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[AMPS] Stability Question

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Subject: [AMPS] Stability Question
From: (
Date: Sat, 6 Feb 1999 22:18:00 -0500

On Sat, 6 Feb 1999 19:39:19 -0600 Jon Ogden <> writes:
>Quiz time!
>I am finally at the point in putting my amp back together where I can 
>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 
>be worst case.

Sure is and not a reliable test of the amps operational stability. Try it
with real world tuning values and then remove the load.  A good layout
should then be stable.
In a perfect world what you are trying should work but the 4-1000A at
that voltage is just asking to take off, why encourage it?  There are
even some commercial ham amps that would not pass that test.

>What happens is I seems to have a housing based resonance problem with 
>one of my tubes. 

Just one tube or all ?
What bandswitch position?

 When the cover is off, every thing is fine.  
>when I put the cover on and bring Ctune close to its minimum value, 
>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 
>would work. 

Yuk !  All you are doing is increasing the chance for problems. On top of
that the 10pf added to the approx 7pf of the Tune cap and the approx 8pf
of the tube puts you well out of range for a reasonable Q on 10M and is
even marginal on 15M. Throw in a few more pf for strays.


 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?  

If you have a GDO what is the oscillating freq in diode mode?

Is the tube base adequately grounded with the required clips? The base is
a shield and MUST be grounded.

Should I add some "walls" 
>the housing to try to break things up?

Some EchoSorb on the walls or cover may help but that is a brute force
fix. Also try a different plate choke on the chance that it has a common
series resonance with the enclosure. The GDO may be of some help.

73  Carl  KM1H

>BTW: My plate voltage is about 5.5 KV on the 4-1000A.
>Any help is appreciated.
>Jon Ogden
>    <--- CHECK IT OUT!  It's been 
>"A life lived in fear is a life half lived."
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