>
>> ? If input contact arcing takes place, transients are fed to the input of
>> the amplifier tube. If these transients stimulate intermittent vhf
>> oscillations, it doesn't matter if the output relay contacts are closed or
>> not since vhf energy can not pass through the low-pass hf tank. > -
>> cheers, Tom
>
>More typical nonsense and distortion of fact by omission, or else
>you don't understand grounded grid PA circuitry.
>
>Either way, your advice is incorrect.
>
>The input circuit is AFTER the relay and BEFORE the tube. If the
>input circuit is properly designed and implemented, VHF energy
>could never make it to the tube.
? I agree that not much VHF energy can make it to the tube. The VHF
energy comes not from the transceiver. It comes from damped-wave ringing
in the anode-resonant circuit. This (VHF) ringing occurs whenever there
is a transient in anode-current. I contend that RF input contact arcing
(hotswitching) during relay closure will produce transients in
anode-current -- and that the resultant ringing in the anode circuit will
be fed back by the feedback C of the amplifier tube. .
>
>Even if there was no input circuit, the tubes have almost no gain at
>VHF when cathode driven.
? Eimac and Amperex rate the 3-500Z for full ratings at 110MHz.
> That's because the grid is parallel resonant at VHF.
? The grid is resonant at one frequency in the VHF region. Around
that frequency, the grounded-grid no longer acts to shield the input of
the amplifier tube from the output of the amplifier tube, so feedback
increases. If feedback increases does it seem more likely or less likely
that an oscillation could take place?
>. Few people use 3-500's as two meter amplifier
>tubes, although they can be used as TPTG oscillators at 180 MHz.
>
>Finally, the VHF energy from the small arc on the input contacts
>would never be large. VHF energy would have to be many many
>times less than the HF energy.
>
? agreed, however that is not the most likely VHF seed.
>
- later, Tom
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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