>
>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.
? It apparently doesn't seem to make a discernable difference if the
tank caps are adjustable. .
>
>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.
>
? Agreed. Anyone who doubts this should take a look at the anode
waveform with a 1000 to 1 HV multiplier probe and an oscilloscope.
>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.
>
? Using a 4kV supply, assuming a min. instantaneous anode pot. of 250v,
the 8877 has an anode potential swing of 7500v p-p. . During the grate
parasitics debate, when we were discussing gold-evaporation from an
8877's grid, I showed that RF grid-current in an 8877 (operating at its
max. ratings of 4kV and 250MHz) exceeds 43RMS amperes. Naturally, this
did not sit well with those who were trying to claim that gold
evaporation could take place with only a few hundreds of mA of grid
current during HF operation.
- cheers, Peter
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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