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.


Got it!  I remember reading many posts some time ago where people were wondering whether that tube was compatible with amps designed for use with KT120’s. I was not aware that ARC had made the change. Thanks for the clarification on that- I bet it sounds great.

@elliottbnewcombjr - That sounds like good advice. Are there specific units you've had experience with for home use? BTY- the tubes I have are wasted! Stubborn me! I thought I'd try to see how far they could go but it's to the point where they won't hold adjustments made for even a day. A new set is going in tonight.

There is NO chance I would try to wring more hours out of the pair not holding bias. When it comes to big power tubes, you want to avoid testing your amp's safeguards when at all possible. 

@mulveling- I hear you and thanks for the warning. My curiosity was driving my actions on this one- I had to know how long they would last. This confirmed that the recommended usage time of 2K hrs was accurate and from what I’ve learned here I’ll probably replace them at 1700-1800 hrs going forward. Anyway, I replaced the entire set earlier this evening and it’s all good! 



I use an Accurate Model 157, this is slightly larger 257.


Don't let the rust on the hinges scare you, they all corrode/rust. 

here's the one I use


You need to know it can test your tube types. I got a 257 for a friend, kept the 157 for myself, so we have both books.

If interested, give me a list of your tube types and I will check for you.