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.
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.
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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!
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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.
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