help with modding a 79' JMP

[dave6]

some help would be greatly appreciated.....

i am an accomplished elect. tech ( in sales now), but would like to mod my own marshall 1979 JMP Master Volume head. i want the traditional marshall "drive", but alot more gain, and a little more bottom end. can anyone help with where to start? the low input has already been disabled.....


all input is greatly appreciated...




dave

[MickRonson]

+1

[Nathan Sanford]

[dave6]

and i'm assuming i have to BUY one of these to try it out?


hmmmm.....

[Shea]

Originally posted by dave6
and i'm assuming i have to BUY one of these to try it out?


hmmmm.....

That's a lot easier than trying to mod that circuit board, which is a one-sided PCB with all the traces on the bottom. It's a sturdy, reliable PCB, but not one that's easy for modding.

You could fatten up the lower end by replacing some of the coupling caps or the bass cap with larger values. You could increase the gain and fatten up the bottom end by putting a bigger cathode resistor bypass cap on V1A. You could get more gain by adding cathode resistor bypass caps to V1B and V2A, and get still more gain by reducing the value of the cathode resistor on V1B.

The problem is, each time you do this, you're likely to run into flubby bass, blocking distortion (which can completely shut down a preamp tube for a couple seconds as soon as you hit one chord), and oscillations. So you'll have to go back and undo what you just did, or maybe split the difference between the original value and the one you tried out, or perhaps you'll be able to tame the problem by adding (or increasing) the attenuation between stages. So for each component you change, you'll probably have to try a bunch of different values, and will probably have to add or change other components in the circuit to compensate for any problems that might pop up.

All trial and error would be a huge headache with that circuit board, and the more time you spend modding it the more you'll run the risk of accidentally lifting a trace or burning the insulation on a lead.

Here's why: Each time you have to unsolder a component from the board, you'll have to undo all the nuts that hold it down, as well as the nuts that attach the pots and input jacks to the chassis, lift the whole thing up as far as you can without breaking anything (which isn't very far), and try to melt the solder on the bottom of the PCB without accidentally burning any leads that get in the way.

A shortcut is to clip off old components leaving their leads sticking up through the holes, so you can solder new components to the old leads. But it's kind of a bandaid solution that can get a little sloppy if you do it with too many parts.

And once you get that all worked out, you still might not have the high gain tone you're after. That circuit has 3 gain stages, and modern high gain tones typically come from 4 or more gain stages. Now, I suppose it's possible to convert V2B into a 4th gain stage, but then the headaches start all over ...

So, it's not really a mod-friendly amp. However, it's easy enough to remove the bright cap on the gain pot . That smooths out the tone, but it won't increase the gain.

Don't get the idea that I'm against mods (as long as they're reversible). I started learning about how amps work from modding silverface Fenders. But those are a lot easier to mod than a PCB Marshall.

Shea

[drbob1]

Plus there's a limited supply of 79 Marshall's and they are a desireable amp. Why not sell it at a profit, get a Hoffman board 50w or a Mojo kit as a place to start (both "PTP" wired) and mod something that won't be missed?

[sabbath90]

you could get a sniper modded boss ge-7 at tone-jam.com and boost the bass and the output. it's done wonders for my jcm800.

[John Phillips]

I have to agree with Shea and Bob.

I'm not against mods either, in principle, but a classic PCB Marshall isn't a good place to start. Actually, if you can find an older PTP Marshall that's already been hacked, that's a good place to start... or even a metal-case PA model (they have almost no commercial value, but use the same basic parts as the guitar amps).

If you boost the input, you could maybe get most or all of what you want, but you'll still be limited by the fundamental bass response of the amp.

Personally, I'd sell it (they're getting quite desirable now) and buy a more modern amp that does more of what you want.

In the reverse, is the task of modding a stock vertical input JCM 800 to JMP specs equally as difficult?

[Shea]

Originally posted by gizmo
In the reverse, is the task of modding a stock vertical input JCM 800 to JMP specs equally as difficult?

I assume by "JMP specs" you mean the 4-input JMPs, because the master volume JMPs had pretty much the same circuit as the early JCM800s.

That conversion would definitely be easier, because at least you'll already know what values of components to use and won't have to go through the trial and error of figuring that out.

If it has the same PCB that was used in master volume JMPs, then it's also the same PCB that was used in 4-input JMPs, and it would have the component values for the 4-input circuit printed right on it. You'd just have to put those correct components in those holes -- still a pain in the neck, but at least it's conceivable that you'd get it all right on the first try and not have to keep fiddling with the PCB over & over.

There's still the matter of re-routing the leads to the input jacks and volume pots. One thing I don't know is whether your chassis already has 4 holes for input jacks underneath the face plate -- my 2204 JMP does, but I don't know if those early JCM800s do.

Shea

[Shea]

Originally posted by John Phillips
Actually, if you can find an older PTP Marshall that's already been hacked, that's a good place to start... or even a metal-case PA model (they have almost no commercial value, but use the same basic parts as the guitar amps).

The previous suggestion of getting a kit was a good one too. He could get a kit for a plexi clone, build the 2204 circuit into it (which wouldn't be too hard), and then start modding. I think that could be a lot of fun, for someone who enjoys that sort of thing.

Shea

Originally posted by Shea
I assume by "JMP specs" you mean the 4-input JMPs, because the master volume JMPs had pretty much the same circuit as the early JCM800s. Shea

Shea...you're a wealth of information. But I actually meant a JMP master volume. I always thought the circuit was the same but I've heard from others that seem pretty knowledgeable that JMP MVs have a little less gain and a more vintage sound to them - noticeable differences.

I wouldn't mind experimenting if it wasn't difficult/risky.

[Shea]

Oops, sorry. No, I don't know. Maybe John Phillips can tell you.

Shea

[dave6]

yeah, i've put my jmp master volume against an 82 jcm 800 and the 800 has quite a bit more gain.

the only reason i haven't sold it and got a jcm 800 to do the mod with was that i was told these are basically the same head, just the 800 has an extra preamp tube....

of course, that's why i posted here because there is a wealth of knowledge and a willingness to help...

again, all input is greatly appreciated.

thank you so much....keep it coming....



dave

[dave6]

i have almost decided 100% to have either Mark Cameron, Michael Soldano, Scott Splawn, or Lee Jackson mod my JMP. any suggestions on which one to go with?

i'll explain the sound i'm going for:

traditional marshall "drive" sound with alot more gain (more than 80's rock)
thicker tone with just a touch more bottom.....WHILE STILL RETAINING THE RESPONSIVENESS AND OVERALL MARSHALL SOUND

i have a 60's ri paul and a mesa oversized recto v30's cab...


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