Yes i know that was a little simple. It depends on the type of filament, Tungsten, Thoriated Tungston, Carburized Filaments, Oxide coated cathodes, Flat ribbon filaments.. Check out Terman's Radio Engineering Handbook, Chapter 7. i used to run my GE base station 24/7 and ran the 4-125's at reduced filament voltage, and kick them up during transmit. Other wise they would lose emission. Paul w0rw