Failure Modes
The emissive layers degrade slowly with time, and much more quickly when the cathode is overloaded with too high current. The result is weakened emission and diminished power of the tubes, or brightness of the CRTs.
The activated electrodes can be destroyed by contact with oxygen or other chemicals (e.g. aluminium, or silicates), either present as residual gases, entering the tube via leaks, or released by outgassing or migration from the construction elements. This results in diminished emissivity. This process is known as cathode poisoning. High-reliability tubes had to be developed for the early Whirlwind computer, with filaments free of traces of silicon.
Slow degradation of the emissive layer and sudden burning and interruption of the filament are two main failure modes of vacuum tubes.
Read more about this topic: Hot Cathode
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