On Jul 12, 12:42*pm, John Byrns wrote:
In article ,
*Patrick Turner wrote:
I already know that a well-designed diode detector works best for me; at
least
since I regressed to tubes, in my IC days I would have answered
differently.
With ICs you get a dc coupled amp and its possible to use an output
diode pointed at an RC circuit with the audio voltage and Vdc fed back
to the FB input port of the IC, and thus get what is supposed to make
the most linear detector imaginable because there is MUCH more NFB
being applied than when using a cathode follower like I do. The IC
input impedance at IF frequency needs to be high though. I have
circuits for doing all that but the CF I use is just fine.
HP had an interesting AM demodulator in their AM/FM modulation meter. *The AM
detector used transistors, not ICs. *A rough description as I remember it is as
follows. *The transistors were configured in what was basically a darlington
configuration. *Feedback was taken from the collector through a capacitor to a
pair of parallel diodes back to a summing junction at the base. *The two diodes
were connected in opposite directions, with a pair of parallel connected complex
cognate networks in series with one diode at the end connected to the transistor
base. *The modulation output was taken across the network that has the form of a
low pass filter IIRC.
It'd be nice to have a schematic John. Then 1,000 words are told with
1 picture.
You say that you ³have tried out all sorts of AM detector ideas², besides
your
spin on the diode detector, and your ill fated attempts at building a
syncrodyne
receiver, what have you tried?
I've tried a lot of variations on normal arrangements with IF coil
feeding an anode in a vari-mu pentode IF amp and following RC circuit.
The CF seems best to me.
I tried to build a version of the DG Tucker Synchrodyne circuit
published in Wireless World in about 1947. Maybe its now online some
place, and what prevented progess was the making of a suitable
balanced synchronous demodulator. Probably the best way is to use a
toroidal transformer with the core µ just right for range of RF
frequencies. But I never had time to explore how to make such a
tranny. The tucker circuit uses lots more tubes than a superhet and
needs much more practical expertise to get running without loud
whistles while tuning. It was a potentially excellent thing but it was
never to become popular because the superhet was king. Then tried
making a synchrodyne based on a self oscillating 6BE6, and there are
some simple circuits of those around, just RF input tube and 6BE6
needed only. That sort of worked a bit but monkey chatter and whistles
and controlling oscillations just right were easier said than done so
I abandoned the idea and proceeded with a good superhet. AGC is
applied to RF vari-mu cascode triode input amp and 6AN7mixer, then
6BX6 sharp cut off IF amp without AGC and using some unbypassed Rk for
local current FB. AF bandwidth is 10kHz, with minimum distance between
IF coils, but even with max distance AF BW is about 6kHz. Tone control
boosts and cuts F above 2kHz. 12AU7 CF detector used, and after
comparing the sound of my kitchen AM radio to countless others I have
repaired, my own design is very much better to listen to.
If you are willing to use an IC you can get a whole synchronous detector circuit
in a single IC in the form of one of the old CQUAM AM Stereo decoder IC chips. *
There are instructions on the web that describe how to do this.
All sorts of things are possible, but the humble pair of CFs for me is
the most appropriate in a tube radio.
If I had the time I'd investigate further but I really have little
need, and I'm perpetually busy fixing people's radios, amps and
speakers. I dunno about you, but I must work or I starve, and find
time to ride 200km a week on a bike.
*There are three binary design
decisions to
be made before I purchase a chassis and begin punching it.
So it seems unlikely you'll ever listen to the benefits of my ideas.
Binary decisions eh. *In a bind are you?
Me thinks you misinterpreted what I meant by the term "Binary decisions".
Er, no, its just my Cents of Hugh Mer.
Here are the three decisions I have to make, perhaps I should flip a coin and be
done with it.
1. The output stage will be of the distributed load type with 50% of the load in
the cathode circuit, and 50% in the plate circuit. *The decision yet to be taken
is should I use the McIntosh output circuit, or an Acoustical style of output
circuit? *This decision will have some impact on the layout of the power supply.
So how long have you been wondering what the F to do? Who is t say
you'll flip a coin tommorrow, then later un-decide what the coin said,
and keep flipping until a better outcome appears. Could Flipper help?
The McIntosh method gives the best measurements, and you don't need
bifilar OPT windings. EAR did a quasi McI amp in the EAR 509 with some
PL509 HORRIBLE output tubes, and it was to get 100W with high NFB and
its sounded terrible, and using a pair of KT88 with UL or CFB of 20%
and aiming to get 50W max with high % of class A1 working, say 15W max
at least, will give excellent performance with one less gain stage.
But during the next 15 years you'll have plenty of time to consider.
2. I must settle on the output tubes to be used, a QUAD of 25L6GTs in push-pull
parallel, or a pair of 7695s. *Actually this is a ternary choice, the KT55 is
another possible choice for the output tubes, while these are cool tubes,
finding some would probably not be easy. *The output tube choice has a major
impact on the chassis size, and layout, so I have to settle it before I purchase
a chassis and begin drilling and punching it.
Its often not what you use, but how you use it that counts.
3. The driver tube must also be specified, the binary choice was between the
6211, and the 12AU7, yesterday I made it a ternary choice by adding the 12AT7 to
the list. *These tubes all use the same socket, so they can all be tried with
only a change of cathode resistor.
Good luck.
I guess we may never know how your project ends up at the present rate
of progress.
I googled * Telefunken AM rebroadcast receiver "Ballempfänger"
Nothing to be seen.
You weren't persistent enough, for a general description go he
http://www.radiomuseum.org/r/telefun...faenger_b.html
If you aren't a member, I'm not, you won't be able to see the schematic.
*The
schematic is available from several other web sites though, you can find it
in
two parts he
http://www.tsf-radio.org/schph.php?f...Ball-Empfanger...
and he
http://www.tsf-radio.org/schph.php?f...Ball-Empfanger...
While the design looks like the ³bean counters² were kept at a safe
distance,
there still isn't a cathode follower in sight!
Those links open but the pictures won't open to reveal what you are
talking about.
I doubt I am missing anything.
I think you are just intimidated by the excellent German engineering in this
receiver, to eliminate any possible excuse of your not being able to open it, I
have posted a copy of the schematic on my web pages at this URL:
http://fmamradios.com/stuff/E1.pdf
Not a bad radio schematic, but I'm not going to adopt any of the
German techniques. Too complex. This radio has UMPTEEN PARTS which
don't need to be there just to get strong local AM stations with hi-fi
quality.
While you won't accept my use of one lousy cathode follower, you are
putting a radio set under my nose which has maybe 200 parts I don't
need to use to attain my simple goals.
AM radios tend to waste huge chunks of my time with very little
profit, and I just do what works for me, and this includes far more
simplicity than what the Germans may have done.
I have several communication radios and trancievers which all need re-
builds. An old radio ham I knew died and his folks sold all they
could, but then there was still a small mountain of collectable stuff
left over in various states of dis-repair. So they rang me to collect
what would have gone to a tip. Took me a week to collect and sort it
then another week to extend my storage areas with shelving to keep the
old junk.
Going right over it all and all would take weeks before even one radio
set could be declared to be "good at what they do", without all sorts
if intermittencies and noises because of old switches and contacts and
crap. But listening on SW is a bit boring, and even if one joins in
discussions with radio amateurs after building one's own little old
radio station, one ends talking to old guys about their latest surgery
and health bothers and old age problems. Kinda puts me to sleep,
listenin' to them droning on nerdishly, saying so many words, but
trying to be un-personal, and not saying much at all. There are many
who just like to broadcast, and their audience never really exists.
They can't convey their creativity with pictures very well.
Rather than put up with droners, I have my own life to do, and a
living to be earned, and fitness to be maintained.
Patrick Turner.