[Amps] Power-off tube cooling

[R.Measures]

**  It depends on the size of the tube.   For example, the 8973 spec 
sheet recommends that forced air cooling on the base continue for two 
minutes after the filament power (16.3v @600a)  is removed

>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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