Adequate cooling of the tube envelope and seals is one of the principal factors affecting vacuum tube life . Deteriorating effects increase directly with the temperature of the tube envelope and ceramic/metal seals. Inadequate cooling is almost certain to invite premature failure of the device.
Tubes operated at VHF and above are inherently subjected to greater heating action than tubes operated at lower frequencies. This results directly from the following:
The flow of larger RF charging currents into the tube capacitances at higher frequencies
Increased dielectric losses at higher frequencies
The tendency of electrons to bombard parts of the tube structure other than the normal elements as the ...