
assist in overall Q which need not be huge. Another way to get low Z
output is to have a tap on one side of PP coil, at say 20% of turns so
you can use 0.01 cap to take out SE signal. No sec winding, and you
still get benefits of the Z transformation and stray output C from
antenna won't worry the coil.
Patrick Turner.

The problem I see with an air core, besides size, is self capacitance
and space winding isn't all that easy. Maybe use Litz with insulation
providing a 'natural' spacing.

At any rate, what seemed 'simple' now has a raft of questions about
just how to accomplish it whereas with single ended I had a couple of
ready made parts that could be tried without adding risk.

With the 100 mW limit I certainly don't need 'more power' so the only
reason I can think of to use a balanced PP in the existing design
would be if the 'balance' significantly improved something else to
make up for the added complexity.

[John Byrns]

In article ,
flipper wrote:

On Fri, 29 Jun 2012 23:49:00 -0700 (PDT), Patrick Turner
wrote:

The 6DT6, yes, because it's simply a dual control pentode. Here's one
http://amradio.freeiz.com/transmitter/6bk76dt6.jpg using the
'conventional' audio to G3 approach.

Gee, that looks really nice and simple! Makes me wanna build one, I
might have a few 6DT6 laying around.

There's a version of it for just about every dual control pentode made
and I think most are just tweaked derivations of Norm's 6888 version.
Linearity depends on the G3 curve.

If you like that one you might also get a kick out of this one
(completely different approach by Robert Weaver )
http://www.radiomuseum.org/forum/a_o...am_transmitter.
html

He gets by with self excite because it's crystal controlled.


I hadn't seen this radiomueseum article before, it is one of the most
interesting radiomuseum articles I have come across. While this transmitter
isn't exactly my cup of tea, it is amazing what the author did with so little,
back to that later!

The use of screen modulation is a departure for these small transmitters as most
seem to use G3 modulation of either a heptode or a Dual Control Pentode, with a
few using Plate modulation as in the series modulation approach, and now the
beam deflection approach used in your unique "Beam Me Up Scotty" transmitter.
Of course screen modulation is nothing unique having been used in real AM
broadcast transmitters, especially those built by Continental, like the 316B 10
kW transmitters installed by CBS as backup transmitters at their O&O stations
around 1960 when they went to remote control for their transmitters.

The manual (23MB), with schematic, for a screen modulated Continental 1 kW MW-AM
broadcast transmitter can be found he
http://louise.hallikainen.org/BH/upl...3141kWAMTx.pdf

There are at least two types of dynamic carrier control, the BBC has written
some good research reports on the subject. One type of DCC, as used in Weaver's
transmitter, reduces the amplitude of the carrier when the modulation level is
low. The second type does essentially the opposite and reduces the carrier
amplitude when the modulation level is high. I believe the second form of DCC,
as opposed to the form used by Weaver, affords greater operating efficiency and
economy as well as reducing the peak power that the transmitter must produce.
The second approach also offers a subjectively lower subjective noise level IIRC.

It is fascinating how Weaver achieves DCC essentially by reducing the bias on
the modulator tube causing grid rectification of the audio and consequent change
in the carrier amplitude. This actually saves a few components that would
otherwise be required to provide normal modulator bias.

While the BBC used DCC for economic reasons, to reduce power and equipment
costs, Weaver's main reason for using DCC seems to be as a poor man's audio
processor. It isn't clear exactly how much compression Weaver's scheme
provides, perhaps as little as 4 dB? It would have been helpful if Weaver had
included trapezoidal oscilloscope patterns for his 300mV p-p and 2.2V p-p test
cases using tone modulation. This would allow judging the actual audio
compression effect, which is produced by the receiver's AGC system.

It would have also have been nice if Weaver had tried some of the pentodes he
mentioned in place of the tetrode he used, so we could have a better idea if it
is true that tetrodes provide more linear screen modulation than pentodes do, as
Weaver reports from the technical literature. References to the relevant
technical literature would also help here, I am particularly curious if this
effect applies to these small receiving tubes, or if it is mainly applicable to
larger transmitting tubes?

Previously I have only read the radiomuseum mini transmitter pages that I have
found through Google, or that people have linked to. Upon looking through their
project index I was surprised, and greatly pleased, to find that Weaver also has
a design for a three tube C-Quam AM Stereo transmitter, although I haven't had a
chance to read the rather lengthy article yet. I was acquainted with the
inventor of the C-Quam AM Stereo system so I have a special interest in this
transmitter design. In the past I have given some thought to designs for tube
based C-Quam receivers. I spent a few minutes Googling a couple of aspects of
AM stereo and was sadly surprised to see that Leonard Kahn, the designer of a
competing AM Stereo System, the Compatible Sideband System, passed away about a
month ago.

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/

[flipper]

On Wed, 04 Jul 2012 18:10:40 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Fri, 29 Jun 2012 23:49:00 -0700 (PDT), Patrick Turner
wrote:

The 6DT6, yes, because it's simply a dual control pentode. Here's one
http://amradio.freeiz.com/transmitter/6bk76dt6.jpg using the
'conventional' audio to G3 approach.

Gee, that looks really nice and simple! Makes me wanna build one, I
might have a few 6DT6 laying around.

There's a version of it for just about every dual control pentode made
and I think most are just tweaked derivations of Norm's 6888 version.
Linearity depends on the G3 curve.

If you like that one you might also get a kick out of this one
(completely different approach by Robert Weaver )
http://www.radiomuseum.org/forum/a_o...am_transmitter.
html

He gets by with self excite because it's crystal controlled.


I hadn't seen this radiomueseum article before, it is one of the most
interesting radiomuseum articles I have come across. While this transmitter
isn't exactly my cup of tea, it is amazing what the author did with so little,
back to that later!

Yes, Robert Weaver is a font of cleverness but if that one amazes you
then his single tube 6ME8 double reflex superhet will blow your mind.

http://electronbunker.ca/OneTubeSuper.html

Talk about making a tube work for it's keep.

Btw, this is the original carrier controlled transmitter he mentions
in the radiomuseum article. http://electronbunker.ca/OneTubeXMTR.html

The use of screen modulation is a departure for these small transmitters as most
seem to use G3 modulation of either a heptode or a Dual Control Pentode, with a
few using Plate modulation as in the series modulation approach, and now the
beam deflection approach used in your unique "Beam Me Up Scotty" transmitter.
Of course screen modulation is nothing unique having been used in real AM
broadcast transmitters, especially those built by Continental, like the 316B 10
kW transmitters installed by CBS as backup transmitters at their O&O stations
around 1960 when they went to remote control for their transmitters.

The manual (23MB), with schematic, for a screen modulated Continental 1 kW MW-AM
broadcast transmitter can be found he
http://louise.hallikainen.org/BH/upl...3141kWAMTx.pdf

There are at least two types of dynamic carrier control, the BBC has written
some good research reports on the subject. One type of DCC, as used in Weaver's
transmitter, reduces the amplitude of the carrier when the modulation level is
low. The second type does essentially the opposite and reduces the carrier
amplitude when the modulation level is high. I believe the second form of DCC,
as opposed to the form used by Weaver, affords greater operating efficiency and
economy as well as reducing the peak power that the transmitter must produce.
The second approach also offers a subjectively lower subjective noise level IIRC.

I'm not sure of the history but think the modern trend is a
combination of both, lowering carrier 'in the middle' and raising it
on the ends. An example:

http://people.wallawalla.edu/~rob.fr...trol/DCC1.html

As I think of it, though, I don't see how the 'second form' you
mention would increase efficiency much since most audio is not at
'high mod' levels.

It is fascinating how Weaver achieves DCC essentially by reducing the bias on
the modulator tube causing grid rectification of the audio and consequent change
in the carrier amplitude. This actually saves a few components that would
otherwise be required to provide normal modulator bias.

While the BBC used DCC for economic reasons, to reduce power and equipment
costs, Weaver's main reason for using DCC seems to be as a poor man's audio
processor. It isn't clear exactly how much compression Weaver's scheme
provides, perhaps as little as 4 dB? It would have been helpful if Weaver had
included trapezoidal oscilloscope patterns for his 300mV p-p and 2.2V p-p test
cases using tone modulation. This would allow judging the actual audio
compression effect, which is produced by the receiver's AGC system.

Yes, his purpose was as 'compressor', despite it appearing to be the
opposite. Receiver AGC does the other half of the work.

I'm not sure if he had a numerical target in mind or simply tweaked
for best 'sound', which I imagine might leave an issue about the AGC
characteristics of the particular receiver.


It would have also have been nice if Weaver had tried some of the pentodes he
mentioned in place of the tetrode he used, so we could have a better idea if it
is true that tetrodes provide more linear screen modulation than pentodes do, as
Weaver reports from the technical literature. References to the relevant
technical literature would also help here, I am particularly curious if this
effect applies to these small receiving tubes, or if it is mainly applicable to
larger transmitting tubes?

He also posts on ARF and there's a thread there where he did try some
of those, including a 6V6.

http://www.antiqueradios.com/forums/...70605&start=40

There was also some discussion about whether the 6CQ8 is really a
tetrode or a 'beam' pentode.


Previously I have only read the radiomuseum mini transmitter pages that I have
found through Google, or that people have linked to. Upon looking through their
project index I was surprised, and greatly pleased, to find that Weaver also has
a design for a three tube C-Quam AM Stereo transmitter, although I haven't had a
chance to read the rather lengthy article yet. I was acquainted with the
inventor of the C-Quam AM Stereo system so I have a special interest in this
transmitter design. In the past I have given some thought to designs for tube
based C-Quam receivers. I spent a few minutes Googling a couple of aspects of
AM stereo and was sadly surprised to see that Leonard Kahn, the designer of a
competing AM Stereo System, the Compatible Sideband System, passed away about a
month ago.

Yes, he posted that one on ARF too. Oh, I see in the radiomuseum
article he brings up the 'beam pentode' tetrode matter that was
discussed on ARF.

Btw, his cabinet work is every bit as good as his tube designs.

[flipper]

On Tue, 26 Jun 2012 22:29:35 -0500, John Byrns
wrote:


Are you trying to play with my mind? I was the first to use the term "dual
control pentode in this thread when I made the statement "Applying the
modulation to G1 doesn't take advantage of the beam deflection capabilities of
the tube and instead uses it in a way that a more ordinary tube, like a dual
control pentode, could serve." when I meant to say heptode. I'm the one that
converted it!

Setting side who's mind is going... hehe. I threw together a quick and
dirty heptode/DC pentode (same pinout so easy to try both)
'substitute' for the 6ME8 to test the theory they should behave
similarly.

Abstract summary is they didn't.

On first power up a 90% mod waveform looked a lot like a mountain with
a huge deep crater where the peak should be and after twiddling around
a bit it occurred to me, uh oh, G3 is relative to cathode, and that's
going up and down with audio, so either the RF amplitude, or 'center
point', needed to swing from full on to cutoff changes. That doesn't
happen with the 6ME8 because the deflection plates are relative to
each other and any change on the cathode is either blocked by the
screen or goes equally to both.

I tried connecting the G3 grid leak to cathode and the result looked
'almost' right but I think it was still distorted. So maybe there's a
way around it but they sure aren't a simple 'plop in' kind of thing.

[John Byrns]

In article ,
flipper wrote:

On Wed, 04 Jul 2012 18:10:40 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Fri, 29 Jun 2012 23:49:00 -0700 (PDT), Patrick Turner
wrote:

The 6DT6, yes, because it's simply a dual control pentode. Here's one
http://amradio.freeiz.com/transmitter/6bk76dt6.jpg using the
'conventional' audio to G3 approach.

Gee, that looks really nice and simple! Makes me wanna build one, I
might have a few 6DT6 laying around.

There's a version of it for just about every dual control pentode made
and I think most are just tweaked derivations of Norm's 6888 version.
Linearity depends on the G3 curve.

If you like that one you might also get a kick out of this one
(completely different approach by Robert Weaver )
http://www.radiomuseum.org/forum/a_o...r_am_transmitt
er.
html

He gets by with self excite because it's crystal controlled.


I hadn't seen this radiomueseum article before, it is one of the most
interesting radiomuseum articles I have come across. While this transmitter
isn't exactly my cup of tea, it is amazing what the author did with so
little,
back to that later!

Yes, Robert Weaver is a font of cleverness but if that one amazes you
then his single tube 6ME8 double reflex superhet will blow your mind.

http://electronbunker.ca/OneTubeSuper.html

Talk about making a tube work for it's keep.

It's too late to blow my mind with that, you already sent us there last year so
my mind is already blown, can't blow it twice.

Btw, this is the original carrier controlled transmitter he mentions
in the radiomuseum article. http://electronbunker.ca/OneTubeXMTR.html

Yes, I had found that myself after I saw his mention of it.

The use of screen modulation is a departure for these small transmitters as
most
seem to use G3 modulation of either a heptode or a Dual Control Pentode,
with a
few using Plate modulation as in the series modulation approach, and now the
beam deflection approach used in your unique "Beam Me Up Scotty"
transmitter.
Of course screen modulation is nothing unique having been used in real AM
broadcast transmitters, especially those built by Continental, like the 316B
10
kW transmitters installed by CBS as backup transmitters at their O&O
stations
around 1960 when they went to remote control for their transmitters.

The manual (23MB), with schematic, for a screen modulated Continental 1 kW
MW-AM
broadcast transmitter can be found he
http://louise.hallikainen.org/BH/upl...3141kWAMTx.pdf

There are at least two types of dynamic carrier control, the BBC has written
some good research reports on the subject. One type of DCC, as used in
Weaver's
transmitter, reduces the amplitude of the carrier when the modulation level
is
low. The second type does essentially the opposite and reduces the carrier
amplitude when the modulation level is high. I believe the second form of
DCC,
as opposed to the form used by Weaver, affords greater operating efficiency
and
economy as well as reducing the peak power that the transmitter must
produce.
The second approach also offers a subjectively lower subjective noise level
IIRC.

I'm not sure of the history but think the modern trend is a
combination of both, lowering carrier 'in the middle' and raising it
on the ends. An example:

http://people.wallawalla.edu/~rob.fr...trol/DCC1.html

Yes, I have seen that one, there is a BBC paper he

http://downloads.bbc.co.uk/rd/pubs/reports/1994-01.pdf

This paper has a bibliography which lists a number of earlier papers, including
several BBC papers from about 10 years earlier which is where I think I first
learned of these techniques. I have mostly forgotten how this stuff works and
will have to dig out some of the references and read them through.

As I think of it, though, I don't see how the 'second form' you
mention would increase efficiency much since most audio is not at
'high mod' levels.

I don't know about that, I remember back in the early 1960s an associate of mine
worked at a station where he proudly pointed out that the modulation monitor
meter rarely dropped off the 100% mark. Maybe the second scheme I described was
just a harebrained scheme I thought up years ago and never really existed
anywhere else. However I would think it could work for stations with very dense
continuous modulation like a rock music station. With this second scheme you
could potentially remove the peak tube from a Doherty style transmitter, saving
some filament power, and perhaps change the pulley ratio on the cooling blower
reducing the power consumed by the blower. Again I will have to read through
some of the references and refresh my memory on all this.

It is fascinating how Weaver achieves DCC essentially by reducing the bias
on
the modulator tube causing grid rectification of the audio and consequent
change
in the carrier amplitude. This actually saves a few components that would
otherwise be required to provide normal modulator bias.

While the BBC used DCC for economic reasons, to reduce power and equipment
costs, Weaver's main reason for using DCC seems to be as a poor man's audio
processor. It isn't clear exactly how much compression Weaver's scheme
provides, perhaps as little as 4 dB? It would have been helpful if Weaver
had
included trapezoidal oscilloscope patterns for his 300mV p-p and 2.2V p-p
test
cases using tone modulation. This would allow judging the actual audio
compression effect, which is produced by the receiver's AGC system.

Yes, his purpose was as 'compressor', despite it appearing to be the
opposite. Receiver AGC does the other half of the work.

I'm not sure if he had a numerical target in mind or simply tweaked
for best 'sound', which I imagine might leave an issue about the AGC
characteristics of the particular receiver.

I got the impression that he was constrained by the circuit an tweaked it for
the best performance he could get out of it. It looks like there might be a few
things he could do to further improve the performance, but the ones that come to
my mind all complicate the circuit. A direct coupled cathode follower between
the modulator and the modulated screen grid might extend the performance range.

IIRC one of the BBC papers, maybe several, delves into the AGC compatibility
issues.

It would have also have been nice if Weaver had tried some of the pentodes
he
mentioned in place of the tetrode he used, so we could have a better idea if
it
is true that tetrodes provide more linear screen modulation than pentodes
do, as
Weaver reports from the technical literature. References to the relevant
technical literature would also help here, I am particularly curious if this
effect applies to these small receiving tubes, or if it is mainly applicable
to
larger transmitting tubes?

He also posts on ARF and there's a thread there where he did try some
of those, including a 6V6.

http://www.antiqueradios.com/forums/...70605&start=40

There was also some discussion about whether the 6CQ8 is really a
tetrode or a 'beam' pentode.

I saw that on one of his radiomuseum pages.

Previously I have only read the radiomuseum mini transmitter pages that I
have
found through Google, or that people have linked to. Upon looking through
their
project index I was surprised, and greatly pleased, to find that Weaver also
has
a design for a three tube C-Quam AM Stereo transmitter, although I haven't
had a
chance to read the rather lengthy article yet. I was acquainted with the
inventor of the C-Quam AM Stereo system so I have a special interest in this
transmitter design. In the past I have given some thought to designs for
tube
based C-Quam receivers. I spent a few minutes Googling a couple of aspects
of
AM stereo and was sadly surprised to see that Leonard Kahn, the designer of
a
competing AM Stereo System, the Compatible Sideband System, passed away
about a
month ago.

Yes, he posted that one on ARF too.

Not sure what you are talking about here, what did he post on "ARF too"?

Oh, I see in the radiomuseum
article he brings up the 'beam pentode' tetrode matter that was
discussed on ARF.

Did he say anything more about it on "ARF"?

Btw, his cabinet work is every bit as good as his tube designs.

It looks that way.

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/

[John Byrns]

In article ,
flipper wrote:

On Tue, 26 Jun 2012 22:29:35 -0500, John Byrns
wrote:


Are you trying to play with my mind? I was the first to use the term "dual
control pentode in this thread when I made the statement "Applying the
modulation to G1 doesn't take advantage of the beam deflection capabilities
of
the tube and instead uses it in a way that a more ordinary tube, like a dual
control pentode, could serve." when I meant to say heptode. I'm the one
that
converted it!

Setting side who's mind is going... hehe. I threw together a quick and
dirty heptode/DC pentode (same pinout so easy to try both)
'substitute' for the 6ME8 to test the theory they should behave
similarly.

Abstract summary is they didn't.

Sounds reasonable, I would be surprised if they did behave the same. I take it
you are saying the 6ME8 behaved differently than the heptode and that the
heptode behaved differently than the dual control pentode?

On first power up a 90% mod waveform looked a lot like a mountain with
a huge deep crater where the peak should be and after twiddling around
a bit it occurred to me, uh oh, G3 is relative to cathode, and that's
going up and down with audio, so either the RF amplitude, or 'center
point', needed to swing from full on to cutoff changes.

By the context you are clearly talking about either the heptode or the dual
control pentode here, did both exhibit this mountain crater effect? Why is
there audio on the cathode? I assume we are talking about a self excited
circuit so that there would be a lot of RF on the cathode, but audio?

Is the plate loading set correctly? Incorrect plate loading would cause an
effect similar to what you are describing.

That doesn't
happen with the 6ME8 because the deflection plates are relative to
each other and any change on the cathode is either blocked by the
screen or goes equally to both.

I tried connecting the G3 grid leak to cathode and the result looked
'almost' right but I think it was still distorted. So maybe there's a
way around it but they sure aren't a simple 'plop in' kind of thing.

I don't follow your reference to "a simple 'plop in' kind of thing", what do you
mean?

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/

[flipper]

On Thu, 05 Jul 2012 21:52:00 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Tue, 26 Jun 2012 22:29:35 -0500, John Byrns
wrote:


Are you trying to play with my mind? I was the first to use the term "dual
control pentode in this thread when I made the statement "Applying the
modulation to G1 doesn't take advantage of the beam deflection capabilities
of
the tube and instead uses it in a way that a more ordinary tube, like a dual
control pentode, could serve." when I meant to say heptode. I'm the one
that
converted it!

Setting side who's mind is going... hehe. I threw together a quick and
dirty heptode/DC pentode (same pinout so easy to try both)
'substitute' for the 6ME8 to test the theory they should behave
similarly.

Abstract summary is they didn't.

Sounds reasonable, I would be surprised if they did behave the same. I take it
you are saying the 6ME8 behaved differently than the heptode and that the
heptode behaved differently than the dual control pentode?

See below.

On first power up a 90% mod waveform looked a lot like a mountain with
a huge deep crater where the peak should be and after twiddling around
a bit it occurred to me, uh oh, G3 is relative to cathode, and that's
going up and down with audio, so either the RF amplitude, or 'center
point', needed to swing from full on to cutoff changes.

By the context you are clearly talking about either the heptode or the dual
control pentode here, did both exhibit this mountain crater effect?

Yes.

Why is
there audio on the cathode? I assume we are talking about a self excited
circuit so that there would be a lot of RF on the cathode, but audio?

No, I'm not talking about audio to G3 with RF to G1, self excite or
not. That's the dime a dozen, pick your tube and there's a ready made
schematic already there to download, way of doing it.

Maybe I misunderstood your "like a (heptode) dual control pentode
could serve" but I took it as suggesting audio to G1 with RF to G3,
'like done with the 6ME8', should produce results 'like' the 6ME8. As
in 'why use a 6ME8 when a plain ole heptode would do the same thing'?
And that's the context all my discussions on them has been in.

So I disconnected the 6ME8, ran RF to heptode/DC pentode G3 with audio
to G1 which, of course, puts audio on the cathode through Rk which, in
turn, is the preamp NFB... just as with the 6ME8. I.E. a 'plop in'
replacement to see if it 'works like the 6ME8' worked.

It didn't.

Is the plate loading set correctly? Incorrect plate loading would cause an
effect similar to what you are describing.

Plate load was fine, and peaked. The problem is G3 being relative to
the cathode, which is going up and down with audio.

That doesn't
happen with the 6ME8 because the deflection plates are relative to
each other and any change on the cathode is either blocked by the
screen or goes equally to both.

I tried connecting the G3 grid leak to cathode and the result looked
'almost' right but I think it was still distorted. So maybe there's a
way around it but they sure aren't a simple 'plop in' kind of thing.

I don't follow your reference to "a simple 'plop in' kind of thing", what do you
mean?

See above.

[John Byrns]

In article ,
John Byrns wrote:

In article ,
flipper wrote:

On Wed, 04 Jul 2012 18:10:40 -0500, John Byrns
wrote:

There are at least two types of dynamic carrier control, the BBC has
written
some good research reports on the subject. One type of DCC, as used in
Weaver's
transmitter, reduces the amplitude of the carrier when the modulation
level
is
low. The second type does essentially the opposite and reduces the
carrier
amplitude when the modulation level is high. I believe the second form of
DCC,
as opposed to the form used by Weaver, affords greater operating
efficiency
and
economy as well as reducing the peak power that the transmitter must
produce.
The second approach also offers a subjectively lower subjective noise
level
IIRC.

I'm not sure of the history but think the modern trend is a
combination of both, lowering carrier 'in the middle' and raising it
on the ends. An example:

http://people.wallawalla.edu/~rob.fr...ontrol/DCC1.ht
ml

Yes, I have seen that one, there is a BBC paper he

http://downloads.bbc.co.uk/rd/pubs/reports/1994-01.pdf

This paper has a bibliography which lists a number of earlier papers,
including
several BBC papers from about 10 years earlier which is where I think I first
learned of these techniques. I have mostly forgotten how this stuff works
and
will have to dig out some of the references and read them through.

As I think of it, though, I don't see how the 'second form' you
mention would increase efficiency much since most audio is not at
'high mod' levels.

I did some digging around on the interweb based on the bibliography in the the
BBC paper I linked to above. I found the following two BBC papers:

http://downloads.bbc.co.uk/rd/pubs/reports/1985-13.pdf
http://downloads.bbc.co.uk/rd/pubs/reports/1988-15.pdf

I think these are the papers I read way back when, I haven't yet reviewed them
again, but I think they both discuss what I called the second DCC scheme because
"Fig. 2" in each paper illustrates the scheme as I conceive of it, I will have
to see what the papers have to say beyond that.

I wonder if there is a poor man's approach to implementing this DCC scheme
analogous to Weaver's implementation of the first DCC scheme?

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/

[flipper]

On Thu, 05 Jul 2012 21:50:23 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Wed, 04 Jul 2012 18:10:40 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Fri, 29 Jun 2012 23:49:00 -0700 (PDT), Patrick Turner
wrote:

The 6DT6, yes, because it's simply a dual control pentode. Here's one
http://amradio.freeiz.com/transmitter/6bk76dt6.jpg using the
'conventional' audio to G3 approach.

Gee, that looks really nice and simple! Makes me wanna build one, I
might have a few 6DT6 laying around.

There's a version of it for just about every dual control pentode made
and I think most are just tweaked derivations of Norm's 6888 version.
Linearity depends on the G3 curve.

If you like that one you might also get a kick out of this one
(completely different approach by Robert Weaver )
http://www.radiomuseum.org/forum/a_o...r_am_transmitt
er.
html

He gets by with self excite because it's crystal controlled.


I hadn't seen this radiomueseum article before, it is one of the most
interesting radiomuseum articles I have come across. While this transmitter
isn't exactly my cup of tea, it is amazing what the author did with so
little,
back to that later!

Yes, Robert Weaver is a font of cleverness but if that one amazes you
then his single tube 6ME8 double reflex superhet will blow your mind.

http://electronbunker.ca/OneTubeSuper.html

Talk about making a tube work for it's keep.

It's too late to blow my mind with that, you already sent us there last year so
my mind is already blown, can't blow it twice.

Btw, this is the original carrier controlled transmitter he mentions
in the radiomuseum article. http://electronbunker.ca/OneTubeXMTR.html

Yes, I had found that myself after I saw his mention of it.

The use of screen modulation is a departure for these small transmitters as
most
seem to use G3 modulation of either a heptode or a Dual Control Pentode,
with a
few using Plate modulation as in the series modulation approach, and now the
beam deflection approach used in your unique "Beam Me Up Scotty"
transmitter.
Of course screen modulation is nothing unique having been used in real AM
broadcast transmitters, especially those built by Continental, like the 316B
10
kW transmitters installed by CBS as backup transmitters at their O&O
stations
around 1960 when they went to remote control for their transmitters.

The manual (23MB), with schematic, for a screen modulated Continental 1 kW
MW-AM
broadcast transmitter can be found he
http://louise.hallikainen.org/BH/upl...3141kWAMTx.pdf

There are at least two types of dynamic carrier control, the BBC has written
some good research reports on the subject. One type of DCC, as used in
Weaver's
transmitter, reduces the amplitude of the carrier when the modulation level
is
low. The second type does essentially the opposite and reduces the carrier
amplitude when the modulation level is high. I believe the second form of
DCC,
as opposed to the form used by Weaver, affords greater operating efficiency
and
economy as well as reducing the peak power that the transmitter must
produce.
The second approach also offers a subjectively lower subjective noise level
IIRC.

I'm not sure of the history but think the modern trend is a
combination of both, lowering carrier 'in the middle' and raising it
on the ends. An example:

http://people.wallawalla.edu/~rob.fr...trol/DCC1.html

Yes, I have seen that one, there is a BBC paper he

Hehe. We must have similar google settings as I saw that one too.

Interesting but in the 'results' they never speak of achieving the
'goals', just perception analysis.


http://downloads.bbc.co.uk/rd/pubs/reports/1994-01.pdf

This paper has a bibliography which lists a number of earlier papers, including
several BBC papers from about 10 years earlier which is where I think I first
learned of these techniques. I have mostly forgotten how this stuff works and
will have to dig out some of the references and read them through.

As I think of it, though, I don't see how the 'second form' you
mention would increase efficiency much since most audio is not at
'high mod' levels.

I don't know about that, I remember back in the early 1960s an associate of mine
worked at a station where he proudly pointed out that the modulation monitor
meter rarely dropped off the 100% mark.

Hmm. Well, in that case, wouldn't it equate to simply having a lower
power transmitter since the 'compression' would be essentially 'always
on' constant?

Maybe the second scheme I described was
just a harebrained scheme I thought up years ago and never really existed
anywhere else. However I would think it could work for stations with very dense
continuous modulation like a rock music station. With this second scheme you
could potentially remove the peak tube from a Doherty style transmitter, saving
some filament power, and perhaps change the pulley ratio on the cooling blower
reducing the power consumed by the blower. Again I will have to read through
some of the references and refresh my memory on all this.

I think it depends on what the 'goal' is and, in the final analysis,
probably not quite as 'simple' as it seems.

To me DAM makes the most sense, if the idea is to conserve power.

It is fascinating how Weaver achieves DCC essentially by reducing the bias
on
the modulator tube causing grid rectification of the audio and consequent
change
in the carrier amplitude. This actually saves a few components that would
otherwise be required to provide normal modulator bias.

While the BBC used DCC for economic reasons, to reduce power and equipment
costs, Weaver's main reason for using DCC seems to be as a poor man's audio
processor. It isn't clear exactly how much compression Weaver's scheme
provides, perhaps as little as 4 dB? It would have been helpful if Weaver
had
included trapezoidal oscilloscope patterns for his 300mV p-p and 2.2V p-p
test
cases using tone modulation. This would allow judging the actual audio
compression effect, which is produced by the receiver's AGC system.

Yes, his purpose was as 'compressor', despite it appearing to be the
opposite. Receiver AGC does the other half of the work.

I'm not sure if he had a numerical target in mind or simply tweaked
for best 'sound', which I imagine might leave an issue about the AGC
characteristics of the particular receiver.

I got the impression that he was constrained by the circuit an tweaked it for
the best performance he could get out of it. It looks like there might be a few
things he could do to further improve the performance, but the ones that come to
my mind all complicate the circuit. A direct coupled cathode follower between
the modulator and the modulated screen grid might extend the performance range.

Possibly but the beauty of the thing is it's utter simplicity and that
he doesn't 'add complexity' but gets a new function almost by just
'rearranging' things. Not quite, of course, because his tetrode isn't
a dual control pentode, like in a 'conventional' broadcaster, but the
number of devices is the same.


IIRC one of the BBC papers, maybe several, delves into the AGC compatibility
issues.

Yeah, but they're being considerably more 'fancy'.


It would have also have been nice if Weaver had tried some of the pentodes
he
mentioned in place of the tetrode he used, so we could have a better idea if
it
is true that tetrodes provide more linear screen modulation than pentodes
do, as
Weaver reports from the technical literature. References to the relevant
technical literature would also help here, I am particularly curious if this
effect applies to these small receiving tubes, or if it is mainly applicable
to
larger transmitting tubes?

He also posts on ARF and there's a thread there where he did try some
of those, including a 6V6.

http://www.antiqueradios.com/forums/...70605&start=40

There was also some discussion about whether the 6CQ8 is really a
tetrode or a 'beam' pentode.

I saw that on one of his radiomuseum pages.

Previously I have only read the radiomuseum mini transmitter pages that I
have
found through Google, or that people have linked to. Upon looking through
their
project index I was surprised, and greatly pleased, to find that Weaver also
has
a design for a three tube C-Quam AM Stereo transmitter, although I haven't
had a
chance to read the rather lengthy article yet. I was acquainted with the
inventor of the C-Quam AM Stereo system so I have a special interest in this
transmitter design. In the past I have given some thought to designs for
tube
based C-Quam receivers. I spent a few minutes Googling a couple of aspects
of
AM stereo and was sadly surprised to see that Leonard Kahn, the designer of
a
competing AM Stereo System, the Compatible Sideband System, passed away
about a
month ago.

Yes, he posted that one on ARF too.

Not sure what you are talking about here, what did he post on "ARF too"?

His 3 tube C-QUAM AM Stereo Transmitter.

Oh, I see in the radiomuseum
article he brings up the 'beam pentode' tetrode matter that was
discussed on ARF.

Did he say anything more about it on "ARF"?

I posted a link to it, up above.


Btw, his cabinet work is every bit as good as his tube designs.

It looks that way.

[John L Stewart]

Another system that managed to save some transmitter power & some BW as well saw some popularity in the late 80s. It is ACSB (Amplitude Compandered Sideband). It did not take off well commercially here as its proponents hoped. I've still got the Aerotron stuff in my file.

ACSB is basically SSB with a 3.1 KHz pilot tone included, so demod is somewhat simpler.

It was directed at the 2-way com market (PMR, Public Mobile Radio) as a replacement for NBFM where channels occupied 25 KHz & the carrier runs continuously. ACSB could be fit into a 5 KHz channel, allowing lots more utilization in a given spectrum.

The local rep pushing the system called looking for suitable test equipment while I was at R&S. It looked like we could do it with the R&S CMT or CMTA if the SSB option was loaded.

Seems like ACSB may still be in use by some amature radio operators. See this link- http://forums.qrz.com/archive/index.php/t-265754.html

Today APCO 25 digital systems C4FM (12.5 KHz BW) & CQPSK (6.25 KHz BW) pretty well solve the BW & power problems. The Aeroflex 3920 can do both.

Cheers to all, John

[John Byrns]

In article ,
flipper wrote:

On Thu, 05 Jul 2012 21:52:00 -0500, John Byrns
wrote:

Why is
there audio on the cathode? I assume we are talking about a self excited
circuit so that there would be a lot of RF on the cathode, but audio?

No, I'm not talking about audio to G3 with RF to G1, self excite or
not. That's the dime a dozen, pick your tube and there's a ready made
schematic already there to download, way of doing it.

Yeah, I forgot exactly what circuit we were talking about, plus I never really
thought about the fact that the cathode in your circuit isn't grounded for audio
frequencies.

Speaking of not being sure what circuit was being discussed, when you were
making those "FMing" measurements for Alex Pogossov I was never clear on which
circuit you were measuring? Was it the full "Beam Me Up Scotty" transmitter
with separate oscillator or something else?

Maybe I misunderstood your "like a (heptode) dual control pentode
could serve" but I took it as suggesting audio to G1 with RF to G3,
'like done with the 6ME8', should produce results 'like' the 6ME8. As
in 'why use a 6ME8 when a plain ole heptode would do the same thing'?
And that's the context all my discussions on them has been in.

As near as I can tell you correctly understood my original question.

So I disconnected the 6ME8, ran RF to heptode/DC pentode G3 with audio
to G1 which, of course, puts audio on the cathode through Rk which, in
turn, is the preamp NFB... just as with the 6ME8. I.E. a 'plop in'
replacement to see if it 'works like the 6ME8' worked.

It didn't.

First let me check to see that I correctly understand where the "craters" you
are observing actually are on the transmitted waveform, as I am interpreting
what you said they are on the positive modulation peaks, is that correct?

I still think incorrect plate loading is the cause of this problem, although if
there weren't audio on the cathode you would just get plain old clipping rather
than craters. The G3 drivelevel may also have some impact on this. While I
think you understood my original question I didn't mean to imply that one of the
other tubes could be "plopped" directly in place of the 6ME8 without tweaking
the circuit parameters to suit the new tube, especially the plate loading and
the G3 drive level.

Another issue that may be compounding the problem is that the G1 control is
presumably like a single ended class A audio amplifier which produces a fair
amount of second harmonic distortion. IIRC, and I hope I don't have this
backwards, the tube output on negative G1 excursions is compressed relative to
positive G1 excursions. If I have this the right way around that would imply
that if you adjust for 90% negative modulation, then the positive modulation
peaks may be considerably greater than 90% and may be limited by "bumping into
the cathode". If this is the case, correct plate loading should fix the problem.

Is the plate loading set correctly? Incorrect plate loading would cause an
effect similar to what you are describing.

Plate load was fine, and peaked.

I understand the "peaked" part, but how are you determining that the "plate load
was fine"? Correct plate loading is important on these grid modulated
transmitters, unlike plate modulated transmitters which are more forgiving of
incorrect plate loading. I have not seen anyone, on radiomuseum, or elsewhere,
delve into the loading issues with these small transmitters. I thought Robert
Weaver was going to get into it in his C-Quam article where he calculates the
impedance of a short monopole antenna, but he balked and didn't discuss the
loading issue.

The problem is G3 being relative to
the cathode, which is going up and down with audio.

I understand what you are saying about the cathode going up and down with the
audio, I'm not sure I understand what G3 has to do with the problem though,
other than possibly the G3 drive level?

That doesn't
happen with the 6ME8 because the deflection plates are relative to
each other and any change on the cathode is either blocked by the
screen or goes equally to both.
screen or goes equally to both.

The question here seems to be did you just get lucky with the loading for the
6ME8, or is the plate somehow blocked from the cathode as you speculate? In any
case any transmitter is going to eventually clip in some way on positive peaks.

I tried connecting the G3 grid leak to cathode and the result looked
'almost' right but I think it was still distorted. So maybe there's a
way around it but they sure aren't a simple 'plop in' kind of thing.

That's an interesting result, I wonder if the change was due to the consequent
change in bias on G3?

I don't follow your reference to "a simple 'plop in' kind of thing", what do
you mean?

See above.

I wouldn't expect it to be as simple as "a simple 'plop in' kind of thing", I
would expect that the circuit parameters would need tweaking for the different
tube.

See above.

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/

[John Byrns]

In article ,
flipper wrote:

On Thu, 05 Jul 2012 21:50:23 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Wed, 04 Jul 2012 18:10:40 -0500, John Byrns
wrote:

There are at least two types of dynamic carrier control, the BBC has
written
some good research reports on the subject. One type of DCC, as used in
Weaver's
transmitter, reduces the amplitude of the carrier when the modulation
level
is
low. The second type does essentially the opposite and reduces the
carrier
amplitude when the modulation level is high. I believe the second form
of
DCC,
as opposed to the form used by Weaver, affords greater operating
efficiency
and
economy as well as reducing the peak power that the transmitter must
produce.
The second approach also offers a subjectively lower subjective noise
level
IIRC.

I'm not sure of the history but think the modern trend is a
combination of both, lowering carrier 'in the middle' and raising it
on the ends. An example:

http://people.wallawalla.edu/~rob.fr...Control/DCC1.h
tml

Yes, I have seen that one, there is a BBC paper he

Hehe. We must have similar google settings as I saw that one too.

Interesting but in the 'results' they never speak of achieving the
'goals', just perception analysis.

Did you see the links in my update post, particularly this one?

http://downloads.bbc.co.uk/rd/pubs/reports/1988-15.pdf

There is considerable discussion of the results in terms of energy savings and
operational experience with actual BBC transmitters for "AMC" which is my
second, and preferred, method. They even mention removing the transmitters peak
tube when using 6 dB of compression, this paper must be where I originally got
my information.

http://downloads.bbc.co.uk/rd/pubs/reports/1994-01.pdf

This paper has a bibliography which lists a number of earlier papers,
including
several BBC papers from about 10 years earlier which is where I think I
first
learned of these techniques. I have mostly forgotten how this stuff works
and
will have to dig out some of the references and read them through.

As I think of it, though, I don't see how the 'second form' you
mention would increase efficiency much since most audio is not at
'high mod' levels.

I don't know about that, I remember back in the early 1960s an associate of
mine
worked at a station where he proudly pointed out that the modulation monitor
meter rarely dropped off the 100% mark.

Hmm. Well, in that case, wouldn't it equate to simply having a lower
power transmitter since the 'compression' would be essentially 'always
on' constant?

I suppose so, except that the AMC scheme on a higher powered transmitter has a
better subjective signal to noise ratio because the carrier power is increased
during quiet periods, causing the receiver's AGC to reduce the gain and hence
the noise.

Maybe the second scheme I described was
just a harebrained scheme I thought up years ago and never really existed
anywhere else. However I would think it could work for stations with very
dense
continuous modulation like a rock music station. With this second scheme
you
could potentially remove the peak tube from a Doherty style transmitter,
saving
some filament power, and perhaps change the pulley ratio on the cooling
blower
reducing the power consumed by the blower. Again I will have to read
through
some of the references and refresh my memory on all this.

I seem to have remembered all this correctly from the BBC research paper rather
than it having been a harebrained idea of my own, or maybe the BBC engineers are
also harebrained.

I think it depends on what the 'goal' is and, in the final analysis,
probably not quite as 'simple' as it seems.

To me DAM makes the most sense, if the idea is to conserve power.

The BBC appears to have chosen to implement AMC, and to have achieved a
considerable energy savings, even with only 3 dB of compression, while my gold
standard is 6 dB which should provide even greater energy savings, although in
my quick scan I didn't notice that the BBC provided any energy savings data for
that mode.

It is fascinating how Weaver achieves DCC essentially by reducing the
bias
on
the modulator tube causing grid rectification of the audio and consequent
change
in the carrier amplitude. This actually saves a few components that
would
otherwise be required to provide normal modulator bias.

While the BBC used DCC for economic reasons, to reduce power and
equipment
costs, Weaver's main reason for using DCC seems to be as a poor man's
audio
processor. It isn't clear exactly how much compression Weaver's scheme
provides, perhaps as little as 4 dB? It would have been helpful if
Weaver
had
included trapezoidal oscilloscope patterns for his 300mV p-p and 2.2V p-p
test
cases using tone modulation. This would allow judging the actual audio
compression effect, which is produced by the receiver's AGC system.

Yes, his purpose was as 'compressor', despite it appearing to be the
opposite. Receiver AGC does the other half of the work.

I'm not sure if he had a numerical target in mind or simply tweaked
for best 'sound', which I imagine might leave an issue about the AGC
characteristics of the particular receiver.

I got the impression that he was constrained by the circuit an tweaked it
for
the best performance he could get out of it. It looks like there might be a
few
things he could do to further improve the performance, but the ones that
come to
my mind all complicate the circuit. A direct coupled cathode follower
between
the modulator and the modulated screen grid might extend the performance
range.

Possibly but the beauty of the thing is it's utter simplicity and that
he doesn't 'add complexity' but gets a new function almost by just
'rearranging' things. Not quite, of course, because his tetrode isn't
a dual control pentode, like in a 'conventional' broadcaster, but the
number of devices is the same.

Yes, I agree that the beauty of Weaver's "DAM" transmitter is its extreme
simplicity with no added complexity. I have come up with two simple poor man's
schemes for implementing my second form of DCC, or "AMC". One is a simple
modification of your "Beam Me Up Scotty" transmitter that actually saves a few
parts. My second approach, which I prefer, is a slight modification of Weaver's
transmitter that requires a few extra Rs & Cs, plus a diode.

Where Weaver's poor man's DAM scheme causes compression of the received audio,
my poor man's AMC scheme would cause expansion of the received audio. Of course
in the real rich man's systems appropriate expansion or compression is applied
to the audio at the transmitter to eliminate the compression or expansion
effects of the poor man's versions.

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/

[flipper]

On Fri, 06 Jul 2012 16:16:26 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Thu, 05 Jul 2012 21:52:00 -0500, John Byrns
wrote:

Why is
there audio on the cathode? I assume we are talking about a self excited
circuit so that there would be a lot of RF on the cathode, but audio?

No, I'm not talking about audio to G3 with RF to G1, self excite or
not. That's the dime a dozen, pick your tube and there's a ready made
schematic already there to download, way of doing it.

Yeah, I forgot exactly what circuit we were talking about, plus I never really
thought about the fact that the cathode in your circuit isn't grounded for audio
frequencies.

Speaking of not being sure what circuit was being discussed, when you were
making those "FMing" measurements for Alex Pogossov I was never clear on which
circuit you were measuring? Was it the full "Beam Me Up Scotty" transmitter
with separate oscillator or something else?

Beamus

Maybe I misunderstood your "like a (heptode) dual control pentode
could serve" but I took it as suggesting audio to G1 with RF to G3,
'like done with the 6ME8', should produce results 'like' the 6ME8. As
in 'why use a 6ME8 when a plain ole heptode would do the same thing'?
And that's the context all my discussions on them has been in.

As near as I can tell you correctly understood my original question.

So I disconnected the 6ME8, ran RF to heptode/DC pentode G3 with audio
to G1 which, of course, puts audio on the cathode through Rk which, in
turn, is the preamp NFB... just as with the 6ME8. I.E. a 'plop in'
replacement to see if it 'works like the 6ME8' worked.

It didn't.

First let me check to see that I correctly understand where the "craters" you
are observing actually are on the transmitted waveform, as I am interpreting
what you said they are on the positive modulation peaks, is that correct?

Yep.

I still think incorrect plate loading is the cause of this problem,

What kind of "incorrect plate loading" do you mean?

although if
there weren't audio on the cathode you would just get plain old clipping rather
than craters. The G3 drivelevel may also have some impact on this.

Well, as I said, "after twiddling around a bit." I tried RF drive
levels everywhere from 1V to 15V.

While I
think you understood my original question I didn't mean to imply that one of the
other tubes could be "plopped" directly in place of the 6ME8 without tweaking
the circuit parameters to suit the new tube, especially the plate loading and
the G3 drive level.

I didn't expect to 'do nothing' either and by 'plop in' I simply mean
the same basic topology.

Another issue that may be compounding the problem is that the G1 control is
presumably like a single ended class A audio amplifier which produces a fair
amount of second harmonic distortion. IIRC, and I hope I don't have this
backwards, the tube output on negative G1 excursions is compressed relative to
positive G1 excursions. If I have this the right way around that would imply
that if you adjust for 90% negative modulation, then the positive modulation
peaks may be considerably greater than 90% and may be limited by "bumping into
the cathode". If this is the case, correct plate loading should fix the problem.

RF amplitude was no where close to 'bumping into the cathode." I wish
it were because that could be easily fixed and would mean I was
getting maximum power generated.


Is the plate loading set correctly? Incorrect plate loading would cause an
effect similar to what you are describing.

Plate load was fine, and peaked.

I understand the "peaked" part, but how are you determining that the "plate load
was fine"? Correct plate loading is important on these grid modulated
transmitters, unlike plate modulated transmitters which are more forgiving of
incorrect plate loading. I have not seen anyone, on radiomuseum, or elsewhere,
delve into the loading issues with these small transmitters. I thought Robert
Weaver was going to get into it in his C-Quam article where he calculates the
impedance of a short monopole antenna, but he balked and didn't discuss the
loading issue.

I don't think it comes up because all of these things end up with
relatively low plate loads.


The problem is G3 being relative to
the cathode, which is going up and down with audio.

I understand what you are saying about the cathode going up and down with the
audio, I'm not sure I understand what G3 has to do with the problem though,
other than possibly the G3 drive level?

G3 is relative to the cathode. Let's say there's 1 mA total cathode
current going through the 5600 Ohm Rk. That puts the cathode at 5.6 V
and, since the G3 grid leak is normally connected to ground, means G3
bias is -5.6 V. Now stick audio into G1. Cathode voltage can swing
from 0 to 11.2 V (assuming no 2'nd harmonic, which will hopefully be
low because we're using NFB to correct it), which means G3 bias is now
changing with the audio from 0 to -11.2 V.

Now, 11.2 V is probably enough to drive all of them to cutoff,
ironically at the point that should be 'max power' (peak cathode
current), so without even getting around to applying RF, and pondering
'drive levels', it seems to me we're already 'in trouble'.

In short, we apply audio to G1, which puts audio on the cathode,
which, in turn, impresses the same audio onto G3. Except, oops, we
don't want audio on G3 and there's no way to filter it off the cathode
because that's necessary for NFB into the preamp driving G1.

You can see why it 'craters' the peaks. Cathode audio drives G3 bias
dramatically negative at the peaks, cutting off plate current.

I feel fairly confident with that analysis because it not only
explains the observed behavior but my 'testable hypothesis' of moving
the G3 grid leak to cathode produced results consistent with
predictions. (All very "scientific method," eh? )

That doesn't
happen with the 6ME8 because the deflection plates are relative to
each other and any change on the cathode is either blocked by the
screen or goes equally to both.
screen or goes equally to both.

The question here seems to be did you just get lucky with the loading for the
6ME8,

It depends on how you define 'lucky'. None of these tube broadcasters
using simply a plate tank have high enough load impedance to get very
large plate swings so generated power is only a fraction of plate
power.

I suppose it's 'lucky' you never have to worry about slamming into the
cathode but it's not so 'lucky' if you're hoping for any kind of
efficiency.

or is the plate somehow blocked from the cathode as you speculate? In any
case any transmitter is going to eventually clip in some way on positive peaks.

I tried connecting the G3 grid leak to cathode and the result looked
'almost' right but I think it was still distorted. So maybe there's a
way around it but they sure aren't a simple 'plop in' kind of thing.

That's an interesting result, I wonder if the change was due to the consequent
change in bias on G3?

Interesting way of putting it as it's due to G3 bias 'not changing'
any more. I.E. assuming impedances work out, which I didn't go to the
effort of trying to refine, G3 then goes 'up and down' in sync with
the cathode going 'up and down' so, as far as G3 is concerned, it's
'static' relative to the cathode. Well, before we apply RF, that is.

That introduces all sorts of impedance and bias current conundrums,
like G3 leakage currents into Rk, and I keep wondering if the screen
should be bypassed to the top of Rk rather than ground, but then
there's screen currents into Rk. Anyway, it became clear it wasn't a
case of simply translating the existing 6ME8 circuit into a heptode/DC
pentode version.

It does make me wonder if, despite the 6ME8 deflection plates seeming
to be being more or less independent of cathode voltage, if doing the
same thing might improve distortion slightly because they're possibly
not '100%' independent. I mean, they need a bias voltage, which also
might be 'changing' because cathode has audio on it, but just not as
much relative to the 35V bias. I'm not sure because it isn't obvious
to me whether that bias is relative to cathode or the screen but
Murphy's Law says it's whichever is worst to deal with, which would be
the cathode. I may try the same idea on Beamus but it's so clean now
I'm not sure I could tell by just looking at the scope, unless it
dramatically screws things up.

I don't follow your reference to "a simple 'plop in' kind of thing", what do
you mean?

See above.

I wouldn't expect it to be as simple as "a simple 'plop in' kind of thing", I
would expect that the circuit parameters would need tweaking for the different
tube.

It isn't a matter of 'tweaking' because G3 is intimately tied to
cathode voltage.

[flipper]

On Fri, 06 Jul 2012 18:23:04 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Thu, 05 Jul 2012 21:50:23 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Wed, 04 Jul 2012 18:10:40 -0500, John Byrns
wrote:

There are at least two types of dynamic carrier control, the BBC has
written
some good research reports on the subject. One type of DCC, as used in
Weaver's
transmitter, reduces the amplitude of the carrier when the modulation
level
is
low. The second type does essentially the opposite and reduces the
carrier
amplitude when the modulation level is high. I believe the second form
of
DCC,
as opposed to the form used by Weaver, affords greater operating
efficiency
and
economy as well as reducing the peak power that the transmitter must
produce.
The second approach also offers a subjectively lower subjective noise
level
IIRC.

I'm not sure of the history but think the modern trend is a
combination of both, lowering carrier 'in the middle' and raising it
on the ends. An example:

http://people.wallawalla.edu/~rob.fr...Control/DCC1.h
tml

Yes, I have seen that one, there is a BBC paper he

Hehe. We must have similar google settings as I saw that one too.

Interesting but in the 'results' they never speak of achieving the
'goals', just perception analysis.

Did you see the links in my update post, particularly this one?

Yes, but it hadn't been posted yet when I commented on the other.

http://downloads.bbc.co.uk/rd/pubs/reports/1988-15.pdf

There is considerable discussion of the results in terms of energy savings and
operational experience with actual BBC transmitters for "AMC" which is my
second, and preferred, method. They even mention removing the transmitters peak
tube when using 6 dB of compression, this paper must be where I originally got
my information.

http://downloads.bbc.co.uk/rd/pubs/reports/1994-01.pdf

This paper has a bibliography which lists a number of earlier papers,
including
several BBC papers from about 10 years earlier which is where I think I
first
learned of these techniques. I have mostly forgotten how this stuff works
and
will have to dig out some of the references and read them through.

As I think of it, though, I don't see how the 'second form' you
mention would increase efficiency much since most audio is not at
'high mod' levels.

I don't know about that, I remember back in the early 1960s an associate of
mine
worked at a station where he proudly pointed out that the modulation monitor
meter rarely dropped off the 100% mark.

Hmm. Well, in that case, wouldn't it equate to simply having a lower
power transmitter since the 'compression' would be essentially 'always
on' constant?

I suppose so, except that the AMC scheme on a higher powered transmitter has a
better subjective signal to noise ratio because the carrier power is increased
during quiet periods, causing the receiver's AGC to reduce the gain and hence
the noise.

Yeah, and when I first saw it I thought improved audio was 'the point'
of it.

Maybe the second scheme I described was
just a harebrained scheme I thought up years ago and never really existed
anywhere else. However I would think it could work for stations with very
dense
continuous modulation like a rock music station. With this second scheme
you
could potentially remove the peak tube from a Doherty style transmitter,
saving
some filament power, and perhaps change the pulley ratio on the cooling
blower
reducing the power consumed by the blower. Again I will have to read
through
some of the references and refresh my memory on all this.

I seem to have remembered all this correctly from the BBC research paper rather
than it having been a harebrained idea of my own, or maybe the BBC engineers are
also harebrained.

I think it depends on what the 'goal' is and, in the final analysis,
probably not quite as 'simple' as it seems.

To me DAM makes the most sense, if the idea is to conserve power.

The BBC appears to have chosen to implement AMC, and to have achieved a
considerable energy savings, even with only 3 dB of compression, while my gold
standard is 6 dB which should provide even greater energy savings, although in
my quick scan I didn't notice that the BBC provided any energy savings data for
that mode.

Well, I have changed my opinion since then

Actually, I was thinking of a modified DAM but the data does suggest
AMC is 'better'.

It is fascinating how Weaver achieves DCC essentially by reducing the
bias
on
the modulator tube causing grid rectification of the audio and consequent
change
in the carrier amplitude. This actually saves a few components that
would
otherwise be required to provide normal modulator bias.

While the BBC used DCC for economic reasons, to reduce power and
equipment
costs, Weaver's main reason for using DCC seems to be as a poor man's
audio
processor. It isn't clear exactly how much compression Weaver's scheme
provides, perhaps as little as 4 dB? It would have been helpful if
Weaver
had
included trapezoidal oscilloscope patterns for his 300mV p-p and 2.2V p-p
test
cases using tone modulation. This would allow judging the actual audio
compression effect, which is produced by the receiver's AGC system.

Yes, his purpose was as 'compressor', despite it appearing to be the
opposite. Receiver AGC does the other half of the work.

I'm not sure if he had a numerical target in mind or simply tweaked
for best 'sound', which I imagine might leave an issue about the AGC
characteristics of the particular receiver.

I got the impression that he was constrained by the circuit an tweaked it
for
the best performance he could get out of it. It looks like there might be a
few
things he could do to further improve the performance, but the ones that
come to
my mind all complicate the circuit. A direct coupled cathode follower
between
the modulator and the modulated screen grid might extend the performance
range.

Possibly but the beauty of the thing is it's utter simplicity and that
he doesn't 'add complexity' but gets a new function almost by just
'rearranging' things. Not quite, of course, because his tetrode isn't
a dual control pentode, like in a 'conventional' broadcaster, but the
number of devices is the same.

Yes, I agree that the beauty of Weaver's "DAM" transmitter is its extreme
simplicity with no added complexity.

I think we should also note that 'affecting' the audio was Weaver's
*goal* because these little broadcasters have no 'compression' like
'the big boys' routinely apply before we get to considering 'power
savings'.

I have come up with two simple poor man's
schemes for implementing my second form of DCC, or "AMC".

Are we trying to save power on a home broadcaster?

One is a simple
modification of your "Beam Me Up Scotty" transmitter that actually saves a few
parts.

I'd love to hear it. Are you thinking maybe a grid clamp on G1?
Although, I don't see that saving parts.

My second approach, which I prefer, is a slight modification of Weaver's
transmitter that requires a few extra Rs & Cs, plus a diode.

Where Weaver's poor man's DAM scheme causes compression of the received audio,
my poor man's AMC scheme would cause expansion of the received audio.

Yeah, and I'm not sure what the audible result would be because, as
Weaver points out, part of the 'problem' with home broadcasters is the
lack of compression causes them to sound 'weaker' than a 'big boy'
broadcast since, to stay below serious clipping, the average program
level has to be lower.

The missing element is AGC action so I don't know if a "poor man's
AMC" would 'enhance' dynamic range or just make it sound even weaker.

Of course
in the real rich man's systems appropriate expansion or compression is applied
to the audio at the transmitter to eliminate the compression or expansion
effects of the poor man's versions.

Yes, but that was the point of Weaver's design: actual audible
'compression' reducing the dynamic range so the end result sounds
'fuller' and 'louder' like 'the big boys' do with direct audio
compression of the source material. In his case you don't want to
'compensate' for it because he's not trying to do a "poor man's DAM"
but a "poor man's audio compressor." Which makes it doubly clever in
using DCC to accomplish a different job.

If I understand correctly AMC must be 'further compressing'
(relatively speaking) the already compressed audio because, after
receiver AGC 'compensates' for it, you still want the audio compressed
like it would be from a 'normal' transmitter.

[John Byrns]

In article ,
flipper wrote:

On Fri, 06 Jul 2012 16:16:26 -0500, John Byrns
wrote:

First let me check to see that I correctly understand where the "craters"
you
are observing actually are on the transmitted waveform, as I am interpreting
what you said they are on the positive modulation peaks, is that correct?

Yep.

I still think incorrect plate loading is the cause of this problem,

What kind of "incorrect plate loading" do you mean?

I mean that the load resistance reflected from the antenna/ground system to the
tube plate is too high.

Another issue that may be compounding the problem is that the G1 control is
presumably like a single ended class A audio amplifier which produces a fair
amount of second harmonic distortion. IIRC, and I hope I don't have this
backwards, the tube output on negative G1 excursions is compressed relative
to
positive G1 excursions. If I have this the right way around that would
imply
that if you adjust for 90% negative modulation, then the positive modulation
peaks may be considerably greater than 90% and may be limited by "bumping
into
the cathode". If this is the case, correct plate loading should fix the
problem.

RF amplitude was no where close to 'bumping into the cathode." I wish
it were because that could be easily fixed and would mean I was
getting maximum power generated.

"Bumping into the cathode" was a bad choice of words on my part and doesn't
correctly reflect what I was trying to say, however I will leave it at that
since I am accepting your analysis further down.

Is the plate loading set correctly? Incorrect plate loading would cause
an
effect similar to what you are describing.

Plate load was fine, and peaked.

I understand the "peaked" part, but how are you determining that the "plate
load
was fine"? Correct plate loading is important on these grid modulated
transmitters, unlike plate modulated transmitters which are more forgiving
of
incorrect plate loading. I have not seen anyone, on radiomuseum, or
elsewhere,
delve into the loading issues with these small transmitters. I thought
Robert
Weaver was going to get into it in his C-Quam article where he calculates
the
impedance of a short monopole antenna, but he balked and didn't discuss the
loading issue.

I don't think it comes up because all of these things end up with
relatively low plate loads.

If it is the case that "all of these things end up with relatively low plate
loads", why do so many of these circuits have to add a resistor across the plate
tank circuit?

The problem is G3 being relative to
the cathode, which is going up and down with audio.

I understand what you are saying about the cathode going up and down with
the
audio, I'm not sure I understand what G3 has to do with the problem though,
other than possibly the G3 drive level?

G3 is relative to the cathode. Let's say there's 1 mA total cathode
current going through the 5600 Ohm Rk. That puts the cathode at 5.6 V
and, since the G3 grid leak is normally connected to ground, means G3
bias is -5.6 V. Now stick audio into G1. Cathode voltage can swing
from 0 to 11.2 V (assuming no 2'nd harmonic, which will hopefully be
low because we're using NFB to correct it), which means G3 bias is now
changing with the audio from 0 to -11.2 V.

Now, 11.2 V is probably enough to drive all of them to cutoff,
ironically at the point that should be 'max power' (peak cathode
current), so without even getting around to applying RF, and pondering
'drive levels', it seems to me we're already 'in trouble'.

In short, we apply audio to G1, which puts audio on the cathode,
which, in turn, impresses the same audio onto G3. Except, oops, we
don't want audio on G3 and there's no way to filter it off the cathode
because that's necessary for NFB into the preamp driving G1.

You can see why it 'craters' the peaks. Cathode audio drives G3 bias
dramatically negative at the peaks, cutting off plate current.

I feel fairly confident with that analysis because it not only
explains the observed behavior but my 'testable hypothesis' of moving
the G3 grid leak to cathode produced results consistent with
predictions. (All very "scientific method," eh? )

If it is the case that "all of these things end up with relatively low plate
loads", why do so many of these circuits have to add a resistor across the plate
tank circuit?

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/

[John Byrns]

In article ,
flipper wrote:

On Fri, 06 Jul 2012 18:23:04 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Thu, 05 Jul 2012 21:50:23 -0500, John Byrns
wrote:

In article ,
flipper wrote:

Yes, his purpose was as 'compressor', despite it appearing to be the
opposite. Receiver AGC does the other half of the work.

I'm not sure if he had a numerical target in mind or simply tweaked
for best 'sound', which I imagine might leave an issue about the AGC
characteristics of the particular receiver.

I got the impression that he was constrained by the circuit an tweaked it
for
the best performance he could get out of it. It looks like there might
be a
few
things he could do to further improve the performance, but the ones that
come to
my mind all complicate the circuit. A direct coupled cathode follower
between
the modulator and the modulated screen grid might extend the performance
range.

Possibly but the beauty of the thing is it's utter simplicity and that
he doesn't 'add complexity' but gets a new function almost by just
'rearranging' things. Not quite, of course, because his tetrode isn't
a dual control pentode, like in a 'conventional' broadcaster, but the
number of devices is the same.

Yes, I agree that the beauty of Weaver's "DAM" transmitter is its extreme
simplicity with no added complexity.

I think we should also note that 'affecting' the audio was Weaver's
*goal* because these little broadcasters have no 'compression' like
'the big boys' routinely apply before we get to considering 'power
savings'.

Yes, the article made it clear that his goal was the compression the scheme
provided, although it is not clear from the data he presented how much
compression he actually achieves. Perhaps a simple compressor circuit could do
better at the expense of an additional tube? I suppose an extra tube would
defile the beauty of it though.

I have come up with two simple poor man's
schemes for implementing my second form of DCC, or "AMC".

Are we trying to save power on a home broadcaster?

No, but since I had originally mentioned two forms of DCC I felt compelled to
offer an idea for applying the second form to a home broadcaster even if doing
so might be counter productive. The simple one tube compressor mentioned above
might ameliorate the problem though, although it would make more sense to simply
apply the simple one tube compressor to a standard home broadcaster.

One is a simple
modification of your "Beam Me Up Scotty" transmitter that actually saves a
few
parts.

I'd love to hear it. Are you thinking maybe a grid clamp on G1?
Although, I don't see that saving parts.

Yes, I was thinking of the grid clamp idea like weaver uses, except applied to
the 6ME8 G1 where it would have the opposite effect of Weaver's grid clamp on
the audio preamp tube. This assumes that the 6ME8 operating conditions could be
suitably tweaked, perhaps G2 voltage, to compensate for the changed G1
conditions. The parts saving would be the elimination of the 6ME8 cathode
resistor and feedback network.

My second approach, which I prefer, is a slight modification of Weaver's
transmitter that requires a few extra Rs & Cs, plus a diode.

Where Weaver's poor man's DAM scheme causes compression of the received
audio,
my poor man's AMC scheme would cause expansion of the received audio.

Yeah, and I'm not sure what the audible result would be because, as
Weaver points out, part of the 'problem' with home broadcasters is the
lack of compression causes them to sound 'weaker' than a 'big boy'
broadcast since, to stay below serious clipping, the average program
level has to be lower.

The missing element is AGC action so I don't know if a "poor man's
AMC" would 'enhance' dynamic range or just make it sound even weaker.

Yes, I agree, as I said it was just an exercise to go along with my mention of
the second form of DCC. I think just adding a simple one tube compressor to the
standard home transmitter is the way to go.

Of course
in the real rich man's systems appropriate expansion or compression is
applied
to the audio at the transmitter to eliminate the compression or expansion
effects of the poor man's versions.

Yes, but that was the point of Weaver's design: actual audible
'compression' reducing the dynamic range so the end result sounds
'fuller' and 'louder' like 'the big boys' do with direct audio
compression of the source material. In his case you don't want to
'compensate' for it because he's not trying to do a "poor man's DAM"
but a "poor man's audio compressor." Which makes it doubly clever in
using DCC to accomplish a different job.

Very True.

If I understand correctly AMC must be 'further compressing'
(relatively speaking) the already compressed audio because, after
receiver AGC 'compensates' for it, you still want the audio compressed
like it would be from a 'normal' transmitter.

Yes, that is my understanding also, assuming I am parsing your words correctly.

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/

[flipper]

On Sat, 07 Jul 2012 22:10:25 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Fri, 06 Jul 2012 16:16:26 -0500, John Byrns
wrote:

First let me check to see that I correctly understand where the "craters"
you
are observing actually are on the transmitted waveform, as I am interpreting
what you said they are on the positive modulation peaks, is that correct?

Yep.

I still think incorrect plate loading is the cause of this problem,

What kind of "incorrect plate loading" do you mean?

I mean that the load resistance reflected from the antenna/ground system to the
tube plate is too high.

Okay. Well, that's what I figured you meant but even the 'high power'
Beamus, meaning with the 2.2k Rk, only did 90 Vp at 100% mod, leaving
65 V to cathode, since B+ came in lower at the higher current draw. On
the '100 mW' Beamus B+ is upwards to 190 V and the lower plate current
doesn't produce as much swing, of course.

The heptode/DC pentode version was doing essentially the same thing
because the load and plate current was the same, by intent.

At the moment I'm not quite sure how much because, somehow, I've
screwed the dern thing up, I mean the reassembled '100 mW' Beamus,
and, after 8 hours of rechecking every connection and component, have
not a blooming clue what's wrong with it.

The only reason I mentioned it is, since I have no idea what nor when
things went wrong, no measurements done since 'converting' it to the
heptode, and back, are reliable.

Odds are I didn't something 'stupid' when restoring it back to the
6ME8 but I'll be damned if I can find it.


Another issue that may be compounding the problem is that the G1 control is
presumably like a single ended class A audio amplifier which produces a fair
amount of second harmonic distortion. IIRC, and I hope I don't have this
backwards, the tube output on negative G1 excursions is compressed relative
to
positive G1 excursions. If I have this the right way around that would
imply
that if you adjust for 90% negative modulation, then the positive modulation
peaks may be considerably greater than 90% and may be limited by "bumping
into
the cathode". If this is the case, correct plate loading should fix the
problem.

RF amplitude was no where close to 'bumping into the cathode." I wish
it were because that could be easily fixed and would mean I was
getting maximum power generated.

"Bumping into the cathode" was a bad choice of words on my part and doesn't
correctly reflect what I was trying to say, however I will leave it at that
since I am accepting your analysis further down.

Yeah, I understand and wasn't going to quibble about minimum plate
voltage either. Near as I can recall maximum swing at 100% mod was on
the order of 50V peak so there was gobs of B+ left.


Is the plate loading set correctly? Incorrect plate loading would cause
an
effect similar to what you are describing.

Plate load was fine, and peaked.

I understand the "peaked" part, but how are you determining that the "plate
load
was fine"? Correct plate loading is important on these grid modulated
transmitters, unlike plate modulated transmitters which are more forgiving
of
incorrect plate loading. I have not seen anyone, on radiomuseum, or
elsewhere,
delve into the loading issues with these small transmitters. I thought
Robert
Weaver was going to get into it in his C-Quam article where he calculates
the
impedance of a short monopole antenna, but he balked and didn't discuss the
loading issue.

I don't think it comes up because all of these things end up with
relatively low plate loads.

If it is the case that "all of these things end up with relatively low plate
loads", why do so many of these circuits have to add a resistor across the plate
tank circuit?

I'm not aware of any plain ole tank loaded dual control pentode
broadcasters that do, including Norm's original 'power monster' 6888
version.


The problem is G3 being relative to
the cathode, which is going up and down with audio.

I understand what you are saying about the cathode going up and down with
the
audio, I'm not sure I understand what G3 has to do with the problem though,
other than possibly the G3 drive level?

G3 is relative to the cathode. Let's say there's 1 mA total cathode
current going through the 5600 Ohm Rk. That puts the cathode at 5.6 V
and, since the G3 grid leak is normally connected to ground, means G3
bias is -5.6 V. Now stick audio into G1. Cathode voltage can swing
from 0 to 11.2 V (assuming no 2'nd harmonic, which will hopefully be
low because we're using NFB to correct it), which means G3 bias is now
changing with the audio from 0 to -11.2 V.

Now, 11.2 V is probably enough to drive all of them to cutoff,
ironically at the point that should be 'max power' (peak cathode
current), so without even getting around to applying RF, and pondering
'drive levels', it seems to me we're already 'in trouble'.

In short, we apply audio to G1, which puts audio on the cathode,
which, in turn, impresses the same audio onto G3. Except, oops, we
don't want audio on G3 and there's no way to filter it off the cathode
because that's necessary for NFB into the preamp driving G1.

You can see why it 'craters' the peaks. Cathode audio drives G3 bias
dramatically negative at the peaks, cutting off plate current.

I feel fairly confident with that analysis because it not only
explains the observed behavior but my 'testable hypothesis' of moving
the G3 grid leak to cathode produced results consistent with
predictions. (All very "scientific method," eh? )

If it is the case that "all of these things end up with relatively low plate
loads", why do so many of these circuits have to add a resistor across the plate
tank circuit?

Same answer: I don't know of any. I imagine that might be due to the
dual control pentode's relative low plate impedance limiting Q and
perhaps it could be more of an issue with heptodes.

[John Byrns]

In article ,
flipper wrote:

On Sat, 07 Jul 2012 22:10:25 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Fri, 06 Jul 2012 16:16:26 -0500, John Byrns
wrote:

First let me check to see that I correctly understand where the "craters"
you
are observing actually are on the transmitted waveform, as I am
interpreting
what you said they are on the positive modulation peaks, is that correct?

Yep.

I still think incorrect plate loading is the cause of this problem,

What kind of "incorrect plate loading" do you mean?

I mean that the load resistance reflected from the antenna/ground system to
the
tube plate is too high.

Okay. Well, that's what I figured you meant but even the 'high power'
Beamus, meaning with the 2.2k Rk, only did 90 Vp at 100% mod, leaving
65 V to cathode, since B+ came in lower at the higher current draw. On
the '100 mW' Beamus B+ is upwards to 190 V and the lower plate current
doesn't produce as much swing, of course.

The heptode/DC pentode version was doing essentially the same thing
because the load and plate current was the same, by intent.

At the moment I'm not quite sure how much because, somehow, I've
screwed the dern thing up, I mean the reassembled '100 mW' Beamus,
and, after 8 hours of rechecking every connection and component, have
not a blooming clue what's wrong with it.

The only reason I mentioned it is, since I have no idea what nor when
things went wrong, no measurements done since 'converting' it to the
heptode, and back, are reliable.

Odds are I didn't something 'stupid' when restoring it back to the
6ME8 but I'll be damned if I can find it.


Another issue that may be compounding the problem is that the G1 control
is
presumably like a single ended class A audio amplifier which produces a
fair
amount of second harmonic distortion. IIRC, and I hope I don't have this
backwards, the tube output on negative G1 excursions is compressed
relative
to
positive G1 excursions. If I have this the right way around that would
imply
that if you adjust for 90% negative modulation, then the positive
modulation
peaks may be considerably greater than 90% and may be limited by "bumping
into
the cathode". If this is the case, correct plate loading should fix the
problem.

RF amplitude was no where close to 'bumping into the cathode." I wish
it were because that could be easily fixed and would mean I was
getting maximum power generated.

"Bumping into the cathode" was a bad choice of words on my part and doesn't
correctly reflect what I was trying to say, however I will leave it at that
since I am accepting your analysis further down.

Yeah, I understand and wasn't going to quibble about minimum plate
voltage either. Near as I can recall maximum swing at 100% mod was on
the order of 50V peak so there was gobs of B+ left.


Is the plate loading set correctly? Incorrect plate loading would
cause
an
effect similar to what you are describing.

Plate load was fine, and peaked.

I understand the "peaked" part, but how are you determining that the
"plate
load
was fine"? Correct plate loading is important on these grid modulated
transmitters, unlike plate modulated transmitters which are more
forgiving
of
incorrect plate loading. I have not seen anyone, on radiomuseum, or
elsewhere,
delve into the loading issues with these small transmitters. I thought
Robert
Weaver was going to get into it in his C-Quam article where he calculates
the
impedance of a short monopole antenna, but he balked and didn't discuss
the
loading issue.

I don't think it comes up because all of these things end up with
relatively low plate loads.

If it is the case that "all of these things end up with relatively low plate
loads", why do so many of these circuits have to add a resistor across the
plate
tank circuit?

I'm not aware of any plain ole tank loaded dual control pentode
broadcasters that do, including Norm's original 'power monster' 6888
version.


The problem is G3 being relative to
the cathode, which is going up and down with audio.

I understand what you are saying about the cathode going up and down with
the
audio, I'm not sure I understand what G3 has to do with the problem
though,
other than possibly the G3 drive level?

G3 is relative to the cathode. Let's say there's 1 mA total cathode
current going through the 5600 Ohm Rk. That puts the cathode at 5.6 V
and, since the G3 grid leak is normally connected to ground, means G3
bias is -5.6 V. Now stick audio into G1. Cathode voltage can swing
from 0 to 11.2 V (assuming no 2'nd harmonic, which will hopefully be
low because we're using NFB to correct it), which means G3 bias is now
changing with the audio from 0 to -11.2 V.

Now, 11.2 V is probably enough to drive all of them to cutoff,
ironically at the point that should be 'max power' (peak cathode
current), so without even getting around to applying RF, and pondering
'drive levels', it seems to me we're already 'in trouble'.

In short, we apply audio to G1, which puts audio on the cathode,
which, in turn, impresses the same audio onto G3. Except, oops, we
don't want audio on G3 and there's no way to filter it off the cathode
because that's necessary for NFB into the preamp driving G1.

You can see why it 'craters' the peaks. Cathode audio drives G3 bias
dramatically negative at the peaks, cutting off plate current.

I feel fairly confident with that analysis because it not only
explains the observed behavior but my 'testable hypothesis' of moving
the G3 grid leak to cathode produced results consistent with
predictions. (All very "scientific method," eh? )

If it is the case that "all of these things end up with relatively low plate
loads", why do so many of these circuits have to add a resistor across the
plate
tank circuit?

Same answer: I don't know of any. I imagine that might be due to the
dual control pentode's relative low plate impedance limiting Q and
perhaps it could be more of an issue with heptodes.

Ops, Sorry, I made an editing mistake and pasted in the wrong text making for
the repeat of the same text from above. The last bit was supposed to be:

"OK, since you are confident in your analysis I will accept it. Given that is
the problem wouldn't replacing the "G3 grid leak" resistor with an RFC between
G3 and the cathode fix the problem?"

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/

[flipper]

On Sat, 07 Jul 2012 22:17:00 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Fri, 06 Jul 2012 18:23:04 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Thu, 05 Jul 2012 21:50:23 -0500, John Byrns
wrote:

In article ,
flipper wrote:

Yes, his purpose was as 'compressor', despite it appearing to be the
opposite. Receiver AGC does the other half of the work.

I'm not sure if he had a numerical target in mind or simply tweaked
for best 'sound', which I imagine might leave an issue about the AGC
characteristics of the particular receiver.

I got the impression that he was constrained by the circuit an tweaked it
for
the best performance he could get out of it. It looks like there might
be a
few
things he could do to further improve the performance, but the ones that
come to
my mind all complicate the circuit. A direct coupled cathode follower
between
the modulator and the modulated screen grid might extend the performance
range.

Possibly but the beauty of the thing is it's utter simplicity and that
he doesn't 'add complexity' but gets a new function almost by just
'rearranging' things. Not quite, of course, because his tetrode isn't
a dual control pentode, like in a 'conventional' broadcaster, but the
number of devices is the same.

Yes, I agree that the beauty of Weaver's "DAM" transmitter is its extreme
simplicity with no added complexity.

I think we should also note that 'affecting' the audio was Weaver's
*goal* because these little broadcasters have no 'compression' like
'the big boys' routinely apply before we get to considering 'power
savings'.

Yes, the article made it clear that his goal was the compression the scheme
provided, although it is not clear from the data he presented how much
compression he actually achieves. Perhaps a simple compressor circuit could do
better at the expense of an additional tube? I suppose an extra tube would
defile the beauty of it though.

I don't remember in which of his posts I read it but modulation was
effectively '200%' so that would be 6 dB, if AGC fully compensated.


I have come up with two simple poor man's
schemes for implementing my second form of DCC, or "AMC".

Are we trying to save power on a home broadcaster?

No, but since I had originally mentioned two forms of DCC I felt compelled to
offer an idea for applying the second form to a home broadcaster even if doing
so might be counter productive. The simple one tube compressor mentioned above
might ameliorate the problem though, although it would make more sense to simply
apply the simple one tube compressor to a standard home broadcaster.

Yes. I'm normally all for 'simple' and 'clever' solutions but, in this
case, think I'd prefer a 'separate' compressor. For one, since I
normally use a PC for audio I can use much more sophisticated software
compression.

I though it sounded great when first tried but, you know, I think I
prefer the 'weaker' uncompressed playback. At least I have a choice
when they're separate.

One is a simple
modification of your "Beam Me Up Scotty" transmitter that actually saves a
few
parts.

I'd love to hear it. Are you thinking maybe a grid clamp on G1?
Although, I don't see that saving parts.

Yes, I was thinking of the grid clamp idea like weaver uses, except applied to
the 6ME8 G1 where it would have the opposite effect of Weaver's grid clamp on
the audio preamp tube. This assumes that the 6ME8 operating conditions could be
suitably tweaked, perhaps G2 voltage, to compensate for the changed G1
conditions. The parts saving would be the elimination of the 6ME8 cathode
resistor and feedback network.

Pulling out feedback is a problem because linearity sucks without it.

My second approach, which I prefer, is a slight modification of Weaver's
transmitter that requires a few extra Rs & Cs, plus a diode.

Where Weaver's poor man's DAM scheme causes compression of the received
audio,
my poor man's AMC scheme would cause expansion of the received audio.

Yeah, and I'm not sure what the audible result would be because, as
Weaver points out, part of the 'problem' with home broadcasters is the
lack of compression causes them to sound 'weaker' than a 'big boy'
broadcast since, to stay below serious clipping, the average program
level has to be lower.

The missing element is AGC action so I don't know if a "poor man's
AMC" would 'enhance' dynamic range or just make it sound even weaker.

Yes, I agree, as I said it was just an exercise to go along with my mention of
the second form of DCC. I think just adding a simple one tube compressor to the
standard home transmitter is the way to go.

Me too. Or the software approach, although I suppose it's kind of
weird to be all hot on 'tube' transmitters and then stick a DSP in the
chain.

Of course
in the real rich man's systems appropriate expansion or compression is
applied
to the audio at the transmitter to eliminate the compression or expansion
effects of the poor man's versions.

Yes, but that was the point of Weaver's design: actual audible
'compression' reducing the dynamic range so the end result sounds
'fuller' and 'louder' like 'the big boys' do with direct audio
compression of the source material. In his case you don't want to
'compensate' for it because he's not trying to do a "poor man's DAM"
but a "poor man's audio compressor." Which makes it doubly clever in
using DCC to accomplish a different job.

Very True.

If I understand correctly AMC must be 'further compressing'
(relatively speaking) the already compressed audio because, after
receiver AGC 'compensates' for it, you still want the audio compressed
like it would be from a 'normal' transmitter.

Yes, that is my understanding also, assuming I am parsing your words correctly.

Its really kind of amazing just how mangled AM transmissions are.

[flipper]

On Sat, 07 Jul 2012 23:47:38 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Sat, 07 Jul 2012 22:10:25 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Fri, 06 Jul 2012 16:16:26 -0500, John Byrns
wrote:

First let me check to see that I correctly understand where the "craters"
you
are observing actually are on the transmitted waveform, as I am
interpreting
what you said they are on the positive modulation peaks, is that correct?

Yep.

I still think incorrect plate loading is the cause of this problem,

What kind of "incorrect plate loading" do you mean?

I mean that the load resistance reflected from the antenna/ground system to
the
tube plate is too high.

Okay. Well, that's what I figured you meant but even the 'high power'
Beamus, meaning with the 2.2k Rk, only did 90 Vp at 100% mod, leaving
65 V to cathode, since B+ came in lower at the higher current draw. On
the '100 mW' Beamus B+ is upwards to 190 V and the lower plate current
doesn't produce as much swing, of course.

The heptode/DC pentode version was doing essentially the same thing
because the load and plate current was the same, by intent.

At the moment I'm not quite sure how much because, somehow, I've
screwed the dern thing up, I mean the reassembled '100 mW' Beamus,
and, after 8 hours of rechecking every connection and component, have
not a blooming clue what's wrong with it.

The only reason I mentioned it is, since I have no idea what nor when
things went wrong, no measurements done since 'converting' it to the
heptode, and back, are reliable.

Odds are I didn't something 'stupid' when restoring it back to the
6ME8 but I'll be damned if I can find it.


Another issue that may be compounding the problem is that the G1 control
is
presumably like a single ended class A audio amplifier which produces a
fair
amount of second harmonic distortion. IIRC, and I hope I don't have this
backwards, the tube output on negative G1 excursions is compressed
relative
to
positive G1 excursions. If I have this the right way around that would
imply
that if you adjust for 90% negative modulation, then the positive
modulation
peaks may be considerably greater than 90% and may be limited by "bumping
into
the cathode". If this is the case, correct plate loading should fix the
problem.

RF amplitude was no where close to 'bumping into the cathode." I wish
it were because that could be easily fixed and would mean I was
getting maximum power generated.

"Bumping into the cathode" was a bad choice of words on my part and doesn't
correctly reflect what I was trying to say, however I will leave it at that
since I am accepting your analysis further down.

Yeah, I understand and wasn't going to quibble about minimum plate
voltage either. Near as I can recall maximum swing at 100% mod was on
the order of 50V peak so there was gobs of B+ left.


Is the plate loading set correctly? Incorrect plate loading would
cause
an
effect similar to what you are describing.

Plate load was fine, and peaked.

I understand the "peaked" part, but how are you determining that the
"plate
load
was fine"? Correct plate loading is important on these grid modulated
transmitters, unlike plate modulated transmitters which are more
forgiving
of
incorrect plate loading. I have not seen anyone, on radiomuseum, or
elsewhere,
delve into the loading issues with these small transmitters. I thought
Robert
Weaver was going to get into it in his C-Quam article where he calculates
the
impedance of a short monopole antenna, but he balked and didn't discuss
the
loading issue.

I don't think it comes up because all of these things end up with
relatively low plate loads.

If it is the case that "all of these things end up with relatively low plate
loads", why do so many of these circuits have to add a resistor across the
plate
tank circuit?

I'm not aware of any plain ole tank loaded dual control pentode
broadcasters that do, including Norm's original 'power monster' 6888
version.


The problem is G3 being relative to
the cathode, which is going up and down with audio.

I understand what you are saying about the cathode going up and down with
the
audio, I'm not sure I understand what G3 has to do with the problem
though,
other than possibly the G3 drive level?

G3 is relative to the cathode. Let's say there's 1 mA total cathode
current going through the 5600 Ohm Rk. That puts the cathode at 5.6 V
and, since the G3 grid leak is normally connected to ground, means G3
bias is -5.6 V. Now stick audio into G1. Cathode voltage can swing
from 0 to 11.2 V (assuming no 2'nd harmonic, which will hopefully be
low because we're using NFB to correct it), which means G3 bias is now
changing with the audio from 0 to -11.2 V.

Now, 11.2 V is probably enough to drive all of them to cutoff,
ironically at the point that should be 'max power' (peak cathode
current), so without even getting around to applying RF, and pondering
'drive levels', it seems to me we're already 'in trouble'.

In short, we apply audio to G1, which puts audio on the cathode,
which, in turn, impresses the same audio onto G3. Except, oops, we
don't want audio on G3 and there's no way to filter it off the cathode
because that's necessary for NFB into the preamp driving G1.

You can see why it 'craters' the peaks. Cathode audio drives G3 bias
dramatically negative at the peaks, cutting off plate current.

I feel fairly confident with that analysis because it not only
explains the observed behavior but my 'testable hypothesis' of moving
the G3 grid leak to cathode produced results consistent with
predictions. (All very "scientific method," eh? )

If it is the case that "all of these things end up with relatively low plate
loads", why do so many of these circuits have to add a resistor across the
plate
tank circuit?

Same answer: I don't know of any. I imagine that might be due to the
dual control pentode's relative low plate impedance limiting Q and
perhaps it could be more of an issue with heptodes.

Ops, Sorry, I made an editing mistake and pasted in the wrong text making for
the repeat of the same text from above. The last bit was supposed to be:

"OK, since you are confident in your analysis I will accept it. Given that is
the problem wouldn't replacing the "G3 grid leak" resistor with an RFC between
G3 and the cathode fix the problem?"

Oh, okay. Well, things are a bit confusing since I've now got the 'its
broke' problem, so I can't be 100% sure about its behavior when I
tried putting the G3 grid leak to cathode, but I'm not sure what the
suggested RFC is intended to 'fix'. With G3 grid leak to ground the
problem is audio on G3 and an RFC isn't going to 'help' there because
it's not in the G3-cathode path, which is 'through the tube'. I don't
think the 'its broke' problem has any impact on this aspect because
the analysis 'is clear' whether you even build it or not.

With G3 grid leak on the cathode I didn't note an 'RF problem' but
can't say much else because I don't know if things were 'right'. I
mean, in theory it had a chance of working, and the results were
'close', so whatever is 'broke' may have affected the result.

However, I *think* the heptode/dc pentode version was 'correct' and
that the 'mistake' came in restoring the 6ME8 but it could have also
happened when I tried putting the 6ME8 deflector bias to cathode, like
I indicated I might try. I don't know which since I didn't 'retest'
Beamus after removing the heptode/dc pentode, because 'we know Beamus
works' rolling eyes. So then I tried the Beamus deflector to cathode
test and the deflector behavior looked as expected with lovely RF, en
block, moving up and down with the audio. But there was a rather large
'floor' to negative mod, below which it would not go. I'm not quite
sure why but suspect stored charge on the bias bypass cap, which is
needed to 'feed through' full audio amplitude to the bias point. Also,
deflector bias behaved oddly. In the 'normal' version there is a point
of 'peak output amplitude' between 35V and 40V but with the revised
circuit it just kept on growing and growing as bias was increased, all
the way to 100 V, as if they were acting as a screen. Anyway, I didn't
see a 'quick fix' and, so, restored it back to 'normal', or so I
thought, but now there's compression of positive mod.

All bias voltages look right, RF amplitude is right, I've swapped
tubes, and even re-ohmed every resistor in the circuit. Everything
looks perfectly fine, except for not working right.

I'm thinking of taking a break to see if the alternate reality
twilight zone black hole that's apparently engulfed my workroom will
drift off to harass someone else.

[Patrick Turner]

Fipper mentioned something everyone wanting hifi from AM should
know :-

Its really kind of amazing just how mangled AM transmissions are.

Indeed. So many stations are not so good to listen to. But then some
are definately more natural true to life sounding than others. We have
a couple of them here which make owning an AM radio worthwhile.

I don't have a huge amount of time to occupy my brain about the 1,001
issues surrounding fascinating use of dual deflection plate pentodes.
If I ever get around to using one, I'll just try something that's
already been done and measure/observe/think/tweak as one does while
wanting to farnarkle around in one's shed because there's FAE2doo.

BUT, I suddenly remembered the type number of a couple of such tubes
given to me about 7 years ago just before when a dear radio ham friend
of 72 died. The tube is the 7360, and if you look it up in TDSL you
find a pdf with a circuit for it.

http://www.tubebooks.org/tubedata/HB...Tubes/7360.PDF

Patrick Turner.

[John Byrns]

In article ,
flipper wrote:

On Sat, 07 Jul 2012 22:17:00 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Fri, 06 Jul 2012 18:23:04 -0500, John Byrns
wrote:

In article ,
flipper wrote:

Possibly but the beauty of the thing is it's utter simplicity and that
he doesn't 'add complexity' but gets a new function almost by just
'rearranging' things. Not quite, of course, because his tetrode isn't
a dual control pentode, like in a 'conventional' broadcaster, but the
number of devices is the same.

Yes, I agree that the beauty of Weaver's "DAM" transmitter is its extreme
simplicity with no added complexity.

I think we should also note that 'affecting' the audio was Weaver's
*goal* because these little broadcasters have no 'compression' like
'the big boys' routinely apply before we get to considering 'power
savings'.

Yes, the article made it clear that his goal was the compression the scheme
provided, although it is not clear from the data he presented how much
compression he actually achieves. Perhaps a simple compressor circuit could
do
better at the expense of an additional tube? I suppose an extra tube would
defile the beauty of it though.

I don't remember in which of his posts I read it but modulation was
effectively '200%' so that would be 6 dB, if AGC fully compensated.

Robert Weaver said in the "Update" to his "A One-Tube Controlled Carrier AM
Transmitter":

"From this trace I made the following measurements:"
"Upward modulation = (7.0 - 2.2)/2.2 * 100 = 218%"
"Downward modulation = (2.2 - 0.2)/2.2 * 100 = 91%"

"The average of the upward and downward modulation is then 154%"

Is this where you got the "200%" figure, or was it elsewhere?

I don't think you can infer, from the modulation percentage alone, the degree of
compression produced by Weaver's scheme in the receiver. Weaver's scheme is
linear with respect to the audio applied to the transmitter; it is the change in
carrier level that produces the compression in the receiver. Using the scope
picture from which Weaver calculated the modulation percentages above, one can
estimate the carrier level at full modulation is about 3.5 divisions, while the
unmodulated carrier is 2.2 divisions in the picture. If the receiver AGC is
fully effective then this would equate to compression of 4 dB.

While Weaver's scheme is clever, it's a good thing it doesn't add any complexity
to his transmitter, as a maximum of only 4 dB compression isn't much, and since
the actual compression occurs in the receiver it doesn't do anything for the
received signal to noise ratio either.

A simple one tube compressor could provide considerably more than 4 dB of
compression and would actually improve the received signal to noise ratio.

No, but since I had originally mentioned two forms of DCC I felt compelled
to
offer an idea for applying the second form to a home broadcaster even if
doing
so might be counter productive. The simple one tube compressor mentioned
above
might ameliorate the problem though, although it would make more sense to
simply
apply the simple one tube compressor to a standard home broadcaster.

Yes. I'm normally all for 'simple' and 'clever' solutions but, in this
case, think I'd prefer a 'separate' compressor. For one, since I
normally use a PC for audio I can use much more sophisticated software
compression.

I though it sounded great when first tried but, you know, I think I
prefer the 'weaker' uncompressed playback. At least I have a choice
when they're separate.

Weaver's transmitter could provide a "choice" with simple modification. By
placing the parallel combination of a resistor, capacitor, and switch in the
cathode circuit of the triode modulator section, one would have Weaver's DCC
operation when the switch is closed, grounding the cathode, and normal constant
carrier operation when the switch is open changing the modulator bias to
eliminate the grid clipping.

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/

[flipper]

On Sun, 8 Jul 2012 03:51:40 -0700 (PDT), Patrick Turner
wrote:


Fipper mentioned something everyone wanting hifi from AM should
know :-

Anyone wanting 'hi-fi' from existing AM is in trouble long before
issues of compression come up because the S/N ratio and allocated
bandwidth are simply insufficient to achieve any reasonable definition
of the term.

You can, however, manage results better than the 'typical' AM radio,
sometimes referred to as 'wideband AM' or some other euphemism, but to
say 'hi-fi' is wishful thinking.

Its really kind of amazing just how mangled AM transmissions are.

Indeed. So many stations are not so good to listen to. But then some
are definately more natural true to life sounding than others. We have
a couple of them here which make owning an AM radio worthwhile.

The topic at hand was AM but it isn't the only medium using
compression. FM is compressed and, unfortunately, so are most 'rock
and roll' CDs. Apparently digital radio is as well.

Having said that, there are 'pro' arguments to compressing, such as
the relatively low S/N ratio of AM, ambient noise level in automobiles
and various 'public' venues, etc. but 'hi-hi' isn't one of them.

What I was hinting at, though, without being explicit, is that AMC
layers 'more distortion' on top of existing distortion and the claimed
result is 'not much worse'. Well, you can 'not much worse' yourself
into garbage if your comparison is always against the last 'not much
worse' degradation over the previous 'not much worse'. For example, AM
audio bandwidth in the US used to be 15 KHz but was reduced in 1989 to
10.2 KHz which, I imagine, was subjectively 'not much worse', at least
to whatever they deemed to be the 'average listener', but it
effectively killed any notion of 'hi-fi', or semi 'hi-fi', AM so we're
now comparing 'not much worse' against "why bother trying?"

Note, though, that means there's actually a useful purpose, beyond
simply choosing your own program material, to a 'home broadcaster'
because, under U.S. Part 15 rules, you're pretty much free to transmit
whatever you like as long as its confined to your own domain and does
not interfere with licensed broadcasters.

So, in effect, you're free to broadcast something that has a fighting
chance of resembling 'hi-hi' AM, including uncompressed if one so
desires, to your old, much wider bandwidth, AM radios.

I had that in mind with Beamus and you'll notice it measured flat to
18 KHz, so it is capable of as much 'fi' as AM can produce.

I don't have a huge amount of time to occupy my brain about the 1,001
issues surrounding fascinating use of dual deflection plate pentodes.
If I ever get around to using one, I'll just try something that's
already been done and measure/observe/think/tweak as one does while
wanting to farnarkle around in one's shed because there's FAE2doo.

BUT, I suddenly remembered the type number of a couple of such tubes
given to me about 7 years ago just before when a dear radio ham friend
of 72 died. The tube is the 7360, and if you look it up in TDSL you
find a pdf with a circuit for it.

http://www.tubebooks.org/tubedata/HB...Tubes/7360.PDF

Patrick Turner.

Thank you but I already have it in my 'beam tubes' folder. But if you
want some more schematics to ponder copying then try this one.

http://jlandrigan.com/files/Receiver...the%207360.pdf

Of potential interest, it has a product detector at the bottom.

You might want to consider selling those on Ebay and tinkering with
less expensive tubes because the 7360 is in short supply these days
and commands prices in the 30 buck range. That, of course, is why I'm
using the $1 6ME8.

[Patrick Turner]

On Jul 10, 10:05*am, flipper wrote:
On Sun, 8 Jul 2012 03:51:40 -0700 (PDT), Patrick Turner

wrote:
Fipper mentioned something everyone wanting hifi from AM should
know :-

Anyone wanting 'hi-fi' from existing AM is in trouble long before
issues of compression come up because the S/N ratio and allocated
bandwidth are simply insufficient to achieve any reasonable definition
of the term.

You can, however, manage results better than the 'typical' AM radio,
sometimes referred to as 'wideband AM' or some other euphemism, but to
say 'hi-fi' is wishful thinking.

Its really kind of amazing just how mangled AM transmissions are.

Indeed. So many stations are not so good to listen to. But then some
are definately more natural true to life sounding than others. We have
a couple of them here which make owning an AM radio worthwhile.

The topic at hand was AM but it isn't the only medium using
compression. FM is compressed and, unfortunately, so are most 'rock
and roll' CDs. Apparently digital radio is as well.

Having said that, there are 'pro' arguments to compressing, such as
the relatively low S/N ratio of AM, ambient noise level in automobiles
and various 'public' venues, etc. but 'hi-hi' isn't one of them.

What I was hinting at, though, without being explicit, is that AMC
layers 'more distortion' on top of existing distortion and the claimed
result is 'not much worse'. Well, you can 'not much worse' yourself
into garbage if your comparison is always against the last 'not much
worse' degradation over the previous 'not much worse'. For example, AM
audio bandwidth in the US used to be 15 KHz but was reduced in 1989 to
10.2 KHz which, I imagine, was subjectively 'not much worse', at least
to whatever they deemed to be the 'average listener', but it
effectively killed any notion of 'hi-fi', or semi 'hi-fi', AM so we're
now comparing 'not much worse' against "why bother trying?"

Our Oz stations began at 10KHz apart but later became 9kHz due to
broadcasters wanting more stations.
But Oz is a big place, so 2CY in Canberra can broadcast with 9kHz of
audio BW without being heard where some other station is on the same
F, or be heard where some other station is only 9kHz away, because
side bands extend out to +/-9 kHz each side of the carrier F.

Note, though, that means there's actually a useful purpose, beyond
simply choosing your own program material, to a 'home broadcaster'
because, under U.S. Part 15 rules, you're pretty much free to transmit
whatever you like as long as its confined to your own domain and does
not interfere with licensed broadcasters.

So, in effect, you're free to broadcast something that has a fighting
chance of resembling 'hi-hi' AM, including uncompressed if one so
desires, to your old, much wider bandwidth, AM radios.

I had that in mind with Beamus and you'll notice it measured flat to
18 KHz, so it is capable of as much 'fi' as AM can produce.

Indeed.

I don't have a huge amount of time to occupy my brain about the 1,001
issues surrounding fascinating use of dual deflection plate pentodes.
If I ever get around to using one, I'll just try something that's
already been done and measure/observe/think/tweak as one does while
wanting to farnarkle around in one's shed because there's FAE2doo.

BUT, I suddenly remembered the type number of a couple of such tubes
given to me about 7 years ago just before when a dear radio ham friend
of 72 died. The tube is the 7360, and if you look it up in TDSL you
find a pdf with a circuit for it.

http://www.tubebooks.org/tubedata/HB...ndustrial_Tube...

Patrick Turner.

Thank you but I already have it in my 'beam tubes' folder. But if you
want some more schematics to ponder copying then try this one.

http://jlandrigan.com/files/Receiver...ircuits%20Usin...

Of potential interest, it has a product detector at the bottom.

You might want to consider selling those on Ebay and tinkering with
less expensive tubes because the 7360 is in short supply these days
and commands prices in the 30 buck range. That, of course, is why I'm
using the $1 6ME8.

Although poor as a church mouse, I could afford a 7360 if I wanted
one.
I wonder what use they could be in a direct conversion receiver or in
an MPX stereo decoder get the L-R signal.
Probably my couple of 7360 are old pulls, not sure, but maybe some NOS
here in Oz somewhere.

I still have work to finish on audio gear before I go "upstairs" -
ie, to work on RF. I also like riding a bike a bit these days.

Patrick Turner.

[flipper]

On Mon, 09 Jul 2012 17:41:55 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Sat, 07 Jul 2012 22:17:00 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Fri, 06 Jul 2012 18:23:04 -0500, John Byrns
wrote:

In article ,
flipper wrote:

Possibly but the beauty of the thing is it's utter simplicity and that
he doesn't 'add complexity' but gets a new function almost by just
'rearranging' things. Not quite, of course, because his tetrode isn't
a dual control pentode, like in a 'conventional' broadcaster, but the
number of devices is the same.

Yes, I agree that the beauty of Weaver's "DAM" transmitter is its extreme
simplicity with no added complexity.

I think we should also note that 'affecting' the audio was Weaver's
*goal* because these little broadcasters have no 'compression' like
'the big boys' routinely apply before we get to considering 'power
savings'.

Yes, the article made it clear that his goal was the compression the scheme
provided, although it is not clear from the data he presented how much
compression he actually achieves. Perhaps a simple compressor circuit could
do
better at the expense of an additional tube? I suppose an extra tube would
defile the beauty of it though.

I don't remember in which of his posts I read it but modulation was
effectively '200%' so that would be 6 dB, if AGC fully compensated.

Robert Weaver said in the "Update" to his "A One-Tube Controlled Carrier AM
Transmitter":

"From this trace I made the following measurements:"
"Upward modulation = (7.0 - 2.2)/2.2 * 100 = 218%"
"Downward modulation = (2.2 - 0.2)/2.2 * 100 = 91%"

"The average of the upward and downward modulation is then 154%"

Is this where you got the "200%" figure, or was it elsewhere?

Looks like I remembered the 218% and forgot the rest.

I don't think you can infer, from the modulation percentage alone, the degree of
compression produced by Weaver's scheme in the receiver. Weaver's scheme is
linear with respect to the audio applied to the transmitter; it is the change in
carrier level that produces the compression in the receiver. Using the scope
picture from which Weaver calculated the modulation percentages above, one can
estimate the carrier level at full modulation is about 3.5 divisions, while the
unmodulated carrier is 2.2 divisions in the picture. If the receiver AGC is
fully effective then this would equate to compression of 4 dB.

Well, that's pretty close to taking the modulation percentage.

4 dB doesn't sound like much but its apparently enough that people who
listen to it are 'impressed'. Maybe he gets more 'effective'
compression with the 'soft limiting' allowing more mod than the 4 dB
implies.

If I remember correctly, when playing with software compression more
than 6 dB was noticeably irritating, or tiring, to me.

While Weaver's scheme is clever, it's a good thing it doesn't add any complexity
to his transmitter, as a maximum of only 4 dB compression isn't much, and since
the actual compression occurs in the receiver it doesn't do anything for the
received signal to noise ratio either.

I don't think that kind of carrier control *can* improve the signal to
noise ratio.


A simple one tube compressor could provide considerably more than 4 dB of
compression and would actually improve the received signal to noise ratio.

That would be because it's compressing the source audio going into the
carrier but the 'noise reduction' is, again, a 'receiver function'. I
mean, since compression moves the average audio level upward the
volume control can be turned downward, which turns down the noise.


No, but since I had originally mentioned two forms of DCC I felt compelled
to
offer an idea for applying the second form to a home broadcaster even if
doing
so might be counter productive. The simple one tube compressor mentioned
above
might ameliorate the problem though, although it would make more sense to
simply
apply the simple one tube compressor to a standard home broadcaster.

Yes. I'm normally all for 'simple' and 'clever' solutions but, in this
case, think I'd prefer a 'separate' compressor. For one, since I
normally use a PC for audio I can use much more sophisticated software
compression.

I though it sounded great when first tried but, you know, I think I
prefer the 'weaker' uncompressed playback. At least I have a choice
when they're separate.

Weaver's transmitter could provide a "choice" with simple modification. By
placing the parallel combination of a resistor, capacitor, and switch in the
cathode circuit of the triode modulator section, one would have Weaver's DCC
operation when the switch is closed, grounding the cathode, and normal constant
carrier operation when the switch is open changing the modulator bias to
eliminate the grid clipping.

Yeah, I thought of that but then the issue of two tubes operating open
loop comes up. With compression active it's 'a necessary sacrifice'
but without it I'd like to put some effort into lowering distortion.
On the other hand, lowering transmitter distortion only to fed
distorted audio into it seems a bit odd as well.

[flipper]

On Sun, 08 Jul 2012 00:52:28 -0500, flipper wrote:

snip


All bias voltages look right, RF amplitude is right, I've swapped
tubes, and even re-ohmed every resistor in the circuit. Everything
looks perfectly fine, except for not working right.

I'm thinking of taking a break to see if the alternate reality
twilight zone black hole that's apparently engulfed my workroom will
drift off to harass someone else.


Ya know, if I didn't have scope pictures I'd be wondering if it ever
worked right.

Turns out it works at 1150 KHz, or thereabouts, but there is a
HUUUUUUUUge difference in drive level needed for 100% mod from the low
to high end of the frequency range. It's almost as if, for some
inexplicable reason, preamp gain goes to nil when RF frequency is
adjusted to the low end of the band.

That makes no sense at all but at least it's a clue, and that's more
than I had before.

[flipper]

On Tue, 10 Jul 2012 00:28:10 -0500, flipper wrote:

On Sun, 08 Jul 2012 00:52:28 -0500, flipper wrote:

snip


All bias voltages look right, RF amplitude is right, I've swapped
tubes, and even re-ohmed every resistor in the circuit. Everything
looks perfectly fine, except for not working right.

I'm thinking of taking a break to see if the alternate reality
twilight zone black hole that's apparently engulfed my workroom will
drift off to harass someone else.


Ya know, if I didn't have scope pictures I'd be wondering if it ever
worked right.

Turns out it works at 1150 KHz, or thereabouts, but there is a
HUUUUUUUUge difference in drive level needed for 100% mod from the low
to high end of the frequency range. It's almost as if, for some
inexplicable reason, preamp gain goes to nil when RF frequency is
adjusted to the low end of the band.

That makes no sense at all but at least it's a clue, and that's more
than I had before.

Okay, I found out why.

Beamus began as a 'since I've got an oscillator' project and that came
about from looking to design an amplitude stabilized OSC for one of
those dual control pentode transmitters.

The original design was a pentode, like now, but had a triode sensing
RF peak and, from that, controlling screen volts to maintain constant
OSC amplitude. Well, I also found that, when loaded (deflection plate
100k), OSC amplitude remained virtually constant over the whole 365 pF
tuning range even without the triode and, so, I didn't use it with
Beamus.

But, *surprise*, when I readjusted the OSC coil from 250 uH to 190 uH,
and padded the 365 pF cap with 100 pF to narrow the tuning range, OSC
amplitude now varies all over the blooming place, say 3 to 1 (maybe
more), with lowest amplitude at low frequency, which is why it works
at the top end but goes to hell at the low. There's simply not enough
RF amplitude to drive the deflectors so it can't 'peak power' the
output.

I didn't notice that, at first, because it was running on the top end
of the dial but, later, when 'experimenting' I switched to the low end
to find a quiet spot.

Boy, that much amplitude variation sure took me by surprise.

[John Byrns]

In article ,
flipper wrote:

On Tue, 10 Jul 2012 00:28:10 -0500, flipper wrote:

Ya know, if I didn't have scope pictures I'd be wondering if it ever
worked right.

Turns out it works at 1150 KHz, or thereabouts, but there is a
HUUUUUUUUge difference in drive level needed for 100% mod from the low
to high end of the frequency range. It's almost as if, for some
inexplicable reason, preamp gain goes to nil when RF frequency is
adjusted to the low end of the band.

That makes no sense at all but at least it's a clue, and that's more
than I had before.

Okay, I found out why.

Beamus began as a 'since I've got an oscillator' project and that came
about from looking to design an amplitude stabilized OSC for one of
those dual control pentode transmitters.

The original design was a pentode, like now, but had a triode sensing
RF peak and, from that, controlling screen volts to maintain constant
OSC amplitude. Well, I also found that, when loaded (deflection plate
100k), OSC amplitude remained virtually constant over the whole 365 pF
tuning range even without the triode and, so, I didn't use it with
Beamus.

But, *surprise*, when I readjusted the OSC coil from 250 uH to 190 uH,
and padded the 365 pF cap with 100 pF to narrow the tuning range, OSC
amplitude now varies all over the blooming place, say 3 to 1 (maybe
more), with lowest amplitude at low frequency, which is why it works
at the top end but goes to hell at the low. There's simply not enough
RF amplitude to drive the deflectors so it can't 'peak power' the
output.

I didn't notice that, at first, because it was running on the top end
of the dial but, later, when 'experimenting' I switched to the low end
to find a quiet spot.

Boy, that much amplitude variation sure took me by surprise.

I thought "Beamus" had an RF drive control to set the correct deflector drive
voltage? Am I confused, why didn't this control insure the correct drivevoltage
across the band? Maybe "Beamus" should have a multifunction meter that could be
switched to measure RF drive voltage to the deflectors, carrier output level,
and the 6ME8 cathode current, to facilitate setting up on different frequencies.

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/

[flipper]

On Tue, 10 Jul 2012 20:38:52 -0500, John Byrns
wrote:

In article ,
flipper wrote:

On Tue, 10 Jul 2012 00:28:10 -0500, flipper wrote:

Ya know, if I didn't have scope pictures I'd be wondering if it ever
worked right.

Turns out it works at 1150 KHz, or thereabouts, but there is a
HUUUUUUUUge difference in drive level needed for 100% mod from the low
to high end of the frequency range. It's almost as if, for some
inexplicable reason, preamp gain goes to nil when RF frequency is
adjusted to the low end of the band.

That makes no sense at all but at least it's a clue, and that's more
than I had before.

Okay, I found out why.

Beamus began as a 'since I've got an oscillator' project and that came
about from looking to design an amplitude stabilized OSC for one of
those dual control pentode transmitters.

The original design was a pentode, like now, but had a triode sensing
RF peak and, from that, controlling screen volts to maintain constant
OSC amplitude. Well, I also found that, when loaded (deflection plate
100k), OSC amplitude remained virtually constant over the whole 365 pF
tuning range even without the triode and, so, I didn't use it with
Beamus.

But, *surprise*, when I readjusted the OSC coil from 250 uH to 190 uH,
and padded the 365 pF cap with 100 pF to narrow the tuning range, OSC
amplitude now varies all over the blooming place, say 3 to 1 (maybe
more), with lowest amplitude at low frequency, which is why it works
at the top end but goes to hell at the low. There's simply not enough
RF amplitude to drive the deflectors so it can't 'peak power' the
output.

I didn't notice that, at first, because it was running on the top end
of the dial but, later, when 'experimenting' I switched to the low end
to find a quiet spot.

Boy, that much amplitude variation sure took me by surprise.

I thought "Beamus" had an RF drive control to set the correct deflector drive
voltage? Am I confused, why didn't this control insure the correct drivevoltage
across the band?

It does have an 'RF drive' adjustment but that was intended to be an
'initial setup' thing and I consider it 'bad enough' you have to
re-tune the antenna on a frequency change.

Maybe "Beamus" should have a multifunction meter that could be
switched to measure RF drive voltage to the deflectors, carrier output level,
and the 6ME8 cathode current, to facilitate setting up on different frequencies.

I'd rather it not be so complicated but even that is moot because the
amplitude variation is so huge that the 'RF drive' pot can't come even
close to compensating for it.

Btw, there's also a secondary effect in that RF OSC current drain is
significant so when amplitude drops B+ rises, which affects deflector
and screen bias. Its a beautiful example of Murphy's Law because as RF
amplitude drops, making it unable to drive 'peak power', B+ rise
increases the power level it needs to drive, because screen volts
increase. That made the LED 'antenna tuning indicator' give the
impression the low end had 'more output power' than the upper end,
because it did.

Those 'problems' will go away again when I solve the RF amplitude
issue, though.

Actually, it could be easily solved with different air variables (in
fact, I was thinking of redoing the antenna matching network anyway in
an attempt to reduce cap circulating currents), because the OSC works
perfectly fine with 250 uH (or 220 uH, I think), but now I'm curious
why lowering inductance made such a dramatic difference. I don't know
a heck of a lot about oscillators but suspect the lower inductance
doesn't have enough 'kick' to sustain both the tuning cap circulating
current and deflector bias resistor (load) current so maybe increasing
those 100ks to 220k might be enough to solve it. I also have another
idea, which is to balanced drive the deflectors off the OSC coil
'primary' but I'm not sure there's enough amplitude there. Anyway, the
problem is solvable, it's just a matter of finding a 'simple', cost
effective, one.

Having said that, B+ rise makes me wonder if I should consider
stabilizing it because that makes the two adjustments to some degree
interactive but maybe the range isn't significant enough to matter
when things are working 'right'. And maybe, adjust deflectors, adjust
RF amplitude, and then go back to 'fine tune' the deflector voltage
isn't enough of an 'extra step' to warrant adding complexity.

[flipper]

On Tue, 10 Jul 2012 23:37:26 -0500, flipper wrote:

snip

Actually, it could be easily solved with different air variables (in
fact, I was thinking of redoing the antenna matching network anyway in
an attempt to reduce cap circulating currents), because the OSC works
perfectly fine with 250 uH (or 220 uH, I think), but now I'm curious
why lowering inductance made such a dramatic difference. I don't know
a heck of a lot about oscillators but suspect the lower inductance
doesn't have enough 'kick' to sustain both the tuning cap circulating
current and deflector bias resistor (load) current so maybe increasing
those 100ks to 220k might be enough to solve it. I also have another
idea, which is to balanced drive the deflectors off the OSC coil
'primary' but I'm not sure there's enough amplitude there. Anyway, the
problem is solvable, it's just a matter of finding a 'simple', cost
effective, one.

I haven't had a chance to do extensive testing but the 'simple'
solutions of changing deflector bias resistors and/or increasing
cathode current didn't have a significant effect. And, for some
reason, even with 250 uH RF amplitude now varies more over the 365pF
range than I originally measured. Also, strangely, the 'best'
deflector bias seems to have become 50 V, vs 35 V, like Weaver
suggested it should, and it looks like RF peak should 'match'. What I
haven't rechecked is that I swapped in a new 6ME8 when trying to find
out "what's wrong" and perhaps the 'optimum' bias point varies by
tube.

At any rate, it's the low end where amplitude drops off and, on top of
that, 50 Vp is near RF OSC maximum so there's not much room to
compensate.

I think the simplest solution is to change air variables, since I was
narrowing the range anyway, and a smaller cap is much less expensive.
For example, radiodaze has a 170 pF air variable for $3.00 vs the
'traditional' $15.00 for a 365 pF. Use the same for both OSC and
antenna tank and then trim the OSC range with the coil and add a cap
trimmer if we need to snug the OSC range slightly under the tank's
tuning range. That removes the need for a 100 pF padder to narrow the
range (making for more efficient use of the air variable) and it's the
'large capacitance' that seems to suffer OSC amplitude.

Considering antenna capacitance that should allow an almost 2 to 1
tuning range, which should be more than sufficient since the purpose
is only to find a 'quiet' spot on the dial (otherwise we'd use a
crystal) and, for antenna efficiency (lousy though it is), the upper
end is preferred.anyway.

I tell you one thing, though, when it's tuned up right she sure sounds
sweet (for AM). Actually, I can't tell 'by sound' when RF OSC
amplitude is low because I think that 'distortion' primarily shows up
at max mod (near clipping) nor is it terribly obvious on the scope
unless you're looking for it, which is why I didn't notice it for a
while.

Coincidentally, I discovered that one of my RF noise problems, in
addition to this location being an RF nightmare, was one of my PCs.
The dern thing splatters RF all over the place even when turned off, I
have to pull the plug to get it off the air, so I guess the power
supply is going out. Fortunately it's not the server.