[Amps] Re: What heats the anode

[Kim Elmore]

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.