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.

If I understand your request, you would like us to abandon the advice of
the tube manufacturers, and instead use our own anecdotal accounts of
tube filament failures?

It is unlikely that any of us have run herd on as large a sample set of
tubes as the manufacturers did. Clearly the chance of filament failure
among transmitting tubes (or any tube for that matter) is pretty small.

If we switch our focus to incandescent lamps, an example of a tungsten
filament with which we all have large experience, we could perhaps
extrapolate that anecdotal experience to tubes in general.

If I eliminate the cases where I dropped, or thumped the lamp bulb (trouble
lamps), I cannot remember a case where a household tungsten filament lamp
has failed at other than turn on or turn off.... and since I have spend my
life living with others, I cannot say for certain that those cases where
I thought the bulb must have failed on turn off, weren't actually cases
where it failed at turn on for a house mate, and they didn't take the time
to replace the bulb, or feel the need to tell me about the failure.

If, however, we wish to ditch the anecdotal evidence, there is ample
statistical evidence, compiled by the lamp manufactures, to allow them
to come up with the following equation for tungsten filament lamp life:

[life at test V/life at rated V] = [rated V/test V] ^ 12

Using a 5% reduction in operating voltage, we get:

[life at test V/life at rated V] = [Vr/(Vr(1-0.5))] ^ 12

or,

[life at test V/life at rated V] = [1.05]^12 = 1.85

Which means by simply lowering the lamp's applied voltage by 5% we increase
its life by nearly 2 times!

Which fits very nicely with the wartime conservation recommendations made
by some of the tube manufacturers.

-Chuck Harris