During the early years of WWII, many of the amateur
radio
publications ran ads from RCA that advised dropping the
heater/filament voltage on transmitting tubes by about 10% and
derating them somewhat. ?Apparently this could result in a doubling
of useful life.
No - it is ham lore only. Many tubes lives will shorten if the
filament is run below spec. In some types, life will drastically drop.
--
Will