Bill said;
>But there is a fundamental mode as well but more often occurs in common
cathode amplifiers ( tuned >grid-tuned plate oscillation) but does show up
in poorly constructed grounded grid amplifiers.
The fundamental TPTG type mode is not one that in my experience is usually
called a 'parasitic', though. I guess a lot depends on your terminology - I
suppose there's no reason why it shouldn't be so called. But are there are
any amplifiers out there that really have true neutralising adjustments on
them? I don't how many commercial grounded cathode amps there are around
these days which are meant for amateur service, and if there are, I'd
expect them to be passive grid with such a low value of grid resistor that
they didn't need neutralisation.
I seem to remember the old RCA transmitting tube manual had a circuit for a
g-g amp with 4 811As, in which there was a feedback winding on the filament
choke fed from the plate via a small cap: whether it really did any
effective neutralising is, I suspect, another matter. If I believe the
Eimac stuff, the it's only above the frequency where the grid lead is
effectively so long that it becomes inductive that the g-g amp goes
unstable, and I would have expected that the inductance inherent in getting
a lead back from the plate(s) would have given enough phase shift at such a
frequency to make the feedback phase somewhat indeterminate. At the very
least, extremely layout dependent.
Bill's 30Hz parasitic sounds like a fun one. I never have been that happy
with ALC loops, except in signal generators!
73
Peter G3RZP
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