Anyone know the answer to my questions

[sevycat]

If you place a tube in the pi position, what would be the tells that the tube is going bad? Can an amp work with a bad pi tube in it or not?

 

[kekam35]

The biggest sign of a bad phase inverter is a drop in overall volume on all channels.

 

[Don]

Can an amp work with a bad pi tube in it or not?

No. Not well anyway. People often replace this tube when they replace power tubes. It's the one tube that has failed in my 1-1/2 year old Express 5:50. My amp stopped working in the 50 watt class AB mode but still worked fine in the 5 watt class A mode when this tube died!

 

[espboogie123]

I've had a phase inverter tube fail in a couple different amps of mine... I had total loss of volume and one with a slight whisper faint of sound.

 

[94Tremoverb]

It depends on which half of the tube fails - the second side of the phase inverter is driven from the first, so if the first side fails you get silence; if the second side fails the first side can keep working (although now biased wrongly, usually) so you get a quiet, badly-distorted sound. I'm not sure how it applies to the switching in the Express though.

 

[littlephil]

Can an amp work with a bad pi tube in it or not?

No. Not well anyway. People often replace this tube when they replace power tubes. It's the one tube that has failed in my 1-1/2 year old Express 5:50. My amp stopped working in the 50 watt class AB mode but still worked fine in the 5 watt class A mode when this tube died!

Thats because it doesn't use the PI in Class A, because its running single ended, not push/pull.