[Amps] Parasitics & Filament Sag

[Peter Chadwick]

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