Tubes and their problems

The idea that a microphonic tube is one that is going to blow is incorrect. Microphonics is a construction issue but not an indication of imminent failure.

@ Joeylawn, its likely that only one of the tubes you replaced was actually the one giving you troubles. To find it, swap the tubes left for right one at a time rather than the whole bunch.

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.

@Bifwynn, we don't have a recommended set of hours on the tubes we use. FWIW power triodes are often good for about 10,000 hours owing to their simplicity. Octal base signal triodes can often do that as well.

We've been warranting our tubes for a year for some 3 decades now and getting away with it, so they must last a good long while.

Power pentodes just don't seem to last as long as triodes, in a nutshell.