Rich had the right basic idea, but he neglected the fundamental
current flowing through the capacitor that transfers energy to the
tank.
Since his method also ignores harmonics and phase shift, I'm not
sure how good an approximation it actually is. Maybe someone
else can offer some insight in to how to make the idea more
accurate but still simple.
If we know the operating load impedance presented by the tank
and the anode RMS voltage, we can calculate the RMS current
flowing into the tank through the blocking cap. Of course that will
be far from the peak current, but that shouldn't matter in most
cases.
So we have the basic current flowing, and the RMS voltage. 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.
It looks like the only problems are phase shift or harmonic currents
(waveform distortion). At HF in a somewhat linear PA maybe that
can be ignored.
With a 3000 ohm resonant load resistance at the anode end of the
tank and 3000 volts RMS, we would have one ampere of base
current PLUS the current required by the capacitance to ground
anode of the tube when excited by the 3000 volt RMS tank swing.
I actually think that might be an easier approximation than the
Handbook offers.
What say Ian, Peter and the rest? Does it look good to everyone
else now that the missing current has been added to Rich's
suggested method?
73, Tom W8JI
w8ji@contesting.com
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