A group of Hivac midget valves
It is inadvisable to run the filament or heater of a valve with no anode voltage applied except for short periods as it causes loss of emission.
Do not run a pentode or screen-grid valve with voltage on the screen and none on the anode, as it may damage the screen or the emission. Do not exceed the anode dissipation limit of a valve. Particularly do not run a valve with a nickel anode up to red heat; this may only be done if the valve has a molybdenum or graphite anode.
In the case of transmitting valves select those which have an anode current, as shown by the characteristic curves at zero grid Volts, at least three times the current at the point of bias or input at which you propose to operate, particularly so where the valve is a modulated amplifier (PA), and bear in mind that the emission from the filament is the anode current plus the grid current. The maximum emission required for 100% modulation will be the normal anode current multiplied by two, plus the grid current peaks, which may reach three times the measured grid current when driven.
It is inadvisable to drive a transmitting valve so hard that a very heavy grid current flows, unless the grid structure is of generous dimensions capable of dissipating the heat generated, otherwise grid emission may occur or the valve go soft.