KT-150 Tubes- Push Farther or Replace?

Hello All-

Seeking advice from others that may have been in this spot before. I’m currently running my trusty Audio Research GS-150 power amp and I’ve just crossed 2200 hours on this set of tubes. I have a new octet of KT-150’s waiting in the wings.

Within the last 200-300 listening hours I noticed that the bias was requiring adjustment after every 3-4 listening sessions. This seems to be driven by two of the tubes continually falling low in bias adjustment.

As the tube bias is slaved in pairs would/could it be beneficial to isolate the two tubes that keep falling low onto the same pair of sockets with the hope that bias can be more evenly maintained or does it just sound like it’s time for these to GO?

The amplifiers manual states that replacement should be around 2K hrs but I’ve read posts here stating 3K might be achievable. The current situation is lower quality bass response and weaker soundstage. Any thoughts would be appreciated.


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’.

@designsfx The Ref 75SE comes with KT150's. I have not tried any other tube types.

@welcher- for some reason I thought the 75’s used KT-88 or 120’s, maybe those were the older models? (Or I may be just completely off base!)

@designsfx The Ref 75 came with KT120's while the Ref 75SE came with KT 150s