Hi Ian,
> I don't have any professional stake in this debate. My only interest is
> to find a technically valid explanation of the confusing mess of
> evidence.
Actually it isn't so confusing. Virtually all RF and tube
design engineers agree about this subject Ian.
Tubes with long thin grid connections are the most likely to
cause trouble. 4-1000A's, 3-1000Z's, 3CX1200 A and D7's, 811A's, and
572B's are among the worse to stabilize, while tubes that work
well at VHF (like the 8877) are the easiest to work with.
Testing PA's for stability, and correcting any problems that show up
is generally not very complex unless the tube tends to oscillate near
or on the operating frequency. In that case, special circuitry
(like neutralization or resonant suppressors) may be needed.
That's why 811's and 572's are so rough to work with on fifteen
meters and higher (they often oscillate near ten meters), while tubes
like 3CX800's and 8877's are simple and very stable (designs using
them are often unconditionally stable).
73, Tom W8JI
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