What are the symptons of a phase inverter going bad? If it does go bad, does it go gradually, or does it go pretty much immediately? I just replaced a microphonic preamp tube and was curious what I should be on the lookout for, because if the preamp tubes go bad, then I guess the PI will eventually, too.
There's no simple answer to this question. It could go gradually or suddenly. It only takes a second (or less) for almost anything to stop working. Symptoms could be loss of power, high distortion, no sound, etc.
A bad PI tube in most cases will reduce output. What usually happends is one side of the tube goes bad increasing even order harmonics, so the amp becomes cleaner sounding.
Also, just like any other tube going south, they can start to pop and spit and/or do the "frying bacon" thing.