> Even well nurtured Tx tubes will eventually go soft after typically many
> thousands of hours operation when the emission / electron supply runs out
> and can no longer meet the peak plate current demand; the emission is
being
> depleted whenever the filament is lit / cathode is hot and streaming off
the
> finite number of electrons in the emissive surface.
>
> Leigh
> VK5KLT
Leigh, I'm sure you didn't mean to convey the idea that a vacuum tube
cathode is a reservoir containing a finite number of electrons which is
eventually depleted. As I'm sure you know, the cathode always maintains
charge neutrality (except for a small space charge effect), with the emitted
electrons continuously replenished by the peripheral circuitry.
As I understand it, the cathode failure mode is caused either by
contamination or "poisoning" by outgassing, an overhot filament, or
impurities in the vacuum, or by a gradual boiling off of the emission atoms
(usually oxides of barium, strontium, or other metals having a low
workfunction.) I have heard that one of the most common causes of cathode
poisoning is from excessive grid current, which overheats the grid, causing
it to evaporate its gold coating, thus contaminating the cathode structure.
73,
Jim W8ZR
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