[HBR] Re: HBR Digest, Vol 55, Issue 5

[match at ece.utah.edu]

On 17 Sep 2008 at 4:02, Peter wrote:

> 
> I'm thinking of running with Walt's advice on the 6EH7 semi-remote
> pentodes in the IF and RF stages. Here's what I've come up with after
> a few evening's brainstorming:
> 
> Since my filament current requirements are going over the top I've
> been thinking of ways to simplify my receiver (less tubes) without
> sacrificing performance.

I would just add another filiment transformer, or toss in a few 5 volt tubes to 
make use of the 5 volt rectifier winding on the power transformer, assuming 
your power transformer has a 5 volt winding.
> 
> What do you folks think about my using the 6JH8 second (HFO) mixer,
> and the 6JH8 product detector in self-excited configuration?
> 
> The second mixer would use the 6JH8 with signal injection to the
> deflector electrodes, while the control grid and cathode would be set
> up as a Colpitts xtal oscillator at 1515kc.

Over on the GlowBugs list we've been having a discussion about using the 
6JH8 as a mixer. The problem with it is that it is rather dificult to drive the 
deflection plates. The capacitance is high, so the higher the frequency, the 
harder it is to drive.

You scheme would work (theoretically) if you make the deflection plates part 
of the tuned circuit coupling them to the previous stage.

> The Product Detector could be set up for self excitation for BFO
> injection.  Would pulling be a problem, or should I stick with a
> separate BFO and buffer stage?

I don't know how bad pulling would be. I suspect it would be large, but the 
only way to know will be to try it.
> 
> Tube line up would be 6EH7 RFA;  6ES8 Pullen Mixer;  6U8 LO and buffer
> (triode) to avoid pulling problems;  6JH8 2nd mixer with self excited
> 1515kc xtal  osc,  two stages IFA using 6EH7s,  6JH8 product detector,
> self excited;  12AU7 infinite impedance AM detector and plate detector
> for AGC,  12AX7 Q-multiplier, 12AU7 s-meter amp and 1st AFA, 6AK6 AFA.

Worth a try.

Marvin
KA7TPH