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?
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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.
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.
Tubes age just like people. As they get older, they slow down and are less dynamic; they dont have the same sparkle and dont 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.
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.