[Amps] why a tube fails ?

[k7fm]

I am of the belief that tube failure is directly related to the amount 
of alcohol consumed by the operator while operating the amplifier.

The only tube failure I can recall being involved in was when I removed 
one of the 3-500Z tubes from an amp to clean the dust out and laid it on 
the workbench.  Tubes are round and it rolled off the bench.  Being 
quick\-footed, I broke the fall with my shoe and the glass did not 
break.  Unfortunately, the filament did.  Lesson learned.  Electron flow 
ceases when the filament breaks.  And, no I had not been drinking.

73,  Colin  K7FM