Marshall JCM 2000 DSL 50 mods

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dehughes
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Marshall JCM 2000 DSL 50 mods

Post by dehughes »

Hi all,

I came across a great deal on an older Marshall JCM 2000 DSL 50 head and am trying to make the best of it. It's yet to display the runaway bias drift issue, and other than having a noisy reverb circuit it sounds pretty good, especially with NOS Mullard, Telefunken, and RCA preamp tubes.

I removed the reverb and that took care of that issue...amp is quiet and feels pretty good. However, I'd like to make this amp the best it can be within reason, and have used Sozo caps in past builds with great success, observing they provide a very clear, clean, "right" sound.

So, I'm looking for help in choosing which coupling caps in the JCM 2000 DSL 50 I should replace for most bang-for-the-buck improvement in tone and feel. The .022uf caps in the tone stack seem like obvious ones, as well as the .022uf coupling caps in between preamp stages as well as after the phase inverter. Maybe a cathode cap or two as well? I'm not too up on the gainier side of that circuit and have not used Sozo's in high-gain circuits, so I'm open to suggestions/input.

Would one of you be willing to take a look at the JCM 2000 DSL 50 schematics out there and let me know what caps you'd suggest swapping with Sozos? It'd be much appreciated....

Thanks in advance,

David
Last edited by dehughes on Sun Aug 27, 2017 6:22 pm, edited 1 time in total.
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dehughes
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Re: Marshall JCM 2000 DSL 50 ... Sozo cap replacement help

Post by dehughes »

Okay, how about this....I found out which 22uf caps I want to replace, but can't sort out which cap is the Presence cap.

Would any of you be so kind as to help?

I'm having a tough time sorting out the various schematics out there...appreciate the help.
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Re: Marshall JCM 2000 DSL 50 ... Sozo cap replacement help

Post by pompeiisneaks »

If I'm looking at the right schematic, it looks like C30 100nF 63v, next to the pot, a VR10 B5k:

http://drtube.com/schematics/marshall/J ... 20iss1.pdf which comes from conn12 that is connected to conn13 on

http://drtube.com/schematics/marshall/D ... 20iss7.pdf

I think that's right

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dehughes
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Re: Marshall JCM 2000 DSL 50 ... Sozo cap replacement help

Post by dehughes »

Awesome. Thanks. That's what I was thinking after making sense of the various boards and schematics....C30 on the "front board" schematic. It looks to be a tiny, white, square, .1uf cap, so that'll be a challenge to replace w/a Sozo. :)

Appreciate the help.
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Re: Marshall JCM 2000 DSL 50 ... Sozo cap replacement help

Post by pompeiisneaks »

Some people just sit the cap on it's end, insert one lead into the hole, then bend the other lead downwards and into the other hole, so it's vertical and can get into tight spots. You could even put some shrink tube on the exposed wire if you want.

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Re: Marshall JCM 2000 DSL 50 ... Sozo cap replacement help

Post by dehughes »

Thanks Phil. I managed to fit it in...put some clear, PVC tubing on the end close to the switch, and angled the cap a bit. The rest fit just fine with one or two needing an altitude change to avoid hitting the other stuff (I hate PCB construction....sorry....).

But yes, it fired up and worked without incident!!!!

When does that ever happen (to me)???

Love it.

Interesting difference, though....with the "stock" setup, the amp was more modern sounding, versatile, even-keeled. After installing a 3hy choke (Classic Tone) and replacing the major coupling caps with Sozo NextGen caps, the amp went way toward the vintage realm. Very interesting...I'm not yet sold on the Green Channel after the swap, but oddly enough, the Red Channel has been improved. Odd, as I preferred the Green Channel by far before the mods.

To sum up the Green Channel tone change post-choke-and-Sozos, it's like the amp went from Green Day to "It's a Long Way to the Top if (You Wanna Rock and Roll)", as the Green Channel seems to magically want to sit, tonally, in that mid-'70s, scooped, (JMP?) tone range. Not sure JMP is the best way to describe it, but I'm tired and that's the first thing that comes to my gut instinct.

Anyway, I'll give it a go and let the caps break in some more before giving a final judgement. One thing is for sure, there is LESS hum now, clearer bass, and more articulate top end present than before. It's not necessarily, qualitatively "better" to my ears at this point, but it sure isn't worse. Very interesting.
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Re: Marshall JCM 2000 DSL 50 ... Sozo cap replacement help

Post by pompeiisneaks »

Very cool!

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Re: Marshall JCM 2000 DSL 50 mods

Post by dehughes »

Phase Two of mods has begun. :)

Removed C12 on the "front board", which served as a bright-cap across the Red Channel's Gain pot. Not a fan of it installed, prefer it somewhat now that it's removed, yet the Red Channel still sounds "congested" for lack of a better word.

I've seen the following mod suggestions:

http://www.marshallforum.com/threads/th ... ead.13424/

and

http://www.marshallheads.com/viewtopic.php?f=25&t=862

HOWEVER, before I start tinkering further, I figured I'd post here as you guys, in my experience, are some of the best on the web.

My goal is to make the amp more open, clear, organic, and less gainy. That is, the Green Side ain't too bad....I feel no particular compulsion to change things on that side unless it's something special. However, the Red Channel just isn't my cup 'o tea, though I do like that Modern Metal thing from time to time and it's better now that the bright cap is out.

I don't usually play with super high gain, and if given the choice I'd always opt for organic clarity over gain for gain's sake.

That said, any suggestions as to how I could tweak the Red Channel to further my goal, to open it up and give it more life and breath? Might like tightening up the low end on that channel as well.

Thanks in advance!
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dehughes
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Re: Marshall JCM 2000 DSL 50 mods

Post by dehughes »

One thought I had was to just INCREASE the value of the "bright cap" on the Red Channel gain pot (C12). I have some higher value caps (.0047uf and .022uf ) and was wondering if throwing one of those in place of the 470pf cap that was C12 might add low end yet not deviate to much from the amp's original design.

Anyone know the low frequency cutoff for a .0047uf vs a 470pf?
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Re: Marshall JCM 2000 DSL 50 mods

Post by dehughes »

I replaced the 470 pf bright cap across the Red Channel gain pot with a .0047uf Sozo NextGen and noticed a definite improvement.

In fact, if I could keep this tonality, yet reduce the overall gain on this channel, I'd be pretty close to content. Any suggestions?
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Re: Marshall JCM 2000 DSL 50 mods

Post by dpin »

Super simple 'mod'
Try replacing the 12AX7 phase inverter with a 12AT7
It will effect both channels, but you might even wind up liking it better for the green channel.
It has tamed a couple high gain builds for me and I now include it in the original plan.
You wouldn't believe how much clipping happens at the PI until you see it on a scope.
AT7 helps
dehughes
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Re: Marshall JCM 2000 DSL 50 mods

Post by dehughes »

Oh cool! I'll definitely try that.

Does the AT7 kill the "Marshall thing", though? I mean, IIRC, the PI circuit is pretty standard setup on the DSL when compared to other Marshalls, yes?
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Re: Marshall JCM 2000 DSL 50 mods

Post by dpin »

Yes, you are correct that most amps, even beyond Marshalls, have a 12AX7 PI.
I see it as a homogenization of topology, in that, everyone does it because it works and always has.
I look at mods and custom amp builds from a stance of "how can I make an amp that will fulfill this particular players needs".
I made this recommendation based on what I read in your posts and what I would personally try if you and this amp were in front of me, without regard for any "Marshall thing".
Sometimes, as modders, we look for complicated answers to simple problems. This is a simple plug and play attempt at getting you closer to what you want to hear from your Marshall.
I have had some success with the JAN AT7s that you can get for under $20.
Please let me know if you try it and what your impression is.
dehughes
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Re: Marshall JCM 2000 DSL 50 mods

Post by dehughes »

Will do, thanks. I have a JAN '80s 12AT7 and an older RCA cleartop 12AU7 ready to try in the PI, once I put my amp back together.

I'm trying an altered version of the Joey Mods....pretty much identical, save for I'm going to run a .68uf cathode cap w/o the 1000pf plate cap on the first gain stage (he recommends a 1uf cathode cap with a 1000pf from plate to cathode, whereas I don't see that on your typical Marshall circuit so I'm rolling the .68uf alone to see if I can get good, true, lower-gain tones out of this thing...not so interested in high gain stuff).
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Re: Marshall JCM 2000 DSL 50 mods

Post by dehughes »

Alright, here's where I am with this:

Main Board: Removed C9 entirely (100pf from the first gain stage plate to the cathode) and replaced C12 (the first gain stage Ck, a 4uf electrolytic) with a .68uf Sozo and lowered the same stage's Ra (R13) from 220k to 100k while raising Rk (R14) from 1k8 to 2k7. I did all that to get that first gain stage closer to a JCM 800 and/or 1959 lead schematic.

Front Board: I installed a 1M to ground on the wiper for VR3 (Red Channel gain knob), lowered C10 to .0022uf (no idea what that does where it is), removed R20 and jumpered (again, not exactly sure what that does), and removed C12 (bright cap across VR3..was a 470pf, might put in a .0047uf cap as that was nice when I tried it).

My thoughts....

I like the "Clean" side on the Green Channel A LOT better now. It's nearly perfect, ESPECIALLY through a straight 4x14 with Heritage Greenbacks.

The "Crunch" side of the Green Channel is alright, but seems to have more midrange now and much leaner bass, maybe too lean. I'll play it more to see how it grows on me, but it might be okay....if not, it's at least close. If I could get more low end here it'd be fine, but for now I'll see if engaging the "Deep" switch will do it for me.

The Red Channel is the odd one....it's better in that it's gain is more useable and the sweep of the gain knob is more even, but there is a distinct midrangey-ness to both Lead 1 and Lead 2 on the Red Channel now that I don't care for. It's like the "R" sound is more pronounced.....a snarly thing in the midrange that is hard to dial out. If I could get it EQ'd more like the Green Channel I think I'd be set...but the network of stuff going on in the Red Channel is beyond my comprehension/skill level at this point.

Oh, and running a 12AT7 in the PI wasn't too different from the 12AX7, but I DO like a 12AU7 in the PI spot...cleans up the amp a bit, yet still has plenty of gain.

Anyway, recommendations for how I could flatten out the midrange on the Red Channel a bit and get it EQ'd more like the Green side? It's getting close...better, but not quite finished. :)
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