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The determining factor is the cathode coating, and whether it looses its ability to emit electrons over time.
I was shown a curve during my 2 week 'retreat' at RACS, which described the useful life of a cathode. It basically showed that an electron gun can be expected to have many years of useful life, provided that quality control was carried out correctly. If a tube was 'over processed' during QC, the cathode would already have experienced an undue amount of use and would already be on the 'downward slope' of it's electron emitting capability. On the other hand if a tube were 'under processed' (excluding any attempt to age the tube), the cathode would never get to it's full potential and would have to be rejected for reprocessing in the plant.
The only thing that has changed over the years is the equipment used to carry out QC on a CRT; the more accurate the testing equipment and methods used, the better you can expect the product to be. In that respect, I would think that a modern CRT could last longer than one built in the 'hit or miss' 40's or 50's. Then again people really knew what they were doing back then, so occasionally you see a used tube still perform quite well despite it's age.
With modern equipment comes modern repeatability, and in manufacturing consistency is key. The more uncontrolled variables, the high the probability of failure becomes. They didn't have computers back then...
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Evolution...
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