On Mon, 7 Apr 2008, Chuck Harris wrote:

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?

Yes. The advice of tube manufacturers comes from experience with fairly
large populations of tubes and what is wrong with us all just actualy
talking about our own actual experience (like I did)?

It is unlikely that any of us have run herd on as large a sample set of
tubes as the manufacturers did.

But a lot of us old timer-types surely had--just like me--several
amplifiers running 2-3-4 tubes in parallele, and for years, and that
starts to add up to a population of 10-20 tubes (at least for me). Not
large, but enough.

Clearly the chance of filament failure
among transmitting tubes (or any tube for that matter) is pretty small.

As I've said before, "chance" is speculative. Out of all of my hamming and
SWLing, I have heard very little on guys turning on their amplifiers and
discovering a tube that didnt' light up. I've heard more stories of guys
smoking their little metal-ceramic forced-air cooled tubes from
overdriving their grids, or "pushing" tubes, or using sweep tubes (I've
had a couple from hamfests where there was hardly any emission left) that
were maybe not tuned up fast enough, or whatever.

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....

Agreed. I've had a few flicker a bit and then a few light turn-ons
later...poof and it goes out.

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!

Yes, and sometimes it is worth doing this. I'll bet that the long life
bulbs out there are made with a little more filament wire so the
temperature is maybe (also) 5% lower, too.

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

Fine.

-Chuck Harris