From: Peter Chadwick <Peter_Chadwick@mitel.com>
Date sent: Thu, 24 Jun 1999 15:30:27 +0100
>
> Tom says:
>
> >The tube also draws less current, because HV is less.
>
> No. The tube draws maximum current when the plate volts are minimum, and
> the grid the most positive (which may not necessarily be positive with
> respect to cathode). So it's swing that gets cramped - the minimum plate
> voltage shouldn't move that far.
You are viewing short term fractional cycle behavior and trying to
use it to explain long term averaged current and power.
Anode current is maximum at minimum plate voltage because you
are viewing the tube at the minimum resistance portion of the RF
cycle. Since it shunts the anode supply to ground, it is natural that
voltage (the across vector) and current (the through vector) are out
of phase.
However, we are talking about the SUPPLY voltage changing while
the tube essentially goes through the same resistance swing. If all
other parameters stay fixed (bias, drive power, screen voltage, etc)
and only HV is reduced, anode current AT EVERY portion of the
cycle decreases. When anode current at every part of the RF cycle
is reduced in conjunction with HV, average anode current and
power gain both drop.
If you have a tube that behaves differently, it is operating in a mode
totally foreign to power grid tubes. There likely isn't an amplifier in
the world that, when all parameters are constant except HV,
increases in current when HV is decreased, at least I can't think of
a case where this is possible unless the other parameters are
varied. There are very few Tunnel diodes used in PA's.
73, Tom W8JI
w8ji@contesting.com
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