OK to replace one power tube for amp?

Should not be a problem since they are individually biased. One question; when you say bias wanders a bit, how much are we talking about?

I have changed one 6C33C on my Ayon Spark, no problem. That is again, as Swampwalker says, because I can bias the tubes myself, not true with fixed bias of course

Short answer, no problem. If the tubes are getting close to their life expectancy, then you may consider changing out both tubes for improved sound performance.

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.

How do I know whether the 6C33s are good? Well, if they are 13 years old, I think that they don't owe you anything.

Well put :)

Most manufacturers estimate tube life in their equipment.

Tubes age just like people. As they get older, they slow down and are less dynamic; they don’t have the same sparkle and don’t have the same frequency extremes. They are just like an aging athlete; they can still play the game, put not at the same level.

The best way to determine if you need new tubes is to buy new tubes and compare for yourself. If there is no sound difference, put the new tubes away until that day comes.

Brf, Speak for yourself :)

Good advice on buying new tubes and compare. This is my phylosophy except I leave the new tubes in and keep the old tubes as spares.

I will argue that you should replace both tubes. The ability to hold bias is only one aspect of tube engineering. The tubes' tone also changes with age and this ought to be considered. I would and have replaced whole quads of EL-34s for thid reason in part. So take the sound quality into consideration a tube is not just a power maker.