On Sat, 5 Apr 2008, COLIN LAMB wrote:

RCA took out an ad in the 1942 Radio Handbook. This, of course, was a
wartime publication.

The title to the article is "How to make transmitting tubes last longer."
They state:

"Long life - not maximum output - is the keynote of transmitting tube
operation today in many services where, because of war restrictions, it may
prove difficult to replace tubes that wear out. For tube types using pure
tungston, a 5% decrease in filament voltage will dobule tube life... Care
should always be taken in starting up tungsten filaments. Never should the
filament current exceed, even momentarily, a value of more than 150% of
normal.

For types using thoriated-tungsten filaments and oxide coated filaments, the
filament may be operated on the low side - as much as 5% below normal
voltage if the loading is light. The filament voltage should be increased
gradually to maintain output. Toward the end of life, additional service
may be obtained by operating the filament above its rated voltage."

The 813 has a thoriated-tungsten filament.

73, Colin K7FM


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.

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.

In my own goofing off, I have raised filament voltage on a 6.3 v
expendable tube up to 18 volts, over a period of about one minute, before
it blew out the filament. It got quite bright. I have had momentary
mistakes where I have put 12.6 volts on filaments of 6.3 volt tubes for
periods of 10-20 seconds (turning off when seeing the filament get
abnormally bright) with no apparent effect on tube lifetime. Sample size:
maybe 3-4 tubes.

I had, once long ago, a tube hi-fi amplifier on which I put a variac and
was somewhat surprised that I still got power output even with 85-90 vac
going in, and over prolonged periods of time (Down at 75-80 vac, audio
droped off and distortion came up).

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.