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A theory which I tend to support is the deterioration of the cathode coating due to foreign gas (air, etc) in the tube. AK member Colortel, an engineer in the field, I recall also supports this theory. Basiclly, the cathode coating, where the electrons are fired from, gets damaged or corroded when the vacuum of the tube is not near absolute. So the more deterioration, the fewer electrons that can get emitted. HOWEVER: The deterioration seems not to happen nearly as badly in CRTs installed in constant filament warm circuits. My personal experience has been that if you have a tube that is starting to weaken, you can keep it alive much longer by rewiring the TV to leave 50 or 60% of the filament voltage on when the TV is off. Worked great in a CT-100! Evidence to this part of the theory is that you can take a perfect working Sony color set from the 1970s (with factory-designed constant filament power) for example... The TV works great with a strong CRT for 20 years, then unplug it for a year and the CRT is about DEAD! Happened to many of my customers...
Bottom line: A CRT new in the box, with excellent vacuum, should stay as new forever. But one with poor seals (15GP22, and any metal/glass combination), or any CRT with less than excellent vacuum (Sony from the 80s/90s, Mitsubishi projection from 90s, etc) will deteriorate no matter what, but can be kept alive longer if the filaments are warm all the time.
Charles
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