Tubes and their problems

Atmasphere, you're right, but I just went ahead and replaced all 6 tubes (which were Electro Harmonix and Tung Sol) with matched and cryo'd JJ's.

Before I take out the many screws to get at my tubes I decided to buy a new rectifier before diving in. I'm hearing a very low thump, or thud, intermittently, and irregular when it shows. The two 6sn7 driver tubes are Shuguang Black Treasure which are only a few months old. Not sure which is the problem, but will switch out the rectifier first since I'm unsure of its age. (Mullard)

Every time when I replaced microphonic tube by new one the microphony of amp dissapeared.

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.