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

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.

My Collins KWM-2 uses a home brew power supply. Step start on filaments -
at correct voltage and no high voltage for 60 seconds. Same tubes in it for
30 years. I think the only two power amplifier tubes that failed were
dropped on the floor. Some amplifiers adjusted the filament voltage to be
correct and some were never measured.

I ran a 3 x 813 amp for years and never changed the tubes or even measured
the filament voltage. Just used a transformer with the rated voltage and
called it good. Unless the filament transformer has a current rating much
higher than the demands of the amplifier tubes, the surge will probably be
ok.

Incidently, the home brew equipment looks practical. I built a vfo using
the Collins 70H3. The problem with it is that it covers 1.5 to 3 MHz with
10 turns. That is 150 kHz per turn. Bad for a linear dial. I used a 8:1
reduction gear and a crystal controlled mixer to cover 5 to 6.5 MHz, then
limited use to 5 - 5.5 MHz. That was coupled in as the second vfo for my
Drake TR-5, so when I switch it in, the Drake digital dial displays the
frequency. Extremely stable. Oh, I converted it to solid state, also,
which is easy to do. There is one minor problem in that there are no stops
on the dial. Not a problem for me, but if some kid came in and started
twisting, he could break something. Oh well, you need to take some risks in
life.

73, Colin K7FM

Nice stories below. Credible. Thanks. I have a few comments below.

On Sun, 6 Apr 2008, COLIN LAMB wrote:

My Collins KWM-2 uses a home brew power supply. Step start on filaments -
at correct voltage and no high voltage for 60 seconds. Same tubes in it for
30 years. I think the only two power amplifier tubes that failed were
dropped on the floor. Some amplifiers adjusted the filament voltage to be
correct and some were never measured.

Out of many dozens of tubes in ham gear, I think I never lost one by any
failure mode. I found an old Halicrafters S-85 at a hamfest and bought it
out of nostalgia. Has original tubes (all marked Halicrafters).

Our B&W TV sets (when I was a kid), however, had maybe half a dozen tubes
go soft and needed replacing. I've bought a number of ac-dc AM/FM radios
with tubes at hamfests and still have original tubes and still work!
Vintage 1950s.

I ran a 3 x 813 amp for years and never changed the tubes or even measured
the filament voltage. Just used a transformer with the rated voltage and
called it good. Unless the filament transformer has a current rating much
higher than the demands of the amplifier tubes, the surge will probably be
ok.

Incidently, the home brew equipment looks practical. I built a vfo using
the Collins 70H3. The problem with it is that it covers 1.5 to 3 MHz with
10 turns. That is 150 kHz per turn. Bad for a linear dial.

Yeah, I know. So, I use gears and don't worry about more than couple kc
accuracy. I'm not a Digital readout freak.

I bought a couple of R-390 VFOs from Fair Radio Sales (Lima, Ohio). Out of
three I bought, two worked and they are something like 2.4 to 3.4 mHz, and
so 100 kc/turn (ten turns nominal, twelve turns real, and still linear,
they go actually up to 3.6 so can use directly on CW part of 80 meters).

I used a 8:1
reduction gear and a crystal controlled mixer to cover 5 to 6.5 MHz, then
limited use to 5 - 5.5 MHz. That was coupled in as the second vfo for my
Drake TR-5, so when I switch it in, the Drake digital dial displays the
frequency. Extremely stable. Oh, I converted it to solid state, also,
which is easy to do. There is one minor problem in that there are no stops
on the dial. Not a problem for me, but if some kid came in and started
twisting, he could break something. Oh well, you need to take some risks in
life.

make your own stops.

73
W4PON

73, Colin K7FM

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

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

A wrote:


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.

Now, the second part of the question is: does it matter?

If the filament life under normal operating voltages exceeds 20,000 hours,
than dropping 5% and gaining an additional 18,000 hours, is probably
not going to result in a noticeable improvement in life. Forever is
forever.

-Chuck Harris

On Mon, 7 Apr 2008, Chuck Harris wrote:

A wrote:


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.

Now, the second part of the question is: does it matter?

If the filament life under normal operating voltages exceeds 20,000 hours,
than dropping 5% and gaining an additional 18,000 hours, is probably
not going to result in a noticeable improvement in life. Forever is
forever.

FYI, FWIW...

Well, that is, kinda, why I was trying to bring the discussion FROM the
"manufacturers recommendations" (on large populations) TO what is everyone
noticing from their own (multiple) amplifier experiences and what everyone
else yaks about in terms of their own (multiple experiences) with filament
power tube lifetimes.

Yeah...20,000 hours is a whole lot of ragchewing. I think I never in my
life spent more than about 10-15 hours/week with amp filaments lit up.
Today, I have the 813 filaments on about 2 hours a week (and drive the
pair with a single 811 in gg and it's filament is on, also, and not
not voltage managed, either). And, I've never blown an 811 filament, ever,
and I had a few of those tubes in amplifiers, too.

FYI, FWIW

-Chuck Harris

A wrote:

Now, the second part of the question is: does it matter?

If the filament life under normal operating voltages exceeds 20,000
hours,
than dropping 5% and gaining an additional 18,000 hours, is probably
not going to result in a noticeable improvement in life. Forever is
forever.

FYI, FWIW...

Well, that is, kinda, why I was trying to bring the discussion FROM the
"manufacturers recommendations" (on large populations) TO what is
everyone noticing from their own (multiple) amplifier experiences and
what everyone else yaks about in terms of their own (multiple
experiences) with filament power tube lifetimes.

Yeah...20,000 hours is a whole lot of ragchewing. I think I never in my
life spent more than about 10-15 hours/week with amp filaments lit up.
Today, I have the 813 filaments on about 2 hours a week (and drive the
pair with a single 811 in gg and it's filament is on, also, and not not
voltage managed, either). And, I've never blown an 811 filament, ever,
and I had a few of those tubes in amplifiers, too.

FYI, FWIW

If we extend the universe of tubes observed to the TV and radio types, I
would have to say that it is very rare for me to find a burned out filament
in the 6.3 and 12V tubes. And very common in the very delicate 35-75V tubes
used in series string sets.

-Chuck

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."