OK to replace one power tube for amp?

You can do it but sound wise one will be very much burned in and starting to die while the other has a brand new sound signature. You can do, it but I would replace both.

Thanks for the responses.

It's wandering 10-15% every day.

With only two output tubes I would replace both. In fact if the tubes have a lot of hours I would replace tubes in both amps if you want the best sound. Tubes wear slowly and we adjust to the sound the same way we adjust to our car when the shocks wear out. I think you will be pleasantly surprised at how much better your amps sound with new tubes. If your amps are push pull the weak tube is placing more wear on the strong tube and wearing it out quicker and since the tubes are not holding equal bias the sound quality is compromised and causing the distortion to rise.

Not to hijack the post, but given Rrog's observation that we adjust to sound as tubes wear, how do you decide when to replace power tubes?

Exactly my question. My amp is a Joule Electra Stargate SET and after just talking with their service guy it looks like the bias problem may be related to the electronics, not the 6C33s. When I get the problem fixed, how do I know whether the 6C33s are good still? The amps sound great but they are probably 13 plus years old and I think the tubes are the originals.