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.


Have you tried swapping out/in two other kt150 tubes from other sockets and compared?

If the manual states 2000hrs, that could be averaged out over a series of tubes they’ve replaced and tested over time. Some +/- hundreds of hours difference for example. I’d try to figure out if its a condition isolated to those two tube sockets or just those two tubes first. Go from there. Or call them if you have not done so already. They may share other helpful self-tests you can do at home.

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’.

@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

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.


this Mercury looks like a variation of the Accurate 157. Perhaps Accurate made it for Mercury, who knows

Seller says it works and takes Paypal, so if it doesn’t work, you have Paypal protection.



same thing, a Jackson, ask seller if your tubes are in the chart.


I don't know about B&Ks, but he shows an old tube and the unit registering tube strength and shorts, also takes PayPal, but it probably doesn't 't qualify as he doesn't 't say 'works' just shows it does seem to.


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. 

A small inexpensive tester is still good to have. Better than none. I think, if you have tubes, it is ridiculous not to have a simple inexpensive one. You leave yourself in the dark.

They test for shorts and tell you, bad, weak, strong, and you can check new matched sets you buy; old ones people give you; stuff you get at garage sales; your existing ones.

Problem shows up, tubes test good, you know to look and find other problems, a bad connection, bad ....

I check all of mine yearly before thanksgiving.

I have a big fancy heavy Hickock or Jackson downstairs. The little 157 and the big one always agreed about shorts and tube strength, so I use the smaller/portable one.

Way back when, when I inherited my Uncle’s Fisher President II: Master Control Panel; FM Tuner; AM Tuner; Viking Tape Deck; Garard Changer; Pair of Mono Amps, Preamps added to the Turntable for MM; Multipath added for Stereo FM: I used to stand in line at the electronic stores until my feet and back hurt, people lined up behind me. When I got my 157, I checked them all at the store, then compared the 157. It agreed with the store’s monster.

Just the basics is confirmation they are not shot, or, they are shot.

My Cayin has 6sl7s and 6sn7s. Same size/look. I switched an L for an N by error, fine for a while: lost a channel, nothing obvious, found the blown tube right away. Without a tester, I would not have found my mistake.


The only thing those emissions testers are good for is maybe telling you if a tube is shorted or dead, the rest of those readings on those types of testers are very suspect.


I'm saying, let go of perfection, go for basic knowledge, I think most people will NOT spend for a big/better one.

IF it just finds shorts, not knowing and continuing using them can be quite harmful to your transformer, amp, 

If they appear to work, like an inexpensive sound meter: they do not need to be accurately calibrated. They will be relatively accurate regarding strength as well as find shorts.

If meter shows bad or weak, even if off a bit, relace them, and test the new ones. the results tell you about your tester, and if they are ’matched’ the meter, if not calibrated, will still show you if they are closely matched.

Better than being helpless, in the dark.


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. 

The problem with some of the cheap emissions testers is they only run the tube at 30 volts, some of the better ones maybe up to 150 volts, the kt150 needs to be tested at a higher voltage. The best way to test them is in circuit. 

Agree with @invalid, some circuits are running up to 600V plate voltage at about 80 - 85 mA current per tube. I ran my KT150s in the 50ma range per tube and they still sound great. Some of the diy folks were talking about this a while back, getting up to 4,000 hours without issue, yet noting some of the older tubes had getters fading faster than more recent production past few years. Best to follow the rec above "best way to test is in the circuit". Keep a close eye on this 2nd set for a while and check how quickly they stabilize at their first bias set point. Best of luck.

After a little reading and putting some more thought into this I’m realizing the reality of this (at least my reality!). As @mulveling first suggested- my system uses 4 different makes of tubes. A gang of 8 KT150 and 11 6H30’s. My DAC uses a couple of the smaller 12AT7’s (which seem to go on forever). With this commonality across the equipment I don’t really see myself critiquing individual tubes as opposed to wholesale changeouts when approaching the recommended end of life hours.

It also seems (pointed out by @mulveling ​​​​@invalid) that a tester really isn’t going to buy me much when it comes to the KT150 in particular due to the voltages required for an accurate test/comparison. The test stations equipped to deal with these are of considerable expense (more than a new set of tubes) and in reality I would just pull and replace.

I liked the thought of having one though- kind of nostalgic but (at least in my case) it doesn’t seem like I would put one to much use. As always- I appreciate the advise given!


You advice is very good, IF people will spend the money for ’real’ testers. I read very little about member’s tube tester results here. My friends don’t have them, they rely on me.

Can you give a short list of testers, perhaps a few to look for, and pick a current eBay listing or two of better ones.

Some newer tube amps have visible and external bias adjusters, many do not, sadly my early Cayin A88T adjusters are internal. Again, I suspect only those with external adjusters use them.

I got the info to adjust mine internally, including advice here, then chickened out and took it to Steve at VAS. When I was younger, I’d have been in there with metal! Joking, I would have left it unplugged a few days, left it unplugged, deoxit, twist back and forth, and left them physically mid-point (i.e. no meter, just guess): ’good enough, close it up!


Hello! I can't speak to which testers would be considered real or better than others- I just did some quick reading on the products suggested by others. Those were the Maxipreamp, Amplitrex and Space-Tech Labs. It seems there are many out there in the used market that ranged from $750 on up and new units going upwards of $1.5K (or more). I have no past experience with these as my amplifiers bias adjustment is performed while reading the meters on front of the amp. 

BIAS Meter.

OK, paraphrasing someone else:

a. setting bias is like setting the idle to a car's engine.

b. It's about when they are doing nothing, ready, but not about when they are busy amplifying, correct?

Bias effects tubes at their idle stage. You run them within a range, cool to hot, yielding more or less life. You can see/feel if they are running hot.

Bias can effect the output sound to a degree.

If amp allows use of different tube types, recommended to adjust bias  


Questions (what does bias tell us about tube strength/matching strength?)

1. using bias meter, adjusting 'idle' of each tube, matching their bias:

does it indicate tube strength?

tube strength matching?

2. not holding bias is definitely a bad tube?

Can you find a 'strong, ok, weak, bad' tube strength via Bias?


What I'm driving at is: my little tube tester has always told me about tube strength, and relative to each other strength, found any weak or bad tube, found any short, since my wife surprised me with it as a birthday present back in 1972, 50 years of answers that were better than flying blind.