Quote:
Originally Posted by Skywave
Quote:
Originally Posted by Boater Sam
AGC is only on the Mixer.
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Really? I am surprised.  Moreover, it was (and probably still is) considered unwise to apply AGC to the mixer stage, since the application of AGC to a mixer can cause detuning effects. |
That's true, AGC on the frequency changer can cause slight change in oscillator frequency as AGC voltage varies. So, from a design point of view, safest not to take AGC to this valve.
Also, the final IF valve, if biased hard back by AGC, signal-handling capability is reduced. And of course, that's just when you want it, when dealing with a strong signal generating lots of AGC. So, it's safest to not apply AGC to the final IF valve.
Trouble is, if you follow both these bits of advice, in the standard 4+1 valve superhet, there's not exactly much left to apply AGC to...
So, in practice, valves intended to be used together have similar grid bases so they both reduce gain gracefully together. With gain control of two stages, you get enough control without the FC being biased hard enough to wildly affect frequency, or the IF to affect inability to drive the detector.
Like Al, I am suprised it s only on the mixer. As a design improvement, I'd be inclined to try applying to the IF valve too. It doesn't reveal whether the tendency to oscillation in this radio is a design fault or not, but you may end up with a better-performing radio for your trouble!