Marshall JCM2000 DSL bias too hot.

[Redline]

Bias is too hot on my DSL. One side is reading around 40ma per tube(fine)but the other side is at 75ma per tube. Pots all turned down and I checked all associated resistors. Is there anything else I should check. This one does not have the bias drift problem.

[g1]

Swap the tubes and see if the problem follows the tube or stays with that side.
Could just be a tube issue.

[Redline]

Swap the tubes and see if the problem follows the tube or stays with that side.
Could just be a tube issue.

Tried that. It follows the tubes. I put brand new quad in and they were also too hot. Again pots turned all the way down.

[Pedro Vecino]

It is unusual because the DSL100 have much room for bias adjustment. At least in my experience, including tubes with very distant degrees. However you can increase the value of R68 (33K) to 47K. This is the common ground reference resistor in the bias circuit.

[ErikStafford]

It is unusual because the DSL100 have much room for bias adjustment. At least in my experience, including tubes with very distant degrees. However you can increase the value of R68 (33K) to 47K. This is the common ground reference resistor in the bias circuit.

Thanks.

[Redline]

It is unusual because the DSL100 have much room for bias adjustment. At least in my experience, including tubes with very distant degrees. However you can increase the value of R68 (33K) to 47K. This is the common ground reference resistor in the bias circuit.

Didn't work. Still too hot

[g1]

Have you checked for ripple on the bias filter caps? Measure AC voltage at the power tube grids with no signal applied.
Also you could try disconnecting the coupling caps that go from the power tube grids back to the phase splitter.

[Jazz P Bass]

Please define 'too hot'.

You can check the general matchyness of the tubes by biasing one tube at a time & making a note of the current draw.

[nevetslab]

Following the suggestions above with altering the voltage tuning range by adjusting R60 (33k) to 47k, what we haven't heard is.....do the two bias pots on the little bias board vary the bias at all. I know you have the pots turned all the way down. I've had issues with that bias board, regardless of what generation, where the pot terminals or the PCB header on that little board OR the mating main PCB have solder fractures. It that's the case, you no longer have control of the power tubes.

The fact that your 'hot tubes' follow the tubes, instead of remaining on one side points away from that scenario. But, when putting in a fresh set of tubes and you're still having the same issue, it is suggesting component problems. Any leakage in coupling caps C22 or C23 will affect the negative bias by leaking in positive plate voltage from the PI stage.

I know it's a total pain to get at that board, as you're dealing with the forest of jumper cables from that main board to all the other boards, removing the rear panel board, all the preamp tubes, screws pan securing the power tube sockets, etc....before you can even get to being able to twist up the main board, and being able to remove the little bias board. But, that's the drill. I normally correct the solder fractures on the bias board, as well as seep in some Caig DeOxit & exercise the two bias pots, instead of replacing the board. Also, I find in the active rental inventory of all our Marshall JCM 2000 and their variants like your amp, there are often solder fractures on the power tube sockets. And, not related to your bias problems....fractures all across your rear panel PCB, that you'd have to remove to get at the main & bias board in the process. After getting down to that level with the corrective procedures, it restores any of the weirdness in biasing that was there before.

[The Dude]

Agreed! Check that bias PCB. I've seen my share of issues with that little board- cracked solder, broken traces, etc. Much of the time, it's from people trying to adjust bias with tools that are not suited for the job. They put too much pressure on the pots and break them loose from the board.

[Redline]

Have you checked for ripple on the bias filter caps? Measure AC voltage at the power tube grids with no signal applied.
Also you could try disconnecting the coupling caps that go from the power tube grids back to the phase splitter.

Which ones are the bias filter caps? Sorry I'm I noob.

[g1]

There may be more than 1 version, but the schematic I have for a JCM2000 DSL100 shows the bias filter caps to be C29, C30, and C35, all 47uF 63V. Note that they all have positive end grounded.

[nevetslab]

On the PCB & schematic I have here, bias filter caps are C42, C36 & C37, and can be seen in the area just to the left of the main HT filter caps (tallest ones). Follow the pink wire down to the board in the picture. up in that region. Then, R77, R67, R68 & R69 are in the region where the 7 conductor cable leaves the board to feed the bias board.