premature death of a tube

[JC@]

What may cause the premature death in a new tube?
Not taking into account shorts or a loss of vacuum.
I've seen this in just one half of a 12ax7 !!
How can a tube go completely flat when the other tubes in the quad are perfectly ok? Or the other half of the tube in the twin triode tube...

[Enzo]

How can one guy have to sneeze when the people sitting next to him breathing the same air don't? Point being that just because something is part of a group - either a quad of tubes or just one section of a multi section tube - doesn't mean it can only do things as a group.

Tubes are mechanical things. They are constructed of little wire posts with even tinier wires welded to the posts as grids. Metal plates surround the inner parts, and these plates are welded or crimped to posts. In the center is the cathode - a little hollow tube, and centered within it is the heater. ALL of these are also then welded to the wires that either exit the tube as pins or are soldered into the pins of the base of the tube.

If ANY of this fails mechanically,the tube doesn't work right or fails completely. If the weld to the base pin cracks, no connection then. If it is the plate or cathode the tube won't conduct. If the weld breaks between support post - which carries the current to the base pins - and the inner element - plate, grid, whatever - then the tube is bad. If the mounting welds, crimps, etc get loose, then the inner partsd can move around, which makes the tube miicrophonic. Worse yet, if one of the tiny grid loops breaks free, it can touch another element - plate of cathode mainly - and short the tube. If the weld breaks between the heater and its supports, no heater. In the case of a dual triode, one side heater can die leaving the other side OK. ANd in octal tubes, the wires come out the bottom of the glass bulb just like on a 12AX7, except in octals, they are then slid into the hollow pins of the base and soldered there. If that solder fails, whatever that pin did doesn't get done. I have repaired any number of octal tubes that needed a pin resoldered.

[JC@]

...If it is the plate or cathode the tube won't conduct. ...

Easy! I mean, now it is easy.
I was really blocked in the idea that this tube was worn-out.

Thank you.