1998 JCM 2000 DSL 50 - One power tube won't bias

velvet_man

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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:

IMG_20221010_152344.jpg

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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FleshOnGear

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I owned a JCM2000 a long time ago, and once one of the 1 ohm current sensing resistors burned out when a tube went bad. That might be a possibility.
 

Ricky Toney

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I agree with fleshongear.I had the same problem with mine put in the bias patch kit from doctor tube and have not had any problem since. The bias pot could of crapped out on that side
 

Pete Farrington

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one of the 1 ohm current sensing resistors burned out when a tube went bad
Normally though, resistors go high resistance / open circuit when they fail due to overcurrent, causing the voltage across them to go higher than normal. Whereas in this case, the voltage across the current sensing resistor is lower than normal.
 

velvet_man

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One likelihood is that its screen grid resistor is blown, open circuit.
No screen grid voltage, no anode current.

I owned a JCM2000 a long time ago, and once one of the 1 ohm current sensing resistors burned out when a tube went bad. That might be a possibility.

Thanks for all of the replies and suggestions. The only problem is, I'm a complete noob with amps, so I don't know which parts the screen grid resistor or current sensing resistors are. Could you explain which parts I'm looking for and how to test whether or not they're working (with a multimeter I assume)?

Here's a pic of the board with the mod installed. Note, I used some isopropyl alcohol and a toothbrush to clean those solder joints on the power tubes, so it looks much better now than it did when I snapped this pic.

IMG_20221013_174645.jpg

EDIT:

Just in case it's needed, here's a pic under the mod kit before it was installed but after I removed the components the guy at Dr. Tube told me to remove:

IMG_20221013_171421.jpg
 
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Pete Farrington

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Adrian R

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This might be a stupid question, but did you try replacing tubes?

Incidentally, the bias issues were not known to exist with the 50w models, only the 100watters.
 
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velvet_man

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The screen grid resistors will be the white ceramic blocks labelled 5W1KJ, R65 and R74 on the schematic.
The cathode current sensing resistors are R6 and R9.
I have no idea whether your amp matches the schematic below that I’m referring to.

Thanks for getting back to me. I think I should be fine to find those now, but how do I test them? And can I test them with everything turned off? I know some things need to be tested with the amp running, but as someone very new to this, I'm not sure how comfortable I am with that at this point.

This might be a stupid question, but did you try replacing tubes?

Incidentally, the bias issues were not known to exist with the 50w models, only the 100watters.

That's the first I've heard of that. I've read about lots of other people having bias drift issues with the DSL 50, and the Dr. Tube kit I used to fix it even came with DSL 50-specific instructions. The problem was with the PCB, which was the same for both amps. My board is the JCM2-60-00 board that is listed as the problem board in pretty much every documentation of the issue I've come across.

At any rate, I did replace the tubes, but that one power tube is still a problem, which leads me to think it's something to do with the socket or board and not the tube itself.
 

Dogs of Doom

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all the boards are JCM2-60-00 pretty much. They named them that, but, then also added an ISS (issue) #

There's 21 different issues of the JCM2-60-00 boards...
 

velvet_man

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Yeah, I know that much. Like I said, I've read a ton about this issue in trying to get this sorted out.

The article on the issue from Dr. Tube (I'd link it but it won't let me post links), and many other sources, specifically say the issue affects the DSL 50 and 100 as well as the TSL 100 and 122. Basically, if it has a board labelled JCM2-60-00 or JCM2-60-02 (any variant of them), it'll have the issue. They even say in the article that the most recent versions of the board that Marshall sells as a fix for the bias drift could still display the issue since the problem is with the layout of the board itself.

And I know mine had the issue because I measured it myself and saw the bias drift every time I played for a few hours. All of this is beside the point. I know the issue was there, I know it caused some damage to the amp, I just need to know how to fix it. Any help with that would be greatly appreciated.
 

Dogs of Doom

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to answer your question to Pete, yes, turn the amp off & test the resistors (R6 & R9) for ohms...

Those tend to fail a lot, & cause a lot of biasing issues. Also, the bias trim pot's fail & also make it unbiasable. They can also cause intermittent drift...
 

velvet_man

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Thank you. Taking it all apart, again, and measuring those resistors will be my job for today then.

Also, the bias trim pot's fail & also make it unbiasable.

I had wondered if that one trim pot could be the issue, but I thought since I can hear the background static noise of the amp increase and decrease as I turn the pot from the left to right it must mean that the pot is working, but maybe that was a flawed assumption. Is there a way to test the trim pot?
 


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