John Byrns wrote:

In article , Fred Nachbaur
wrote:

Patrick Turner wrote:

John Byrns wrote:

In article , "John Walton"
wrote:

If you don't go this route, the most difficult item to find for a super-het
will be the ganged 365uF (they called 'em 365mmF) capacitor and the IF
transformers.

Doesn't a TRF receiver also require a "ganged 365uF capacitor"?


For TRF, 4 single tuned circuits are required, so that means two
double gang tuning caps, and then you have to get them all to track
properly. The Q of each tuning circuit dosn't want to be too high, or else
the final BW will be so narrow that so will the AF BW.

The bigger problem is that the Q changes across the band, tending to be
higher at the top end because of the higher reactance (and therefore
higher ratio of reactance to resistance). So there'a compromise between
good sound but lousy selectivity at the low end, and good selectivity
but muddy sound at the high end. The AM broadcast band is bad that way,
since it's about a 3:1 frequency range.

When you speak of the "low end", and the "high end", are you talking about
wavelength, or frequency?

Actually you want the Q to vary across the band, you want the Q to be
proportional to frequency, so that you can maintain a constant bandwidth
as you tune from low frequencies to high frequencies. This effect can be
approximated by making as many as possible of the lossy elements in the
tuned circuits, series elements. This means the tuned circuit should be
driven from a pentode with a high output resistance, and care should be
taken in the design and layout of the amplifiers to make the grid
conductance as low as possible. series resistors can then be added to the
tuned circuits to set the desired circuit Q. Double tuned circuits, using
two tuning condenser gangs for each stage, can also be used to improve the
filter shape factor and achieve steeper skirts without excessive narrowing
of the nose response. Complex coupling impedances can also be used to
maintain the desired coupling as you tune across the band, and also to
provide increased coupling at the low frequency end of the band to help
offset the effects of the remaining parallel losses in the circuit. These
techniques can greatly reduce the compromises necessary in TRF AM
broadcast receivers. The famous Western Electric No. 10A Radio Receiver
is an example of this type of design. J.W Miller also offers a series of
radios and components based on this design technique, and the J.W. Miller
design was used in Altec's early AM tuners.

They invented the superhet to overcome the TRF troubles.

Indeed. Only one frequency to worry about, as far as the selectivity vs.
bandwidth situation is concerned. Also makes it practical to do
stagger-tuning to give a more square-shouldered bandpass characteristic.

Why even bother to stagger tune? Double tuned IF transformers, and even
higher order filters, like the fourth order filters used in some of the
H.H. Scott AM tuners, can accomplish much the same thing by judicious
choice of Qs and coupling coefficients.

Regards,

John Byrns

Interesting.

I have two tuned LC input circuits in the front end of my superhet reciever.
they are loosely coupled to the antenna.
I found that the BW was smallest at the lowest RF and there was significant side
band cutting
and would have been reduction of audio bandwidth, had I not used two LC coils, tuned

slightly apart, to give a wide enough BW.

It is necessary to have at least 60 dB attenuation when 40 kHz off the wanted F,
and 80 to 100 dB is better, and a couple of tuned circuit don't give enough
attenuation away from the wanted F, even if the LC circuit has a high Q.
If a single 455 kHz IF LC circuit has a BW of 20 kHz,
then the Q is only about 22.7.
When 4 such circuits are arrayed, the resultant Q is a lot more, and BW is a lot
less.
Such LC circuits have only say 15 dB attenuation at say 40 kHz off the wanted F,
so you need a few LC circuits to get stations 40 kHz away from the one you want to
be inaudible when there is silence in the programme.
We have a sports station of 300 watts at 1008 kHz, and
there is an entertainment station of 10,000 watts KHz less than a Km away,
at 1,053 kHz, and I know a radio is OK if I can tune to 1017,
and not hear the 1053 in the bacground of 1008 which
will be noisy, and distorted.

Patrick Turner.


Surf my web pages at, http://users.rcn.com/jbyrns/

"Patrick Turner" wrote in message
...


Yves Monmagnon wrote:


I also tried an all tube direct conversion synchrodyne,
but that was a flop for many reasons, and I see why it never became
popular.

Hi Pat !
Could you tell a bit more about that ?
Yves.

Crikey, it'd take a bloody book.

The first real proper synchrodyne afaik was D.G.Tucker's circuit, in
1947,
with an RF amp using two pentodes with NFB and inptut tuning circuit,
then a locked oscillator,
third pentode, then a balanced detector using a toroidal transformer
and 4 diodes. I never got the detector coils to work, and the RF amp
teneded to be be very unstable, and the locked oscillator tended to be
modulated
by the synchronising injection from the input amp, and
all in all, it seemed like a real PITA.
Then when I eventually got a sychrodyne to work with a self oscillating
6BE6,
the output was tiny, it was unstable, and the supposed selectivity due to
AF filtering
still left plenty of weird tones and crap in the AF output.

Commercially viable synchronous detection had to wait until the days of
chips and PLL,
and then it works OK.

And tuning a synchro means lots of whistles and howls until the set locks
on a station.

Was just to satisfy my curiosity since have never seen such designs with
tubes, done !
I expected the synchrodyne was in place of the IF, that means a superhet
with synchrodyne detector.
Have done that (with chps, not tubes !) in a VHF traffic receiver to detect
SSB, AM and FM.
In FM, audio in the PLL error voltage.

We have chance that regulation authority left room for good (for me) radios
in FM band along all those
commercials playing heavy compressed beats !

so more tubes are needed for a complex muting circuit.
In this country, there were never any built commercially, afaik.
Ppl stuck to plain old 5 valve superhets.
Then eventually, after an absurdly long wait due to government bungles
and
vested interests in radio, we got FM in 1972, and the tube era was over
then.
The RDH4 has some FM circuits, which very few ppl could understand, let
alone build.
And very few know how multplex decoders work.

As an Ham, I explain multiplex to myself as a supressed carrier dual side
band 38khz, with a 19Khz pilot to rebuid the carrier.

Then there was tubed TV, and that was really baffling.

But many radio amateurs at least made their own receivers, and there were
kits you could buy,
but none of the parts used in 1960 are still available except as pulls
from other gear.

There are mountains of old but not very collectable valve radios about.
something made about 1960 will have fairly miniturised parts,
and have at least a two gang tuning cap.

At the end of the day, what use is an AM radio? All the commercial
stations here are talkback amounst blue collar uneducated folks, so
its the blind leading the blind, or it pop music, usually from a small
selection of records.
So basically, commercial radio is complete crap.
But we at least have the print handicapped radio station, where they read

the papers each day.
Then we have the ABC, the govt rum station, and the afternoon talkback
is amoungst those with degrees fro a uni, and its actually interesting to
listen in.
The music shows on the ONE station and public interest interviews demand
a decent
AM reciever, so I built one from scratch, because the usual
5 valve superhet has so many inherent problems of small AF bandwidth,
and high thd/imd. These problems can easily
be addressed, and AM can nearly be as good as FM to listen to.

The transmitted signal is usually subject to a filter at 9 kHz,
and since OZ stations are all 9 kHz apart, with DX,
one gets 9 kHz whistles, with chatter each side due to the
beating of the station you are on with sidebands of a nearby station.
A 9 kHz notch filter only removes the 9 kHz whistle, and not the
other whistly chatter.
To avoid this interference AF bandwidth has to be limited to about 4 kHz,

and this is the usual AF BW of most tube radios.
Many only have 2 kHz.
2 Khz is typical of many SS radios, because the IFT are not transformers
but single
winding coils, so the shape of the selectivity curve is lousy.
at leat 5 tuned circuits are needed for a radio, and
4 of the 5 shoulod be those in an IF amp, where the critical
coupling of the IFTs result in a flat topped bandpass filter.

Three IFTs are good, but each IFT further reduces AF bandwidth,
so to get more AF BW with steep sided bandpass filter action, a higher
frequency
has to be used for the IF, hence the idea of 2.5 MHz.
So that if the staion recieved is at 500 kHz, the oscillator is at 2 Mhz,

just above the top station frequency. at a staion of 1,600 Khz,
the oscillator is at 3.6 Mhz. The oscillator F range is easily tuned.
The coils for IFT at 2.5 MHz might be easier to wind than 455 kHz,
and they could be wound on existing 455 kHz formers.
They need less turns, and less capacitance.

With an IF of 2.5 MHz, the filtering of RF out of the recovered audio
would be very easy.
The pentodes used for IFamps will work just as well.
With two IF amps, only the first one really needs to be subject to AVC
for MW locals.
But for short wave, obviously, the 2.5 mHz IF would restrict you to
stations above
3 Mhz.
Once one considers SW, up to say 30 Mhz, one has to think about
quitet inputs, quiet mixers, maybe doble conversion perhaps mechanical
or crystal filters, then there is the BFO, for sideband reception, and so
on.

A good SW radio will pick up a few radio hams on the bands.
But for SW, antennas become important, and for the amateur bands,
rotating beam antennas are the go, so that someone using a single
807 in a transmitter in japan, can be heard OK in Canberra.

Or even here !

The Racal communications radio was one of the best ever made,
with a cast and machined aluminium chassis, to seperate all the stages,
and this had 22 valves.
Digital techniques dominate HF radio now, and receivers just digitise the
antenna signal,
whatever it is, and get a computer to count what is in there, and just
extract certain numbers,
and there is your siganl, at any requency selected, its more like a noise
analyser.

I know hams who built everything in the shed, all the test gear,
and all the transmitters and modulators, etc.
They are mostly retired, and getting old, and they craft will die with
them

MMMH ! I don't feel such old !! and not ready to die soon !!

because HF radio is becoming an old fashioned way to communicate
long distance.
And that communication is subject to ionsphere condtions in 11 year
cycles,
and lots and lots of bloody noise and interference.
Then there are all the problems of restrictions on radio gear in built up
areas.

I doubt I will ever become a ham radio guy.
I find too much activity at AF frequencies, which prevents me going
higher.

Yes, but we need DG to talk about what we're doing.
The main specificity of amateur radio is to use the rig to speech about the
rig !
And I will certainly never had the opportunity to listen your amps except if
you tie one as a modulator for an AM transmitter !
With those 12x6550, you could easily modulate a full RF KW .
Then I'll need a good variable selectivity receiver

Patrick Turner.

P.S.
Oh ! what a beautifull red tie and assorted handkerchief !!
Sure, with radios it's not so easy to have pictures from antipods !

Cheers, Yves.

Yves Monmagnon wrote:

"Patrick Turner" wrote in message
...


Yves Monmagnon wrote:


snip for brevity....



Yes, but we need DG to talk about what we're doing.
The main specificity of amateur radio is to use the rig to speech about the
rig !
And I will certainly never had the opportunity to listen your amps except if
you tie one as a modulator for an AM transmitter !
With those 12x6550, you could easily modulate a full RF KW .
Then I'll need a good variable selectivity receiver

Well maybe you will have to get busy to build the Miller TRF
reciever detailed at
http://users.rcn.com/jbyrns/Miller570.html

To make such a wonderful radio, REAL craftmanship is required.



Patrick Turner.

P.S.
Oh ! what a beautifull red tie and assorted handkerchief !!
Sure, with radios it's not so easy to have pictures from antipods !

Cheers, Yves.

It was the only time I wore that suit last year......

Anyway, if I used the 300 watt amps to modulate a nice linear RF amp,
the atmospheric conditions between here and France would
ruin the fidelity.
But if I can borrow a few tubes from Radio America,
and do a deal with the power company, and borrow one of Radio National's
antennas, maybe you get me pretty good.

BTW, watch out for our Oz stars in Le Tour, I hear they are giving some guys
a hard time.
But maybe Lance will win again, who knows?

But that Miller radio is not a bad idea, and I tried a similar front end,
but its a bit hard to get the flat topped response.

The synchrodyne simply is supposed to add a constant level RF
tone of the same F as the recieved station, to that station signal.
Then all that is passed through a low pass filter, and hence audio is made.
if there was a station 9 kHz away, then you get a 9 kHz AF tone detected
in the audio from the wanted station.
Then if you have a very steep LP AF filter with a cut off of 8.5 kHz,
the adjoining carrier is kept out of the wanted station signal
But if the station at 9 kHz away from the wanted one has AF modulation,
then it is in the form of a 9 kHz carrier modulated by its AF content,
and the sharp cut off AF filter cannot keep out the sidebands either sider of 9
kHz,
and it sounds bloody awful.
So synchronous, or direct conversion is good where there are no
other strong signals, but needs to have a small AF BW where stations
of similar strength are close to each other.

I think a superhet is MUCH easier to build than that Miller radio.
And I still think 2.5 MHz IF would be practical, and have 3 IFTs.
This would allow a high Q for the total IF coils system,
and for 25 kHz AF BW, the Q could be 100.
And with a total of six tuned IF circuits, the slope away from the
cut off F would indeed be nice and steep.

There would not need to be much gain in the tubes used for IF.
The secert to getting clean performance from a frequency converter
is to have a fair degree of selectivity prior to the converter, so
the signal you want isn't modulated by strong stations nearby, also entering the
converter.
so the double LC tuned stage with mutual L or mutual C coupling
in the bottom of the L of each LC circuit
will make a nice front end system.

Patrick Turner.