[AMPS] blocking cap current

[Peter Chadwick]

Tom says:

>We can calculate the current flowing from the tank back into the tube's 
>capacitance because we know the anode to ground voltage and 
>anode to ground capacitance.

That turns out to be frighteningly high on 10 metres - take say 3000v pk and
20pF, that's over 10 amps peak. I must admit that I've never actually
thought about that. 

As far as phase shift at the fundamental is concerned, I would say that's
pretty negligible. A 1000pF blocker is only 88 ohms at 1.8MHz. For a low Z
circuit (6 paralleled sweep tubes) you might want something a bit bigger on
160.

The second harmonic component of a 180 degree conduction angle plate current
is 6dB down on the fundamental current. I think one has to treat the tube as
a current source here, so that the harmonic current into the tank circuit is
in phase with the fundamental current. Since the  plate to ground reactance
is one half at the second harmonic, the amount of harmonic current flowing
into the tank will alter depending on frequency i.e. the ratio of the
capacity of the pi tank tuning C to plate-to-ground C. This suggests that
there will be a frequency where the two effects add to give maximum current
through the capacitor, although my feeling is that they won't get much above
the peak capacitor current caused by the plate to ground capacity at the top
end of the frequency range.

This suggests that a big GG triode on 2 must have pretty enormous RF grid
currents flowing. An 8877 with 10pF Cpg and 3000 volts pk - pk plate swing
will have 13.5 amps peak flowing through that capacity. 

73

Peter G3RZP



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