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 asked:

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.

Depends on the type of tube -and the service it's in. For instance -
many large transmitter tubes are run at 100% filament voltage for their
1st 100 hours - then reduced to (usually) 92% and run until their
emission starts falling off - usually seen in PAs when the tube is
having a hard time making full power (and grid current has dropped off
from "usual"); - or in the case of modulators - when peaks get
clipped... then the filament gets raised back towards 100% to restore
proper operation. When 98% is reached - time to order new tube(s) to be
sure they're available when the installed one(s) croak.

Running tubes such high power tubes (like the 3CX2500, 4CX3000, etc.) at
92% for as long as possible adds years to their life.

Keep in mind, however - these tubes are designed for such service.
Under powering many small signal tubes can cause damage to the emitting
surface (cathode) - so it's best to run them closer to design value.
Basically - if a tolerance for the filament voltage isn't given -
assuming a tolerance of +/- 5% is conservative - while +/- 10% is
pushing the the tube pretty hard; and shouldn't be done.

Tubes designed to be run "cool" like very large power tubes will be so
noted by the manufacturer or rebuilder as the case may be.

best regards...
--
randy guttery

A Tender Tale - a page dedicated to those Ships and Crews
so vital to the United States Silent Service:
http://tendertale.com

On 2-Aug-2005, Don Bowey wrote:

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.

Forgive me if this comment is out of place, but I think I see a trap here.
It is very easy to use the terms "heater" and "filament" interchangeably.
They are two entirely different animals and should be treated accordingly.
The comments regarding reduced life at reduced voltage would probably apply
more to indirectly heated cathodes (heaters) rather than directly heated
cathodes (filaments).