... ...
>Something at the back of my mind says that I've seen somewhere that one
>shouldn't run a thoriated filament in saturation, but I can't remember
>why - the penalty of getting old, I guess. If you run it too hot, then
>you boil out the thoria,
The c. 1.5% thorium content of the tungsten filament remains unchanged.
However, run the filament too hot and the emissive tungsten carbide layer
gives up it's carbon atoms at an accelerated rate--- leaving tungsten
behind. Each 3% excess in fil.V =s 1/2 the emissive life. 6% excess
fil. V =s 1/4 the emissive life--et cetera.
- However, by temporarily introducing acetylene gas (C-two, H-two) into
the envelope, the tungsten carbide layer can be restored to full emissive
capability.
>but doesn't something nasty happen if you run
>it too cold, besides not getting enough emission?
>
If the vacuum is somewhat less than perfect, and the filament is run
below the rated minimum V, gas atoms can be absorbed into the filament,
which interferes with emission. However, by briefly running the filament
V slightly above the normal range, the gas atoms are easily driven off
and emission returns.
Rich---
R. L. Measures, 805-386-3734, AG6K
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