[HBR] Receivers

[waltah at earthlink.net]

Kees wrote:

> I have really enjoyed the interchange going on. Great discussion. 

Can't tell you how much I enjoy having a few kindred spirits to 
discuss this stuff with ... if anyone else is doing this I don't know 
who or where.

> Just a little different thought flow................
> 
> I like experimenting with and building all the analog tube rigs, but with
> the problems associated with heterodynes, and the advent of super stable
> PLL oscillators, isn't there a possibility of applying direct conversion
> here, regeneration even ? I know regens are usually poo-poo'd. 

I don't think they should be poo-poo'd.   People *do* still build them 
-- a lot more of them than HBR-scale superhets.

> I would continue to focus on a very low noise highly tuned LC and
> tube frontend for sensitivity and selectivity with a focus on high
> Q. Somewhat like a tunable Q multiplier frontend ..... I guess
> that's basically what a regen is in part. 

Yep, a regen is a Q-multiplier that has enough more feedback and 
is so configued that it gets out of the linear range and become a 
detector.   

> So what are the pitfalls of regeneration in addition to lack
> of stability and how do you solve them ? Maybe, back off on the
> regeneration and add the best solid state has to offer for the non signal
> source osc, and finish out with tubes for the audio, ? 

Bite yo' tongue ... solid state indeed ... Hi.

Although people do still build regens, I'm not sure many of them 
have thought through the theory.    The approaches seem to divide 
into "I built an exact replica of this 193X regenerative set described 
in ..." and "I built this regen set that uses modern tubes and has 
the following features not found in early sets."   

Nor am I really sufficently theoretical to do the subject justice.   
But when did that ever stop me from trying?

Stability breaks into two parts:  *frequency* stability, meaning that 
the set will stay correctly tuned to the selected station -- I don't 
immediately see why it wouldn't be possible to do that with SSB, 
since we build VFOs at up to 10 Mcs that do it.

Frequency stability is exactly the same game as for a VFO and 
that game is familar.   Use a high gain tube and decouple as much 
as possible from the tank circuit.   Use stable components and 
keep heat away from them, so far as possible.   Put the tank 
circuit in a box with the tube outside and use tiny wire to connect 
the two, taking care that the tiny leads don't flop around.   

I would certainly use a good dial mechanism; bandspread 
requirements for tuning SSB are the same regardless of the 
detector type.

Then there's *amplitude* stability -- when regeneration control is 
adjusted, it *stays* adjusted, regardless of tube parameter 
variations and signal strength.   I have less understanding of the 
issues there.   I'd certainly want to try one of the variable-mu tubes 
like the 6ES8 or 6EH7 but whether that would really be better or 
something like a 6GK5 would be best, (high gain VHF TV RF tube) 
I don't know.   I've never heard of an attempt to discuss the 
requirements for the ideal regenerative detector in terms that could 
be translated to a tube type.

I'd start with a 6ES8 push-pull oscillator set up to oscillate at pretty 
high grid leak bias (small plate current) and reduce the voltage until 
it went out of oscillation.   That *should* (says the guy who never 
built one) give you a regenerative plate detector.   Plate resistor 
100k or up.   Then an audio amp after that.  

An interesting variation would be a crystal controlled converter 
ahead of the detector; in effect the regen detector would be a 
tunable IF.   The converter stage would get rid of antenna effects 
and the consequences of trying to tune more than one band in the 
detector -- bound to be worse than a multiband VFO.

Doing it this way you could apply audio-derived AGC to the 
converter stage.

I believe the Handbooks about 1965 had a three or four tube 
receiver consisting of a tunable converter stage and a fixed-tuned 
regenerative detector.   Was this the Super Simple-X receiver?   
And there was an article published in CQ sometime in the 60's I 
think about a crystal controlled regenerative detector.

Walt 
KJ4KV