mopardave Posted April 28, 2019 Share Posted April 28, 2019 Bought this Douk 12w/ch KT88 amp back in January. Set bias to .70 and it was fine til about a month ago the bias would drift up, way up to like 100 til I caught it and reset to .70. Today I turned amp on and it only went to .40 after warm up on right side only with R bias turned all way up wont go any higher. I set L bias to .50 for now. Switched out the KT88's for a different brand and same issue. What could be taking a crap here? Quote Link to comment Share on other sites More sharing options...
mopardave Posted April 28, 2019 Author Share Posted April 28, 2019 12 hours ago, mopardave said: Bought this Douk 12w/ch KT88 amp back in January. Set bias to .70 and it was fine til about a month ago the bias would drift up, way up to like 100 til I caught it and reset to .70. Today I turned amp on and it only went to .40 after warm up on right side only with R bias turned all way up wont go any higher. I set L bias to .50 for now. Switched out the KT88's for a different brand and same issue. What could be taking a crap here? I turned the amp on this morning and everything is back to normal. Drifted way up and had to turn the bias settings way back. I don't get it. I'll pop the bottom plate off today and see if anything is out of place, but there's really nothing I can fix. Quote Link to comment Share on other sites More sharing options...
Schu Posted April 28, 2019 Share Posted April 28, 2019 You should have posted this in the tube section... I'd like to know more about drifting bias also Quote Link to comment Share on other sites More sharing options...
Marvel Posted April 28, 2019 Share Posted April 28, 2019 It could be as simple as a bad solder joint. That can make things at all sorts of weird. Is there a tech who is close to you who could go over it and not charge as much as a new amp? If it were mine I would reflow/resolder all the connections I could get to. Seems like I remember someone else having the same issue with one of these. Bruce Quote Link to comment Share on other sites More sharing options...
mopardave Posted April 28, 2019 Author Share Posted April 28, 2019 1 hour ago, Marvel said: It could be as simple as a bad solder joint. That can make things at all sorts of weird. Is there a tech who is close to you who could go over it and not charge as much as a new amp? If it were mine I would reflow/resolder all the connections I could get to. Seems like I remember someone else having the same issue with one of these. Bruce That's interesting. The amp has been on for 4hrs and bias on L and R side first started drifting up as it got some heat in it, so I reset to .60 and now that the amp has been on for awhile R side bias has fallen to .20 and L is steady at .60. Still sounds sweet thou. Also notice the R power transformer is 127* and the L is 106*. Gonna shut it down in a few and check it out. Yeah I did post in the wrong place. May move it. Quote Link to comment Share on other sites More sharing options...
mopardave Posted April 28, 2019 Author Share Posted April 28, 2019 3 hours ago, mopardave said: That's interesting. The amp has been on for 4hrs and bias on L and R side first started drifting up as it got some heat in it, so I reset to .60 and now that the amp has been on for awhile R side bias has fallen to .20 and L is steady at .60. Still sounds sweet thou. Also notice the R power transformer is 127* and the L is 106*. Gonna shut it down in a few and check it out. Yeah I did post in the wrong place. May move it. Ok, I pulled the bottom cover and took a look inside. Visually I seen nothing out of place. Took some measures with the multimeter and set bias from inside to .55 just to be safe. Now both power transformers are running closer to same temps. left is 94* and R is 92*. Looks like the MA meters are not working properly with left meter reading 110ma and right reading 10ma. Something in the meters path must have took a crap, so can't set bias from outside of it anymore. That was one of the reasons I bought the amp was for easy external bias setting. Quote Link to comment Share on other sites More sharing options...
kevinmi Posted April 28, 2019 Share Posted April 28, 2019 14 minutes ago, mopardave said: Ok, I pulled the bottom cover and took a look inside. Visually I seen nothing out of place. Took some measures with the multimeter and set bias from inside to .55 just to be safe. Now both power transformers are running closer to same temps. left is 94* and R is 92*. Looks like the MA meters are not working properly with left meter reading 110ma and right reading 10ma. Something in the meters path must have took a crap, so can't set bias from outside of it anymore. That was one of the reasons I bought the amp was for easy external bias setting. another thing to look out for is your tube sockets. If they get loose on the pins from swapping out tubes, it can make the bias act funny, and even lead to a tube red plating. Take a tiny screwdriver and snug up the female pin connectors in the socket. Make sure the amp is off and unplugged first. Quote Link to comment Share on other sites More sharing options...
mopardave Posted April 28, 2019 Author Share Posted April 28, 2019 1 hour ago, kevinmi said: another thing to look out for is your tube sockets. If they get loose on the pins from swapping out tubes, it can make the bias act funny, and even lead to a tube red plating. Take a tiny screwdriver and snug up the female pin connectors in the socket. Make sure the amp is off and unplugged first. I'll keep that in mind Kevin, but I haven't rolled manner tubes in this amp yet. Either the meters took a crap or something inline with them did. Both power transformers are running same temps now, but the meters read opposite each other. May see if Craig is interested in workin on it. Maybe get some better caps in it while i'm at it. Quote Link to comment Share on other sites More sharing options...
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