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