Maintenance for Tube Preamps

Other than periodically replacing the tubes (and they generally last longer than most people think), I would just occasionally treat all of the contacts with Caig Deoxit or Cramolin. You don't need much of it at all, but I think it's a good idea. For a quick treatment, just spray a tiny amount of deoxit into the end of your interconnects and then plug it in and rotate the end to spread the love.

I'd also advise testing your tubes before replacing them. Don't replace based upon some arbitrary limit. If something starts to sound wrong or you blow a fuse, test all the tubes. You can probably find an old Eico dynamic tube tester on eBay for about $200 or less.

The only other thing I could think of is just opening it up and making sure to keep it clean/dust free inside.

By the way, I mentioned Cramolin but haven't used it for at least a decade. Cramolin violates some EPA regulations and was not rated as safe for plastics. Cramolin is no longer readily available, and I only mentioned it in case you happen to have some lying around. The Caig Deoxit Gold 5% solution/Pro Gold are much better. Interestingly, you cannot legally ship Deoxit/Pro Gold through the air - it must travel by ground. And if you order more than one can, it must travel in a separate package. I think this only applies to the aerosol version.

Anyway, the point is if you already have a can of Cramolin lying around, go ahead and use it, but sparingly, wiping off any excess with a lint-free cloth. Since you probably don't have it, order a can of Deoxit Gold 5%. You can find it any number of places online if it's not carried in your area. Musician's Friend had a good deal on it recently, and you can definitely find it on Amazon.