Tubes and reasons / circumstances of wear

[KT88]

Is it so that a tube as in the case of an unused half (one of two triodes) lets say of a 6SN7 or a 12AX7 in some circuit does not wear out but only the used half (one single triode)? So if at all only the heater wears out a bit which always runs for both partial triodes?

 

How is it regarding power tubes like e.g. a KT88. Would it virtually not wear out when it is switched on (again except for the heater) as long as no signal is amplified? So the electrons are only "washed out" of the cathode when a signal is present?

If this is true can I then further conclude that the power tube is good for longer time if I only listen to low level music or does it not matter how loud you listen so it's only about the hours that are consumed regardless of demanded volume level?

I know that tube wear also depends on how much you stress a tube in the circuit, how high the anode voltage is rated, etc. But my question is more basic.

 

 

[captainbeefheart]

When the cathode is heated emissions release an electron cloud, with a negative enough bias or no plate voltage the cathode will wear out much less than if the emissions are being pulled out of the cathode and up to the plate. This is why a high idle bias current like in Class A amps they wear out faster vs a very cold biased amp running Class B operation. Of course if you are pushing the Class B amplifier to maximum output all the time the tubes will wear down. McIntosh amps are known to have their power tubes last very long because of the latter, cold bias heavy into Class B operation. What can happen is with the hot cathode and no current flow to plate the cathode becomes poisoned and behaves like a degenerated common cathode stage, high output impedance low gain.

 

With a tube like the 12AX7 you can actually heat just one triode via using the center tap. The 6SN7 does not have a heater center tap so when the filament is powered it is always heating both triode sections cathodes.

 

 

[captainbeefheart]

Forgot to answer your question about low signal listening and tube life.

 

An operational tube is DC biased so current always flows through the tube with no signal. If the tube is say a preamp tube and biased well below the max plate dissipation and or max cathode current then it will last a very long time regardless of signal or not.

 

If you have a power tube, say single ended triode output, this is a Class A circuit so bias current will be high to keep it in conduction the full 360° signal waveform and they are almost always biased near max plate dissipation. These will wear out the soonest no matter what.

 

A Class AB amp biased for decent power will have relatively cool bias, say 60% of max dissipation, yes the tubes are passing current with no signal but most likely half of what it would be in a Class A circuit so they last longer. Run that amp at high powers for long periods the amp goes into Class B with large current swings and so yes the tubes will last longer here if you are just listening to it at lower power/signal levels.