There are a number of things I don't understand about the parasitic suppression
business. According to Terman, a tuned amplifier with a plate circuit tuned LF
of the grid circuit is unconditionally stable because the grid has a positive
input resistance. If the grid is tuned LF of the plate, it has a negative input
resistance. On this basis then, a tube with a good short grid cone (8877?) with
a collet type connection has a good chance of being stable without any
suppression. A 2C39 should be pretty good, too....As an aside, why were there
amplifiers in the 1950s with series inductors in the grid for parasitic
suppression? That appears, on the face of it, to be ridiculous.....
The 'conventional' parasitic approach is, if I understand the various
arguments, to lower the circuit gain at VHF by reducing the plate load
impedance. I don't see how a resistance wire suppressor does that, since a
resistance and inductance in series have an increasing impedance, at least
until the parallel resonance caused by the self capacity of the inductor is
reached. So the circuit gain would increase, while with the shunt inductance
and resistance, it tends towards whatever the impedance of the resistor with
its strays is. However, the plate parasitic frequency would decrease, thus
meeting the criteria above for parasitic suppression. It would act as a sort of
glitch resistor though....
Or do 'conventional' parasitic suppressors actually do both? The inductance
lowers the parasitic frequency, but provides another VHF parasitic possibility,
damped by the shunt resistors?
73
Peter G3RZP
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