Sadly, when you have a tube amp you have to take the bad with the good. FWIW the only tube that I've had fail (knock on wood) in my UFO/UFO25s is a high dollar Mullard GZ34 that should have outlived me. But that's the thing with tubes, you just never know.

Russian tubes in general, unless you have a large batch to select from and test and use only the best testing, are sort of a surprise. Surprise if you get 2,000 or more hours out of them, or a surprisingly bad result when you don't want one.

I've had the best luck with those that come from Steve either with the amp or off his tube for sale page. That said I have about twenty plus pairs that I collected over the years gasthering dust, and only one amp that uses that tube! So if a tube starts to age, I replace it at the first pop or fizzy noise and every three or four months I'll compare a tube in use to one that has just a few hundred hours on and see if the sounds are in the same. ballpark. I find that there's a period of time that the tube starts to sound suboptimal before it really gets bad and fails. I tend to get a year to 16 months or so out of a pair.