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.

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.