If the plate circuit is tuned lower in frequency than the grid circuit,
then it doesn't matter how much feedback capacity there is, the tube cannot
oscillate. See Mr. Terman on the subject. Which is why some suppressors
have got away with no resistance at all. With really big tubes, the
internal size is such that cavities can form - K5PRO has told us about
that. But the principle is still correct about getting the plate parasitic
resonant circuit tuned lower in frequency than the grid parasitic circuit.
As has been pointed out many times, tubes like the 572B with its super long
grid leads make this a bit (sic!) more difficult.
73
Peter G3RZP
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