Tubes and their problems

I agree that microphonic tubes do not indicate failure but in fact I have used these tubes with great success, some of which sound pretty darn good. If they are too microphonic you can tame them a bit with tube dampers.

Does anyone know for sure what a tube exhibits before it dies. Maybe that can vary from one piece of gear to another. In my case the tubes in a preamp slowly lost gain and the gain would suddenly drop. I have heard that tubes in power amps can go out with a big bang and would require more frequent testing to ensure safety.

These days arc-over is a frequent symptom of power tube failure. They can also fade away. If you see the silver that is on the inside of the glass turn brown or transparent, it means that the tube is gassy and will run hotter and with less output. If the silver area has turned white it means the tube has lost its vacuum.

In signal tubes they can develop intermittents; crackling, noise, etc. Sometimes they respond well by thumping the tube hard but sooner or later the noise is back if this is a problem. Microphonics is not a source of failure but it is annoying and hand-picking is really the only way to get rid of it- dampers help but only slightly. Finally, signal tubes can just fade away, often replacing output level with some noise (hiss).

Cryo-ing a tube will not improve its performance, FWIW.

Atmasphere, thank you for your in depth answer. I was hoping that you could explain arc-over and its cause.

Daveyf,

I wouldn't presume to offer a tech answer when the likes of Ralph (Atmasphere) provides such comprehensive advice. That said, being a fellow "tube-o-phile," I would only mention that many manufacturers suggest that tubes be replaced within a certain life span. I surmise that tubes are more susceptible to failure or performance degradation if use exceeds recommended replacement points (as measured in hours in service).

In my case, ARC recommends that 6H30 signal tubes be replaced at the 4000 hour point. I understand that the new KT-150 power tube has a 3000+ hours life span.

I'm sure every manufacturer has its own recommended re-tube specs. Ralph, what do you recommend for your amps??

Bifwynne, of course, let us pray!