> From: Rich Measures <measures@vc.net>
To: <amps@contesting.com>
> Date: Mon, 11 Aug 97 01:54:57 +0000
> Apparently, there are two types of gold.
<snip>
> Ordinary electrons obey the formula P=I*E.
> However, the other type of electrons apparently don't.
> Welcome to the weird world of (grid) Dissipation Science.
Nice to have you welcoming people as they arrive at "weird world".
;-)
While all charges follow the rules of dissipation in a resistance,
this NOT the primary heating mechanism in a power grid vacuum tube
when. The current density is so low that resistive heating can be
ignored (except that caused by filament current).
The primary source of heating is the kinetic heating of the element's
surface as it is struck by high velocity electrons. That is why we
calculate the quiescent dissipation of a tube as P=IE where E is the
accelerating potential and I is the current flow. If a tube has 3000
voltas of anode voltage and has a steady dc anode current of 1
ampere, the anode dissipation is 3000 watts.
That's also why we don't have to consider heating of the vacuum or
heating of the cathode by plate or grid current, during operation.
Heating caused by the distributed resistance, even of a graphite
anode, is negligible compared to kinetic heating. The same is true of
the control grid (except for a small amount of heat transferred from
other elements).
If the anode or grid current is pulsed or is time varying, the
dissipation is the time integrated value of dissipation at every
point during one complete cycle.
For example...
If we took a "snap shot" of anode (or grid) to cathode voltage and
the current at 360 point (every degree) of an RF cycle, and
integrated these power levels into one average power level, that
would be the dissipation. That would be one way to calculate anode
power dissipation in an operating PA. It would produce the same
figure as subtracting the power transferred out to the tank and load
from the anode dc power.
That's why tubes operating with shorter conduction angles--
and tubes that switch off and on faster or harder--- dissipate less
power and produce higher efficiency. Less heat is produced because
the tube is "on" in a high power dissipation state for less time.
Using your "method", a 3-500Z would idle with the same anode color if
it was passing 400 mA at 500 volts anode voltage.... or 400 mA at
4000 volts. After all, the I^2 R loss would be the same.
73 Tom
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