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

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