>>but what I can say
>>for sure is that without anode suppressor a 3CX1500 in GG is quite
>>stable and much less prone to VHF parasitic than other common tubes
>>with conventional VHF suppressors.
The danger is to generalise too much, and say 'All tubes must have parasitic
suppressors under all circumstances'. Some tubes are such you can never get away
without suppressors; others, in some circumstances, you can. Sometimes you can
with tubes from one production batch.......
Provided that the parasitic circuit between grid and cathode resonates higher in
frequency than the plate parasitic circuit, there won't be an oscillation. Now,
ensuring that situation occurs in any particular amplifier is another
thing........Certain of the books have suggested that adding the plate
suppressor choke alone may, in some circumstances, be enough to do that without
any damping resistor.
Yet again, I've seen HF tx's of considerable power (100kW) where parasitic
suppressors consisted of 50 ohm carbon resistors, 12 inches long, 1 inch
diameter, with one end attached to each plate terminal, and the other in free
air, connected to nothing. Brown's book 'Radio Transmitters', published some 35
years ago by Peter Peregrinus for the IEE, covers this well.
73
Peter G3RZP
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