Thanks to all of you for your insightful replies. This all appears to be a
bit like an efficiency issue: how much of the available energy can we
actually use? The electrons possess kinetic energy, obtained via the
acceleration from the cathode to the grid. Running over to my old Halliday
and Resnick physics text, I find that the kinetic energy obtained by an
electron traversing an electric field of V volts in a vacuum is simply:
K = qV,
where q is the charge in coulombs and K is in Joules. If we're dealing
with an electron, it's the quantum charge, e, and we get that neat-o
sounding unit if energy, the electron-volt. Well, let's work this out a
bit further (this is the first time I've bothered to do this)...
Current is coul/sec. So, using a 3-500Z with anode current of 0.4 A and
anode voltage of 4 kV, I have a charge flow of 0.4 coul/sec beging
accelerated across 4000 V. The available kinetic energy, then, is
0.4*4000/sec = 1600 J/s = 1.6 kW.
Obviously, not all of the kinetic energy gained by the charge is dissipated
as heat in our application, else a 3-500Z running 400 mA at 4 kV would get
very bright for only a very brief time, and we would extract no power from
it. So, given the way we build our devices and matching networks, we must
be able to get about 1100 W out of the system for the tube to remain within
its dissipation ratings. This requires an efficiency of 1100/1600 = 0.685,
or 68.5%. Wow! This is about what we see in practice with AB2 amplifiers!
So, I guess what a tube physically does is convert some (not all) of the
kinetic energy gained in the charge transfer from cathode to anode into
what we want (RF) while the rest is lost as heat. Efficiency, then, boils
down to how well we can control how much we lose as heat and how much we
convert to RF. This explains a bit why higher anode voltages will yield
more output: we've increased the amount of available kinetic energy with
which to work. Kinda neat in that I never thought of it this way before.
73,
Kim Elmore, N5OP
Kim Elmore, Ph.D.
University of Oklahoma
Cooperative Institute for Mesoscale Meteorological Studies
"All of weather is divided into three parts: Yes, No, and Maybe. The
greatest of these is Maybe" The original Latin appears to be garbled.
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