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[AMPS] Stability Question

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Subject: [AMPS] Stability Question
From: (Jon Ogden)
Date: Sat, 6 Feb 1999 19:39:19 -0600
Quiz time!

I am finally at the point in putting my amp back together where I can run 
some stability tests.  And I have come up with a question/problem.

Here's how I am testing:

Key up amp, no RF, Ctune at min, Cload at max, open circuit on output 
(ie: no antenna or dummy load - no coax attached).  I figured this would 
be worst case.

What happens is I seems to have a housing based resonance problem with 
one of my tubes.  When the cover is off, every thing is fine.  However, 
when I put the cover on and bring Ctune close to its minimum value, the 
plate current starts to rise.  I must have a resonance in the tank 
circuit housing.

I tried adding a 10 pF vacuum cap in parallel with Ctune as some have 
suggested to improve stability in the past.  On paper you would think it 
would work.  Effectively, you'd be raising the value of Ctune by 10 pF 
and hopefully, it will never get low enough then to set the amp in to 
oscillation.  Well, no such luck.  The amp still oscillates even with 
this cap in place.

So, what have you guys got for ideas?  Should I add some "walls" inside 
the housing to try to break things up?

BTW: My plate voltage is about 5.5 KV on the 4-1000A.

Any help is appreciated.



Jon Ogden
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