A wrote:

The key issue which is not discussed at all above is that you have to look
at _some_ sample population to see the effect of lower filament voltage on
increased filament life. The question I ask, again, is how many guys out
there with filament cathode power tubes have had filaments blow out
(compared, say, with decrease in power output, or internal shorts that
blow fuses) in some manner that one might hypothesize is due to filament
warmup being too fast. Remember, I had one 813 filament open up on me even
though I was going from zero to ten volts, slowly over about ten seconds,
and on a Chinese 813 about one year old. So, I conclude, on a sample size
of one, that a filament can blow even if you raise filament voltage
slowly. Oh, yes, I also slowly decrease filament voltage to zero, too,
when I shut down the station. And, I'll remind everyone that I've had lots
of amplifiers in my ham career, and with no inrush protection, and never
lost a filament. I'd still like to hear from others about this.

It depends entirely on the application and how the tube fails in that
application. But yes, Varian has done statistical analysis and there is
an Eimac white paper on filament voltages.

Regarding higher filament voltages, I remember as a kid that we had a TV
set where the picture tube cathode emission deteriorated after some years
of use thus causing a weak picture (white areas were gray, gray areas were
black) and they had these little filament voltage boosters that caused the
filament to heat up to hotter temperatures thust restoring cathode
emission for some unknown additional period of time (I think the voltage
boost was from 6.3 vac to maybe 7.5 or so) since a hotter filament should
burn out sooner. Interestingly, we were still using that TV, every night,
for another three years. Again, a sample population of one. However, the
picture brightness did come back to fully normal levels.

Yes, the picture tube brightners will get a little extra life out of a
CRT with poor emission. So will a "CRT Rejuvenator" which will throw
a large reverse voltage on there to blow debris off the cathode.

Surely there are a few people out there who have done some actual
(experimental) playing around with their tubes and can tell some stories
instead of passing on technical rumors.

I suggest getting the Eimac white paper. Or for small signal tubes, a
copy of "Subminiature Electron Tube Life Factors," an NTIA report authored
by some Raytheon folks.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."