Marshall JCM2000 stable bias mod

[Randall]

I installed the stable bias mod from Dr. Tube in a JCM2-60-00 equipped DSL Marshall that had the dreaded run away bias issue. Fortunately it didn't burn up, although it did burn out the tubes. The install was successful, and I separated pins 5 from the board with a diamond hollow tip drill bit. All good. I got a fresh matched quad of JJ's, and the bias is now stable. Problem is, with the bias pots at their extremes, the lowest I can get is around 18/19 watts per side, and that is too hot for me. I have inquired via email as to if there is something I can change on his board to get me more negative bias voltage, but who knows if or when he will respond.

Can anyone suggest something to adjust on the main board that might get me there? I hate these Marshall schematics.

https://www.thetubestore.com/lib/the...-Schematic.pdf

[Jon Snell]

If I had sight of the schematic for the bias, I could advise but it suffices to say, decrease the feed resistor value and increase the load resistor value to increase the -ve bias voltage.
The bias voltage must vary with the HT voltage otherwise the output stage will not perform properly.

[g1]

(edit: following component numbering is for board revision of schematic in post #1)
Like Jon said, decrease value of R71 (feed), or increase R60 (load), or do both.

But you also want to be sure the issue is not due to a circuit problem. C36 (&41) to the right of OT on schem., C29,30, &35 in bias circuit shown left of power tubes.

Bias pot board is shown on pg.5 of schem.

[Randall]

g1, that makes sense, however the big problem here is Dr. Tube's mod involves adding a small satellite board and moving 8 resistors and two caps to this new board. Since he didn't include a schematic, I don't really know which ones have been moved.

[g1]

problem here is Dr. Tube's mod involves adding a small satellite board and moving 8 resistors and two caps to this new board. Since he didn't include a schematic, I don't really know which ones have been moved.

Can you list the values of the 2 electrolytics and 8 resistors?

[g1]

Never mind, I found a picture.
Refer to post #3, all is valid except you can ignore C29 and C30.

[g1]

Now I see they have changed the component names in the later revision.
Does your board match the schematic you posted? Or more like this one attached.

[SoulFetish]

g1, that makes sense, however the big problem here is Dr. Tube's mod involves adding a small satellite board and moving 8 resistors and two caps to this new board. Since he didn't include a schematic, I don't really know which ones have been moved.

So, this is an industry wide problem when it comes to installing mod kits, and annoys the sh*t out of me. Every mod kit that I've ever installed for a customer, regardless of vendor, has never come with the accompanying schematic. Without exception. They've only come with "instructions".
This is by far the most inefficient method of installation for anyone with some technical literacy, and can really hamstring a technician if there needs to be any troubleshooting. It needlessly takes up valuable time to trace a circuit, when the simple solution is cheap and available: VENDORS–PROVIDE A SCHEMATIC OF YOUR MODIFICATION

Sorry, I'm done with my rant. I just got a little fired up after reading that this morning. What a way to start the day . Cheers!

[Mick Bailey]

The last time I modified one of these boards it worked fine and no bias issues. The reason for doing this was at the time Marshall was out of stock of the boards and the customer needed the amp for gigging, so against my better judgement I 'fixed' it. The amp came back to me 2 years later with what I thought was an unrelated problem; after about 20 minutes the volume would drop and the sound distorted really badly, plus there was a lot of crackling. The problem turned out to be HT voltage leaking around the preamp tubes and the only fix was to replace the board. The power section was still good with no further problem there.

[Jazz P Bass]

Yeah.
I had one that I did and after being on for about an hour the output starting humming.
It was not more than 90 mv's but still.
Annoying.
Bandaid is all it is.
You can't shine sh*t.

[Randall]

Sorry for the pause. Everything is taking so long to ship these days, at least where I am. It was recommended by Dr. Tube to change C43 and C44 (22uF/50v) (lower right), and since I was in there C42 47uF/63v (left of the bottom EL34). I did this and there is no change. He also sent me this schematic, and even though my board is a JCM2-60-00, this new one (JCM2-60-02) seems to be more accurate. I will never understand the Marshall numbering system.

So since I need to find out from the vendor exactly what he has moved to the new satellite board, could someone point out which are the bias feed and load resistors on this new drawing, so that I may communicate clearly with Dr. Tube? I am still at 19 watts at idle with the pots cranked to full.

JCM2-60-02 (2001) iss7.pdf

[nickb]

R77 (10K) is the feed. The loads are the two 22K trimmers on the rear bias trim PCB. Try a 10K in parallel with R77 as a starting point. It might be enough to get you to where you want to be.

[g1]

The loads are the two 22K trimmers on the rear bias trim PCB.

& R68 as the series 'safety' resistor?

Schematic of rear adjust trimmer board attached.

[Randall]

I Have asked the question. I trust Dr.Tube will respond, he has been good with this. I will report.Customer is getting antsy.

[g1]

I am still at 19 watts at idle with the pots cranked to full.

Sorry if I missed it Randall, but have you verified that the 1R cathode sense resistors (R6 & R9) are still exactly 1 ohm?