Tube aging, in-circuit test?

[Skorpio]

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?

[Merlinb]

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.

[DF96]

The 95% could refer to anode current or gm (or anything else really). As Merlinb says, normal cathode resistor bias automatically adjusts for valve ageing (and parameter spreads on new valves).

[Skorpio]

Ok, thank you for your answers, it covers half of waht I wanted to know ;-)

The other thing, is there some objektive test (measurement) I can do in-circuit to "test" the tubes condition?

Or is the subjective feel of something is not so good anymore the way to go?

[Merlinb]

Originally Posted by Skorpio

The other thing, is there some objektive test (measurement) I can do in-circuit to "test" the tubes condition?

Well, you could perhaps just measure the voltage across the cathode resistor and so calculate the anode current. Over time it will fall, and this will reflect the fall in gm.

[DF96]

Drop the heater voltage a bit (10%?) and see what this does to the anode current (or voltage drop across cathode resistor). The change should get worse as the valve ages.