Re: aspen pittman tube amp book
As a guy that has changed a LOT of caps over the years...I'd have to say it depends on a lot of factors.
Generally speaking, the lower the rating temperature, the longer the cap will last. I have personal equipment with low temp metal cans that are over 50 years old that function perfectly. I still get the occasional late 60's amp with original caps that don't need replacing. A little relaxatiion in value can really sound good but it depends on the circuit it filters.
Newer caps are less predictable and they tend to fail before their value reduces. I used to replace the Mallory caps in old Fender amps at 30 years and LCR caps in old Marshall amps at about 20 years...today's caps don't have the same indicators of imminent failure like the older ones, at least as I've found.
All in all, I really don't see a lot of cap failures but replacing every 10 years is way excessive.