If you leave the unit on all the time, like I do with my Prima mk2, those tubes won't last more than 2 years. I just replaced my Prima's myself last month after 17,000 hours and the improvement that new tubes brought was shocking. So I suggest you do it at least that often if left on 24/7.
The manual says do it every 10,000 hours but normally 6021 tubes last much longer. They were designed for circuits subjected to exteme g forces, such as missile guidance systems, and to have likewise extreme reliability. I have read that some can last 100,000 hours. But it seems AA makes them run very hard because my old tubes had obvious signs of space charge degradation (leaves a sooty film on the inside). So 17,000 hours was too much already IMO. It may be worse in the Capitole, not sure.
Also, the Sylvanias are not made the same as the Philips. Perhaps part of my improvement was due to the change in brands. The stock Philips have gray open plates whereas the Sylvanias have boxed black plates. So the are NOT the same construction which means they could well sound different. This has been my general experience with tubes in any case.
Replacing these tubes is easy if you are a patient person and have access to a high-quality soldering iron. Having the right tool is everything so don't attempt this with a Radio Shack special. Let me know if you want detailed directions.
Arthur
