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

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

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.

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