velvet_man
New Member
- Joined
- Nov 7, 2016
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Hi everyone, sorry in advance for the long post.
I bought a JCM 2000 made in '98 without knowing any of the issues they're infamous for. I used it for weekly band rehearsals for a few months before I came across the bias drift issue on this forum. So I took my multimeter to rehearsal and reset the bias before each session. It never got really out of hand. It only seemed to increase by 2-5 mV each session.
I tried to get my amp into a local tech, but they were so backed up that I'd have to leave it with them for 4-6 weeks, and then I wouldn't be able to rehearse, so that was out of the question. So I kept using it and rebiasing, but then my multimeter broke, and I figured it only drifted a few mV each time, so I should be OK for a while. But I think I let it go too long. Eventually, the left-hand power tube (the one farther from the main) went dim and I started losing sound. I bought another multimeter and tried to set the bias, but that tube wouldn't bias at all. No matter how I turned the trim pot, it only read 0.1-0.6 mV. The other tube would bias, but it fluctuated wildly.
I purchased a bias drift fix kit from Dr. Tube. After watching a bunch of videos and reading a bunch of articles about the install, I finally went ahead and did it. I've never worked on amps before, but I've wired up lots of guitars and things, so I figured it should be pretty straightforward.
However, when I opened the amp up I realized someone had already modded it. I should note that when I bought the amp, I was told it was a back-up touring amp for a local band that got big for a minute back in the early 2000s, so it looks like his amp tech tried to fix the issue already (and changed out the transformers and did some mod to the effects loop, too).
Here's a pic of what the modded part of the board looked like:
I sent the picture to Dr. Tube, and this is what they told me: "Ah, yes, that's the common (attempted) fix for the bias drift problem which only partially fixes half of the problem..." He then gave me the following instructions for how to proceed with installing the Dr. Tube kit:
"Remove R66 & R70 (and R7 & R10) (5k6 in plastic sleeve).
Remove R67 & R69 (the big fat white 150k resistors).
Remove C36 & C37 (the two blue electrolytics)
and install the SB kit.
R68 (33k) MUST stay on the board!"
So I did all that, desoldered and cleaned up the connections to the power tube pins, and installed the Dr. Tube Kit.
I put everything back together and turned it on, and it seemed to work. It came on, the tube all lit up and got hot (even the pre-amp tubes), and I get a good sound out of the speakers. The right-side power tube biased perfectly and was rock steady, no fluctuating at all. So the new bias kit seems to be working.
BUT, that left-hand tube will still not bias. I fiddled with the trim pot, but could not get a reading higher than 0.9 mV. If it matters, I hear the background static noise through the speaker increase and decrease as I turn the trim pot but the voltage isn't changing.
I thought maybe the tube had died, so I purchased a new matched set of JJs and put them in, but still get the same thing. Then I thought it could be a fuse, so I checked the mains fuse, HT fuse, and all 4 heat fuses for continuity, and they all came out fine and look fine, too.
Any idea what could be causing this?
I bought a JCM 2000 made in '98 without knowing any of the issues they're infamous for. I used it for weekly band rehearsals for a few months before I came across the bias drift issue on this forum. So I took my multimeter to rehearsal and reset the bias before each session. It never got really out of hand. It only seemed to increase by 2-5 mV each session.
I tried to get my amp into a local tech, but they were so backed up that I'd have to leave it with them for 4-6 weeks, and then I wouldn't be able to rehearse, so that was out of the question. So I kept using it and rebiasing, but then my multimeter broke, and I figured it only drifted a few mV each time, so I should be OK for a while. But I think I let it go too long. Eventually, the left-hand power tube (the one farther from the main) went dim and I started losing sound. I bought another multimeter and tried to set the bias, but that tube wouldn't bias at all. No matter how I turned the trim pot, it only read 0.1-0.6 mV. The other tube would bias, but it fluctuated wildly.
I purchased a bias drift fix kit from Dr. Tube. After watching a bunch of videos and reading a bunch of articles about the install, I finally went ahead and did it. I've never worked on amps before, but I've wired up lots of guitars and things, so I figured it should be pretty straightforward.
However, when I opened the amp up I realized someone had already modded it. I should note that when I bought the amp, I was told it was a back-up touring amp for a local band that got big for a minute back in the early 2000s, so it looks like his amp tech tried to fix the issue already (and changed out the transformers and did some mod to the effects loop, too).
Here's a pic of what the modded part of the board looked like:
I sent the picture to Dr. Tube, and this is what they told me: "Ah, yes, that's the common (attempted) fix for the bias drift problem which only partially fixes half of the problem..." He then gave me the following instructions for how to proceed with installing the Dr. Tube kit:
"Remove R66 & R70 (and R7 & R10) (5k6 in plastic sleeve).
Remove R67 & R69 (the big fat white 150k resistors).
Remove C36 & C37 (the two blue electrolytics)
and install the SB kit.
R68 (33k) MUST stay on the board!"
So I did all that, desoldered and cleaned up the connections to the power tube pins, and installed the Dr. Tube Kit.
I put everything back together and turned it on, and it seemed to work. It came on, the tube all lit up and got hot (even the pre-amp tubes), and I get a good sound out of the speakers. The right-side power tube biased perfectly and was rock steady, no fluctuating at all. So the new bias kit seems to be working.
BUT, that left-hand tube will still not bias. I fiddled with the trim pot, but could not get a reading higher than 0.9 mV. If it matters, I hear the background static noise through the speaker increase and decrease as I turn the trim pot but the voltage isn't changing.
I thought maybe the tube had died, so I purchased a new matched set of JJs and put them in, but still get the same thing. Then I thought it could be a fuse, so I checked the mains fuse, HT fuse, and all 4 heat fuses for continuity, and they all came out fine and look fine, too.
Any idea what could be causing this?
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