how does one tell when a tube is truely dead?

[alexmoose]

As I grow more familiar with the world of tubes, I think i'll need to figure out how to tell when one of my tubes goes. my question is. As long as:

A. the vaccuum is still good

B. The heater still works
And
C. there is nothing visibly wrong

is there any other way for a tube to break?

 

[jeff mai]

A tube could develop a short circuit between electrodes.

The emissive cathode material could run out.

 

[lndm]

Try biasing it to a known typical Vg-k / B+ / Ra, and notice the current drawn. I have had problems getting good results out of output devices that only put out 60% of what I expect a good one should.

They will put out more when you reduce Vg-k but pushing a worn device may bring about its ultimate demise.

In the case of diodes, they will typically arc over from time to time to indicate they are worn (they may tick and flash blue for example). I assume triodes might too but haven't seen it.