> Now take the maximum RF RMS voltage at the tube plate. It
> will be quite
> close to the DC plate voltage, so for simplicity you might
> even cheat
> and use the DC voltage.
The dc plate voltage is the peak RF voltage in a very hard
switched PA, so RMS plate voltage would be about .71 times
anode voltage but this ignores harmonics.
In linear PA's it less, since the anode cannot swing from
full HV rail to zero.
So it seems the DC rail would probably be a very
conservative estimate when the PA is properly loaded.
When the PA is not properly loaded the circulating currents
can be higher, and the peak voltage can be higher than the
anode voltage!
I also agree the unloaded Q has little to do with the
problem, neither does the operating Q. It is strictly the
amount of RF voltage across the capacitor and the capacitive
reactance that causes the current....but we have to be
careful of ignoring harmonics and also careful of situations
with improper loading.
73 Tom
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