Randy or Sherry Guttery wrote:

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

The reason for dropping a high power transmitter tube's heater voltage
when running under power is to compensate for the heat added to the
cathode by the anode current, and the reflected heat from a toasty hot
plate. The hot anode acts like an oven surrounding the heater, and as such
forces its temperature up. If you don't reduce the heater voltage from the
nominal value, the heater will overheat, and cathode life will be reduced.

-Chuck

Whether operating with reduced filament voltage is detrimental seems to
depend on whether the tube is an oxide-cathode type.

Pure tungsten or thoriated tungsten filaments do not suffer damage from
filament under-voltage, while oxide cathode types do.

Thanks to W8JI for the info:

http://www.w8ji.com/vacuum_tubes_and...e_failures.htm
Vacuum power tubes, using old valves, and vacuum tube failures


Chuck

Randy or Sherry Guttery wrote:

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

snippage

You haven't posted anything contrary to the +/- 10% rule given by RCA.

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."

wrote:

You haven't posted anything contrary to the +/- 10% rule given by RCA.

Geez, Frank:
I was answering a specific question posted earlier: "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 noted that the answer was yes - and that it is a common (recommended)
practice with certain tubes in certain types of service (I.E. running at
reduced voltage until the inevitable emission fall-off required
gradually restoring full filament voltage).

I'll further note that many tube testers (B&K 700 series, for instance)
have a "Life Test" where the filament voltage is reduce by some amount.
If the tube is "fresh" emission falls very little. If the tube is
"long in the tooth" - emission falls significantly. This shows that
tubes that are in trouble at reduced voltage can still (for a time)
perform OK at "rated" filament voltage.

This is all I was addressing in my earlier post; and demonstrate even
further in the above.

--- So what's your point? Where did I imply I was going to comment on
RCA's "rule" - contrary or otherwise?

But now that you make an issue of it -- and so as to not disappoint you
- I will post something "contrary" to RCA's "rule":

Mullard's general recommendation was +/- 5%.

Were RCA's tubes that much "better" than Mullard's that they could stand
twice the variation?

Or was Mullard more concerned with quality and how their stuff was
used-- while RCA rated their tubes as being just mass-market junk?

That oughta stir some crap... Happy now Frank? Something to sink your
ever present teeth into?

Sheesh.
--
randy guttery

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

chuck wrote:
Pure tungsten or thoriated tungsten filaments do not suffer damage from
filament under-voltage, while oxide cathode types do.

If you reduce B+ and screen voltages proportional to the fil. voltage
reduction, you negate the problem of depleting the space charge and
stress-cracking the oxide coating (assuming there is a decent amount
of emission left, of course). Yes, output is reduced.
But I think most of us know the difference between
100 watts and 70 watts isn't worth mentioning.

73 Dave S.