Hi Peter,
I accidentally deleted your message, so can't repost it.
Typical amateur amps are actually rather modestly shielded at
VHF. They rely heavily on low impedance on the anode and
cathode of the tube to hold direct VHF radiation on TV channels to
a minimum.
If the capacitor was a lousy as "Rich's theory" requires for
parasitics to develop enough voltage to cause arcs, you can bet
the amp would wipe out VHF TV channels for hundreds of feet. The
amp might squeak by FCC, but even that would be unlikely.
Your guess about the VHF and UHF impedance agrees with what I
measured. The cap does go through a dip in reactance at upper
VHF, but it is a broad dip. When it does go into a parallel resonant
(or perhaps transmission line) mode up in UHF, the impedance still
remains low. The SIGN of the reactance changes however, from
capacitive to inductive.
That does not surprise me at all, looking at the cap.
Of course for the PA to arc the bandswitch at VHF, the VHF
impedance shunting the tank would have to be very high. If the VHF
impedance was high, even Rich's "magic nichrome" would have
little effect on stability. In order for a suppressor to work effectively,
it has to be the dominant impedance in the anode path. That's
because it is the SYSTEM Q, and not just the parasitic
suppressor's Q, that controls loss or dampening.
The lower the impedance of the stuff connected to the other end of
the suppressor, the more effective the suppressor becomes.
If the cap was "parallel resonant" with a high impedance, the amp
would likely be a nightmare in many ways. Few of Rich's theories
make any sense, most of them argue against themselves.
73, Tom W8JI
w8ji@contesting.com
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