Hi...
I have very little experience with vacuum tubes and would like to know
what is an acceptable tolerance on the filament voltage. I am
planning on using some 6U8A tubes in a homebrew receiver. The data
sheet that I saw mentions no tolerance on th 6.2V filament. Would +/- 1V
be OK? Any help would be appreciated....

Thanks..
Jim
WB5KYE

Jim Flanagan wrote:

Hi...
I have very little experience with vacuum tubes and would like to
know what is an acceptable tolerance on the filament voltage. I am
planning on using some 6U8A tubes in a homebrew receiver. The data
sheet that I saw mentions no tolerance on th 6.2V filament. Would +/- 1V
be OK? Any help would be appreciated....

Thanks..
Jim
WB5KYE

Thats a bit too much...but not catastrophic either. Figure on about +/-
10% max with a fresh tube. An older wornout tube might not like even
that much reduction.

-Bill

In article ,
-ex- wrote:

Jim Flanagan wrote:

Hi...
I have very little experience with vacuum tubes and would like to
know what is an acceptable tolerance on the filament voltage. I am
planning on using some 6U8A tubes in a homebrew receiver. The data
sheet that I saw mentions no tolerance on th 6.2V filament. Would +/- 1V
be OK? Any help would be appreciated....

Thanks..
Jim
WB5KYE

Thats a bit too much...but not catastrophic either. Figure on about +/-
10% max with a fresh tube. An older wornout tube might not like even
that much reduction.

Agreed: the filament is just like a lightbulb, and increasing the supply
voltage will *drastically* shorten the life. (Reducing the voltage will
extend the life, similarly, at the expense of reduced output. I think
it's something like 12% overvoltage will cut the working life by 50%,
but I may be mistaken on that.

Chris.

--
Service with a capital "Bugger Off".

Agreed: the filament is just like a lightbulb, and increasing the supply
voltage will *drastically* shorten the life. (Reducing the voltage will
extend the life, similarly, at the expense of reduced output. I think
it's something like 12% overvoltage will cut the working life by 50%,
but I may be mistaken on that.

Chris.

I kind of agree that higher filament voltage is not good. But 50% of
the working life really doesn't say anything. If a tube is good for
only 2000 hours then it would drop to 1000 but if it good for 100,000
hours and drops to 50,000 I would say we really do not have a problem
because 50,000 hours is far more than I will ever have any boatanchor on.

So what are the mean time before failure of most tubes ?

Ron WA0KDS

Ron wrote:

I kind of agree that higher filament voltage is not good. But 50% of
the working life really doesn't say anything. If a tube is good for
only 2000 hours then it would drop to 1000 but if it good for 100,000
hours and drops to 50,000 I would say we really do not have a problem
because 50,000 hours is far more than I will ever have any boatanchor on.

So what are the mean time before failure of most tubes ?

Depends on the circuit, but I recall that one of the premium industrial
brands were called 10,000 hour tubes. They had gold plated pins, an
extra large getter, and a special high purity cathode that wasn't susceptable
to "cathode interface", the nemesis of tektronix vacuum tube oscilloscopes.

10,000 hour tubes were identified by having type numbers from 6000 to 6999.

-Chuck

Chris Suslowicz wrote:

Agreed: the filament is just like a lightbulb, and increasing the supply
voltage will *drastically* shorten the life. (Reducing the voltage will
extend the life, similarly, at the expense of reduced output.

NOT. The resulting reduced emission shortens, not lengthens, tube life
in most cases.

wrote:

NOT. The resulting reduced emission shortens, not lengthens, tube life
in most cases.

I've heard that, and it makes me wonder if the reduced life is only as viewed
with the tube running at the reduced voltage. If you take a tube that has
reached end-of-life with a reduced heater voltage; I wonder if it would still
have a bunch of time left at the correct heater voltage.

-Chuck

On 8/1/05 9:07 PM, in article , "Chuck Harris"
wrote:

wrote:

NOT. The resulting reduced emission shortens, not lengthens, tube life
in most cases.

I've heard that, and it makes me wonder if the reduced life is only as viewed
with the tube running at the reduced voltage. If you take a tube that has
reached end-of-life with a reduced heater voltage; I wonder if it would still
have a bunch of time left at the correct heater voltage.

I suspect it would. In the early days of radio, filament voltage rheostats
were sometimes used to control volume. Also as tubes aged the voltage was
cranked up to keep it usable.

Don

Chuck Harris wrote:
wrote:

NOT. The resulting reduced emission shortens, not lengthens, tube life
in most cases.

I've heard that, and it makes me wonder if the reduced life is only as viewed
with the tube running at the reduced voltage. If you take a tube that has
reached end-of-life with a reduced heater voltage; I wonder if it would still
have a bunch of time left at the correct heater voltage.

I think this also depends on the tube.... as I recall it is much less of
an issue for power tubes than small signal tubes....
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."