I've watched this reflector for some time in silence but now find a
question that seems to need to be asked....
What is it about amplifier parasitics that is so foreign to so many
experienced amplifier people here? I don't bill myself as any sort of
expert, but do have a bit of experience in amplifier maintenance on the
job, and have built a few as a ham. And I'm not an engineer, So I'll
call myself "informed and experienced", and not anxious to engage in
arguments with the many here who are better informed and more
experienced. But---
We all know that any amplifier is just waiting for the chance to
become an oscillator, and will leap at the opportunity when it sees an
attractive set of resonances and at the same time sufficient gain and
feedback are present. Even the cathode driven type. At VHF resonances
and feedback occur much easier than at HF. When all these conditions
lean far enough in the same direction at the same time....POOF! instant
selfoscillate/selfdestruct, and when the cleanup is finished, the
correct conditions no longer exist to make it selfoscillate. The fact
that it seems to occur rarely may just mean that is too often
misdiagnosed.
I think Rich Measures is to be commended for his extensive
investigative work into this phenomena whether or not he is totally
correct in every aspect of VHF parasitics. I have a stock original-tubed
full-output MLA2500 that has never, in the dozen years I have owned it,
showed any tendency to selfoscillate. I can see that this fact doesn't
mean such will always be the case. Some circuit parameters can change
with aging alone. I believe that a few bucks spent on prevention could
be very cost effective in avoiding the expense of a set of new tubes.
Whether it "needs" it or not.
]
73, Dick W0EX
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