OK to replace one power tube for amp?

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.