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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.

designsfx

Showing 3 responses by mulveling

mulveling's avatar
mulveling
1,774 posts
 

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's avatar
mulveling
1,774 posts
 

Those old emission tube testers aren't going to tell you much about how your KT150 (or really any tube) are doing. A MaxiMatcher and MaxiPreamp can be pretty useful tools ($830 + $990), but if you only have a quad of KT150 and 6H30 to worry about (the latter typically being very reliable), it's more cost effective to just keep track of the hours (check bias periodically) and replace when prescribed. 

mulveling's avatar
mulveling
1,774 posts
 

@elliottbnewcombjr

We’re saying it’s not good data from those testers. OP sees a pair of KT tubes not holding bias in his amp, and knows the hours are in excess of 2000 - and that alone is far better quality data than an emissions tester will tell you. It’s not adding any information, and in fact might make one draw the wrong conclusions (i.e. that a bad tube is still good).

If you buy a grab bag of old unknown tubes and want to quickly screen out the dead-dead ones - then that’s maybe a use case for these testers.

A tester is not going to predict an oncoming short. Nothing can. Staying within tube usage guidelines and being an attentive user (check bias occasionally) is how you reduce the chances. If it happens, you can only hope the amp's protection circuitry kicks in in time to prevent board damage. 

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