KT-150 Tubes- Push Farther or Replace?

Not holding bias is a clue that those tubes are done. Time to replace. 

I agree with @russ69; I had to recently replace 1 Kt150 in my Ref 75SE because it would not hold bias after 400 hours of use.

@welcher - that is really unfortunate and a little suspicious. I haven’t heard a Ref 75SE with KT-150’s, how has the presentation changed for you since moving to that tube?
 

@decooney- Thank you for the suggestions sent. After noting the previous bias levels I moved the tube in question two times and the problem followed the tube. I guess it’s just a bad apple! What’s interesting is that although all of the tubes had factory measurements written at the base this problem tube was the only one w/o a slot number noted on it. I moved it to where it shares the bias adjustment with the next weakest tube and was able to get that pair to within range. If this holds up I will continue their use to see how far they can go before becoming unbearable.

With all that is going on now with availability I didn’t want to jump to a full changeout but as @russ69 noted I don’t think it will be too much longer.

Thanks Again!

Sounds like you've got it figured out.  A while back I watched a tour of the factory and how they batch those groupings of matched kt150 tubes for your amp.  I recall thinking I'd likely not get too hung up on keeping matched quads or octets if I owned that amp, and maybe just keeping a few matched pairs around and replace those pairs when needed.  Hopefully you can replace them in some kind of way where you're not replacing all of them at once.  Good luck with the bias checking and longevity! 

"The current situation is lower quality bass response and weaker soundstage. Any thoughts would be appreciated."

that’s it, new tubes!

I’ll add my advice, get your own simple tube tester, to test existing tubes; new tubes you buy; and any time you need ’it’s not the tubes’ confidence answer when something else is ’off’.