Peter sez:
> 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 phase shift could be an issue on ten meters, where series L
and shunt C forms a delay line.
> 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.
It might then be advisable to add a slight safety factor to the
estimated current. Say fundamental current (given by the absolute
power leaving the tank and the voltage at that point) added to
circulating current (given by ALL capacitances at the tube end of
the blocker) plus a small additional safety margin added for
harmonic currents.
At ten meters, where the tube-end of things has several times less
reactance than the tank's input impedance, tube capacitance will
dominate the blocking cap current. At lower frequencies,
fundamental current calculated by the current driving the tank
would be very important.
> 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.
Fortunately that current is involved only is I^2 R heating and not
electron kinetic energy heating of the grid.
Rich loses sight of that fact, and thinks because the tube can
handle 10 or more amperes of capacitively coupled current n the
grid that means it can also handle 10 amperes of grid current driven
by an accelerating voltage of a hundred volts.
In the case of grid current from electron bombardment by cathode
emission, dissipation is given by E times I.
In the other case, it is simply I^2 R losses of the material in the
grid itself with the current transfer handled by displacement
currents rather than electron bombardment.
When you calculate cuurent, the power leaving the tank must be
included or else on lower frequencies there is a very large error.
73, Tom W8JI
w8ji@contesting.com
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