In serious need of help, what's been modded in my -74 JMP MKII?

FretDance

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Hey guys,
new guy in town here, from Sweden.
I recently posted a thread in the main forum,trying
to determine the age of a Marshall amp I acquired.
Anyways, it turned out to be a 1974 JMP MKII 50 Lead (1987 model).

Beautiful amp, with killer potential! However, it has been modded
in the past, probably in 1980, as there is a service tag inside stating this date.

What I'd need your professional help with is determining what mods have been done to it, as I am a total noob as far as amp guts are concerned, have worked as an instrument tech, by never serviced amps to any extent.

First of, it has a master volume mod, so a new knob has been placed where the indicator light was, and the light has been moved to a new hole, drilled over the "P" in JMP. Also, I see some resistors are missing from the PCB-board, and what looks like an additional wire soldered in as well.

What I want to do, is to remove the master volume mod, and instead do the "cap-mod" myself, to use on the first input, but loose the bright cap, have tried several amps with this mod, and I think it sounds brilliant for pedals.
Is it the cap on the Channel 1 volume pot I should de-solder, or is the the resistor in the input jack?

Are there and decent schematics for these amps out there, and I'm talking more like PCB schematics, no the "electrician type" schematic, which I can't read for my life :confused:

I would love to further mod the EQsection of this amp to become more like a Superbass, since I also think these sound superior to the standard Lead when it comes to pedals, you loose that nasty upper mid.

I'm including gutshot below, and sincerely hope someone can help me out here, or at least point me in the direction of someone that might be able to help me :)

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Looks like someone did the "one wire" mod and added a pre-phase inverter master volume. The one wire mod converts the Super Lead's "mixed" front end channels (normal and bright) into a cascaded preamp where the bright channel's output is fed directly into the normal channel's input rather than having the normal and bright be their own separate preamp and "mixing" their outputs at their volume control outputs like the stock circuit.

If you wanna keep that mod you can probably get the front end a bit tighter by dropping the value of that 330uF bypass cap (or 250uF...whichever value they decided to use on that particular one) on the input end of the board to something like a 2uF.

Another thing I find interesting is the 82K grid leak resistors on the power valves at the phase inverter output, yet judging by the bias range resistors it looks as if this one is configured for EL34s?
 
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Here is the stock Super Lead circuit -

SLStock.jpg


You can see that 1/2 of V1 is the Bright channel (Channel I) while the other 1/2 of V1 is the Normal channel (Channel II). They are then "mixed" together via after their respective volume pots via the two 470K resistors into the input of V2.

Here is the SL circuit with the "1 wire" mod -

SL1Wire.jpg


Here they've taken the Bright channel's volume control output along with its mixer resistor and re-routed it directly to the Normal channel's input to "cascade" the stages for more distortion. They've also added the master so that you can run your Volume I and II controls higher than normal at low master settings.

The beauty of this mod is that with the exception of the added master control, this mod is easily reverse able.
 

FretDance

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Thanks for the replies!
I have experienced that it does break up fairly early, and your description of the cascade gain mod, explains that part. I should have mentioned that yes, this is indeed the EL34 model, so that part is also right.

However, I am not qualified to read these charts well enough so that I can understand how to actually reverse the input and master volume mods :(

ANy chance someone could give me a step-by-step instruction on how to do this? And, the so called "bright cap" mod for channel 1, is that then done simply by removing the large cap situated on the volume pot?

Oh, and I wonder, when and if I do make a successful removal of the current mods, will I have to re-bias the amp?

Sorry for all the noob questions :) I guess I should purchase that Weber amp book at some point :hmm:
 
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First you would remove the 470K resistor and blue wire you see on the board. There's an empty spot on the board that says 470K where that resistor came from. Reinstall that 470K resistor there...if the leads aren't long enough just use a different 470K. Also replace the missing 500pF cap that's right next to it on the board.

Take the pink wire from Volume 1 and resolder it where it was originally connected to the board (right next to where you see it coming up through the board from the bottom).

To remove the master, look at the amp with the front panel facing you. Starting from the left end of the board (the input jack end), find the 4th brown wire. Cut it. Then remove the wire that is currently on the center leg of the treble pot and solder the brown wire you just cut to that terminal.
 

FretDance

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Wilder, I really appreciate you taking time to help me out here!
Unfortunately I don't have a suitable 500pF cap atm, but the amp should still play right?
I must also say, that thanks to your help, this turned out to be much less of a nightmare than I anticipated :fingersx:

Might I also ask you the last two questions I had?
1.) Will I have to re-bias the amp now that the master-mod is gone?
2.) Will removing the cap situated on the Volume 1 lug do the "bright-cap mod" I talked about, that sounded really good on other amps I've tried.
 
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Wilder, I really appreciate you taking time to help me out here!
Unfortunately I don't have a suitable 500pF cap atm, but the amp should still play right?

Yeah the 500pF is just a "treble peaker" that peaks treble in the 2-4kHz range...you'll just have limited brightness on that channel until you put in on there. Once you get the resistor in, if you want to wait till later to install that cap you can just do it from the top of the board via soldering its leads onto the exposed leads of the 470K that it normally would install next to.

FretDance said:
I must also say, that thanks to your help, this turned out to be much less of a nightmare than I anticipated :fingersx:

Anytime! Always good when it turns out to be a pretty minor deal.

FretDance said:
Might I also ask you the last two questions I had?
1.) Will I have to re-bias the amp now that the master-mod is gone?

No you shouldn't have to since this is all in the preamp section but always a good idea to check anyway.

FretDance said:
2.) Will removing the cap situated on the Volume 1 lug do the "bright-cap mod" I talked about, that sounded really good on other amps I've tried.

Yep...you can either pull it or even try a smaller value...something in the range of 100-500pF usually works pretty well there, or you can just omit it altogether. It's one of those "season to taste" components. ;)
 

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