July 17, 2017
March 23, 2015
My understanding is:-
Metals and other substances evaporated from the surfaces of the valve elements get deposited on the glass surface. You will notice the effect in power valves more than low current types. The PL81 valve suffers from this quite noticeable but the PL81 very often work ok.
Quite happy to have a different explanation.
February 17, 2015
That is basically correct. Power valves like line output, audio output, video output and boost diodes etc. ran hot…sometimes very hot so the metal electrodes would eventually deposit particles on the inside of the glass. It didn’t necessarily mean the valve was no good but a badly stained one might indicate that it’s had a lot of use or been overrun for long periods. I have a number of badly stained PL81’s but they are all good.
Is it supposed to do that?
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