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They are like most other electronic parts. They might last a few years or a few decades. When they fail, your nose will know it!
For the most part, I think Zenith (as well as most others) went to silicon by the mid '60's. I do have a cheap low end Zenith table AM/FM radio from '67 that had a selenium rectifier. The chassis is an older design; but, the tubes have '67 date codes.
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