Re: Valve question
In certain types of electronic organ using separate oscillators with HT keying, the oscillator valves run with full heater and no HT until required to speak. Certain notes are so seldom played that for every thousand heater hours, anode current might flow in the relevant oscillators for all of a minute or two.
When first checking over my 4-rank Miller, which arrived in unplayable condition, most of the valves that run continuously with full anode current were found to be completely life-expired with woefully low emission. This was true regardless of whether they were original or replacements. In contrast, many of the oscillators are still running on their original Brimar 12AU7s, registering 50-75% emission on test; I have not yet investigated how much of the measured reduction is due to conventional wear on the cathode surface and how much to interface resistance or other deterioration mechanisms. More conspicuous was the low insulation resistance; hundreds of kilohms being typical, in some cases only tens of kilohms. I would like to study the valves further to discover whether this is related to the unusual operating conditions.
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