We also do not know where the oscillation is occurring. Is it in the VHF
range? HF? LF? All that's important, because if the oscillation is in the
HF or LF frequency ranges, screwing with the VHF parasitic resistor ain't
gonna do a bit of good. An amplifier doesn't necessarily always oscillate
at VHF.
Starting with construction analysis is the best solution to start with as
Tom pointed out. Look at improving grounds, shortening lead lengths, etc.
Before I rebuilt the RF deck on the amp I bought from an estate sale, the
first pass had a horribly long anode lead in it. All the nichrome in the
world and "lower Rp" suppressors could save it. Then I gutted it, rebuilt
it with my own design shortening things, improving grounds, etc. and now
there's no nichrome in it and it's stable.
The difference between an amplifier and an oscillator is a very thin line.
Start with the basics and work from there is my first suggestion.
Also, some people raised the question of terminating the input in 50 Ohms
during an oscillation test. I disagree. To me, the worst case test of the
stability of an amplifier is to test the amplifier on the bench with nothing
at all hooked to the input or output - both open circuits. Then key the amp
and rotate the tune cap through its paces along with changing the inductor
positions, etc. If you can get through doing all that without any increase
in grid or plate current beyond the zero signal currents, then you are
likely pretty stable. Also, the tube should obviously not start to glow
brightly either!
73,
Jon
NA9D
on 5/7/01 9:54 PM, Tom Rauch at w8ji@contesting.com wrote:
>
>
>> I guess I'm confused. If this tube is prone to self oscillation--which
>> is what I get by reading this thread--why not try a different
>> suppressor? If it works you have saved yourself a lot of aggravation
>> it seems.
>
> I think the problem is he never described the construction in any
> detail that would give a clue what to do.
>
> It might be something as simple as grid lead lengths, or it could be
> the suppressor or any number of other things.
>
> One guess is as good as another without knowing anything about
> important things like grid lead lengths, distance from the tube to
> the tuning cap, how the caps are mounted, etc.
-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)
Member: ARRL, AMSAT, DXCC, NRA
http://www.qsl.net/ke9na
"A life lived in fear is a life half lived."
--
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