Ron wrote:

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 ?

Depends on the circuit, but I recall that one of the premium industrial
brands were called 10,000 hour tubes. They had gold plated pins, an
extra large getter, and a special high purity cathode that wasn't susceptable
to "cathode interface", the nemesis of tektronix vacuum tube oscilloscopes.

10,000 hour tubes were identified by having type numbers from 6000 to 6999.

-Chuck