It's ironic that some state that "rotating the tubes" is a waste of time, when Eimac stated (although they haven't made a 3-500Z in a very long time) that any tube older than one year is suspect -- thus implying you really need to use the tube within a year of its manufacture date. So, if someone bought a "spare tube" and let it just sit on the shelf unused, Eimac's policy was after a year the warranty would expire and if the tube doesn't work you're out of luck. Solution? Use the tube. When you have a spare, the only way to assure the spare works is to use it; and the only way to assure the "other" tube works is to use that one, also. In a single-tube amp you can't use them both at the same time, so cycling the tubes through the amp every year or so is the obvious solution. I agree some tubes seemingly last forever in storage or light use. But the 3-500Z is one commonly used tube for ham amplifiers that is of different design and construction than most of those "last forever" tubes. I've never had a shelf life issue with external anode ceramic tubes, and never had a shelf life issue with 4-400s, 4-1000s etc. either, but those are of very different design.