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[Amps] Power-off tube cooling

To: amps@contesting.com
Subject: [Amps] Power-off tube cooling
From: RFlabnotes@aol.com
Date: Thu, 15 Jan 2004 10:55:54 EST
List-post: <mailto:amps@contesting.com>
I am curious about the origin of this mythology. It would be nice to see some 
 factual data, or, at least, logic to support it.

My simple-minded physical reasoning says that when the power is all removed, 
there can be no additional heat-rise. So, at least the filament for a short 
time is no better or worse off than during steady-state standby. You might then 
be tempted to argue that thermal stresses induced by the cooling down period 
will go to work, possibly causing dangerous differential stresses. If true, the 
forced air will actually accelerate such processes. Letting everything slowly 
equilibriate back to ambient would clearly be less stressful of everything in 
the thermal 'circuit'. What good is accomp[lished by forcing it quickly back 
down to ambient (of course, in the process, it will then spend a few minutes 
less time at its operating temperature - is that what this is all about).

Is there really some good physics to rely on here, or is this just another 
piece of folklore that won't go away?

Eric von Valtier K8LV
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