Agreed: the filament is just like a lightbulb, and increasing the supply
voltage will *drastically* shorten the life. (Reducing the voltage will
extend the life, similarly, at the expense of reduced output. I think
it's something like 12% overvoltage will cut the working life by 50%,
but I may be mistaken on that.

Chris.

I kind of agree that higher filament voltage is not good. But 50% of
the working life really doesn't say anything. If a tube is good for
only 2000 hours then it would drop to 1000 but if it good for 100,000
hours and drops to 50,000 I would say we really do not have a problem
because 50,000 hours is far more than I will ever have any boatanchor on.

So what are the mean time before failure of most tubes ?

Ron WA0KDS