On 10/08/2013 12:17 PM, Fuqua, Bill L wrote:
There can either be a feedback path that causes the problem due
to the fact that by-pass capacitors are chosen to bypass HF RF
not low frequencies
That is what worries me. I wonder it it was on the edge of
instability all along until a minor change in the plate choke pushed
it over the edge. I wonder if I should think about upgrading the
by-pass capacitors? The by-pass at the cold end of the plate choke
is only 1000 pF which has always worried me a little. Some others
are the same or not much better.
I neglected to mention this is cathode driven, swamped with a
resistor. Gain is 13 dB. The grid is at RF ground (6000 pF) and
screen clamped to the chassis.
The difference in the chokes is likely to be their Q. If the
choke had lower Q the amplifier would be less likely to
oscillate.
That makes sense.
73,
Paul
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