[Boatanchors] LINE VOLTAGE, FILAMENT VOLTAGE and SANITY

[Gary Peterson]

“Somewhere in the deep recesses of my mind, I seem to remember that it is possible that the potential difference between plate and indirectly heated cathode CAN be enough to "rip" the electrons off that cathode if the cathode isn't heated enough to supply the current being drawn.  As I mentioned, this is something I heard or read MANY years ago. MAYBE, this is behind the earlier quoted statement about the 4CX250B. 
Mac, K2GKK/5”

That pretty much matches the explanations that I have read.  Application of high voltage to the plate and screen, before the cathode has come up to temperature, may damage the metallic oxides coating the cathode that are responsible for electron emission.  Most amplifiers, that contain tubes with indirectly heated cathodes, have time delay relays to prevent premature application of high voltage.   As I recall, the manufacturer recommended five minutes warm up for an 8877.  I always waited for at least three minutes on a transmitter with 4CX250B drivers.

Gary, KzeroCX