Re: Valve Testers
Yes, indirectly heated.
As you say, there is lots written about cathodes but from my reading by the 1940's it was all pretty much known and standard. Whether this is still so for the new valves I don't know. So temperature, on European or US valves is still the defining emission limit.
If the temperature is known and emission is low then what more do you need to check?
Without going back to the books, the cathode material will energetically emit electrons at a temperature. It will do this until the whatever material in the cathode is depleted, poisoned or whatever. At this point the maximum emitting current drops from the data sheet value, or whatever is calculated to be a sensible limit, the valve is worn out.
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