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[AMPS] Dissipation Science

To: <amps@contesting.com>
Subject: [AMPS] Dissipation Science
From: w8jitom@postoffice.worldnet.att.net (w8jitom@postoffice.worldnet.att.net)
Date: Mon, 11 Aug 1997 07:16:01 +0000
> 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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