> 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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