[Amps] Suppressors

[Peter Chadwick]

Steve said:
>Yes, and maybe I should have said 'higher' rather than just 'high' - but 
are amplifiers unstable if you disconnect the blocking cap and introduce 
infinte R?<
As usual - it depends!  Because there's inherent inductance and shunt capacitance, there's obviously an impedance at VHF. The higher the impedance, the higher the gain. Stability depends on the phase and magnitude of feedback. An infinite anode load means that gain will become gm divided by Yload, where Y load  is the sq. rt of  R squared plus X squared. R is the load resistance in shunt with the ra of the valve. [a variant on 'muRL/(ra+RL)]. So the gain will have a maximum value that's likely to be higher than that achieved with a shunt L-R network. Whether or not that leads to oscillation then depends on the Bode plot parameters for the feedback.
I seem to remember that the shunt network should only get the resisitors hot from fundamental frequency current, because there isn't any parasitic current if the the thing hasn't got parasitics. 100nH at 28MHz is about 17 ohms, so with about 1 amp rms effective anode current flowing, that suggests a shunt 47 ohm resistor needs to be able to disipate 6.5watts. As it's in a hot environment, this suggests that a 'Globar' type resistor rather than a couple of plain old carbon comps is required. That is one advantage of the nichrome wire - it doesn't go high resistance with time - well not unless you're getting it red hot repeatedly!
73
Peter G3RZP