• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tube aging, in-circuit test?

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Is it possible to test (and compensate) a circiut for aging tubes?

The actual circuit is my 12BH7A common cathode amplifier, with +B=300V, Va=150V, Ra= 15K, Ia=10mA, Rk=470Ohm (decoupled)...

If I test these values e.g. each 6 month, I will likely see a drift from original values, will this drift be due to aging...(?)

Could this drift be compensated (like bias adjustment in power amps), and by this increase tubelife in the circuit? This could be by changing the Rk to keep Va/Ia at original values...

Are there any guidelines for tube aging? Sometimes I read that a tube is measuring 95% of specified values, what parameters does this "95%" refer to?

If not bias point at predefined values, could it be gm?
 
With age gm falls, so you will see the quiescent anode voltage rise. The absolute voltages are not important though (unless it is a DC coupled circuit), but the bias point is.
A cathode biased stage naturally adjusts the bias point and will maintain roughly the same headroom over most of the valve's natural life, so there isn't really any extra life you need to squeeze out of it.
 
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