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[AMPS] Chasing undesired oscillation, query...

To: <amps@contesting.com>
Subject: [AMPS] Chasing undesired oscillation, query...
From: W8JI@contesting.com (Tom Rauch)
Date: Thu, 7 Sep 2000 09:11:39 -0400
> With the amp all assembled I stick some
> white noise on the input. Just a little, say from
> one of those Palomar antenna tuner tuners.
> Now I sweep the range where the tube has
> gain listening with the receiver. Won't I hear an
> increase in the noise level at the frequencies
> where I've got some feedback???
> 
> The triode in my little regen RX behaves this way.
>

You can use a broadband noise source, and the sweep the output 
with a receiver. That will tell you something, if you don't have 
standing waves or filters between the points you are measuring.

To check for the "alleged" VHF instability common in amplifiers, 
connect the noise source to the tube cathode (nothing else 
connected) with a termination resistor across the cable at the 
socket.

You can then measure the signal at the anode with the anode 
driving a load matching the low impedance detector.

If you sweep the frequency range, you will see a definite peak in 
feedthrough at the frequency where the grid isn't effectively 
shielding the anode.

If you want to measure actual feedthrough to calculate stability, 
you have to terminate the anode in it's normal load and the input in 
the normal source and measure loss across the tube. The 
complication is the grid becomes the second port in the amplifier at 
the frequency of VHF instability ( the cathode is usually the 2nd 
port for HF instability) and you can't easily measure that S21.

It's easy to tell WHERE the VHF problem will occur but not how 
much of a problem it is.

Hope this helps.




          


   
73, Tom W8JI
w8ji@contesting.com

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