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Anybody have any tone/gain mods for the JMP-1 rack preamp?
I know the Voodoo mods sound great, but yeah - those aren't published and I want to mod it myself.

I also am interested in any good JMP1 mods, the schematics are available at mt tube website if you haven't already got them.

Bumping , I'd like to see any mods as well
It's not a bad preamp but not an end of the world pre either

Looking around didn't see much detail about any mods for the jmp-1
some reference to removing or replacing the diode bridge [ goes to ground before
one of the tube stages ]

okgb wrote:Looking around didn't see much detail about any mods for the jmp-1
some reference to removing or replacing the diode bridge [ goes to ground before
one of the tube stages ]

The diode bridge is doing exactly the same job of clipping the signal as the diodes in all diode to ground distorion circuits plus the is also an extra diode connected in the middle of the bridge. You could remove the bridge rectifier and the extra diod and experiment with other types of diode and/ or configurations of diodes.

I think a good way to change things could be to try and change the gain of that stage to drive the 12ax7 valve more, rather than get the clipping from the diodes. There is also a lot of options for altering the tone stack which is made up of active filter stages with digital potentiometers.

The JMP1 is quite a simple circuit to get to grips with if you look at it ignoring the midi/digital/switching stuff it becomes quite simple to understand whats going on.

This thread http://foros.guitarramania.com/viewtopic.php?t=26024
is analyzed and clarified with schematic here
http://forum.gitarnorge.no/topic/14714- ... ro-vecino/

First post here. I've been making and modding amps and pedals over the last few years so this is a great site to find and I am very fortunate to find this post in my first look. I just picked up a broken JMP-1 this month and after reviving it with a new transformer, I just tried out some of these mods with parts on hand. I played around with all these mods except the IC31 pin which would be hard to reverse.

I used a 150K Carbon Comp in R126 and 1.5K Gold lead Mepco in R124. I'd say this is kind of a middle ground on the gain changes I would think (plus some parts I had on hand). It does take some of the over the top OD edge off. I think this is the area where you'd want to experiment the most with some values if you want significant change.

In pulling some of the tone caps, I found a couple that did not test very good. For example, the 1u Presence measured as 8u on my meter. Even so, it was kind of hard to tell if there was much of a difference because of cap type changes but it is hard to A/B without two units.

But put back together and putting a new JJ803S in V1 and a Mesa 12AX7A in V2, everything sounds pretty good. I'm not big Marshall guy, but I like to have fun once in a while with the crunch!

thanks for the update , changing the plate & cathode resistors will affect the gain of that stage [ lowering it in this case ]
and not suprizing that caps may be off , often rated +/- 10% . I haven't opened it up yet , But along with changing the diodes
on the bridge to ground before the tube stage , I was wondering if a trimmer would " massage " the influence of this .
the silver jubilee sounds pretty good and uses diodes for I'll compare that schematic .

That is an interesting thought about trying different diodes. It might smooth things out a little. I had a Ceriatone 2550 clone for a while. There are a couple of red LEDs involved in that circuit.

Here's a French article I found a while back. I haven't tried any of the mods yet but will probably give replacing the ceramic caps in the signal path a go sometime (and the elec caps in the power supply), doing that to my Marshall 9005 improved the tone a treat. Love my jmp-1, it's really versatile, especially with the gain low so you can back off the guitar volume to clean it up, wish the clean channel wasn't so sparkly clean though, my fave cleans always have a bit of dirt in 'em... I've read that removing the diode bridge can damage IC12 4053 as the diodes limit the voltage to the following stages, I don't know how true this is though - then again it's a Marshall, I like it to sound like a Marshall so the diodes stay ha!






And a dodgy Google translation:
http://www.latentlemon.co.uk/LLimage/jm ... lation.rtf

Nice find. I'm going to have a go at the BR1 and BR2 filters. Maybe it will cut out some of the extra noise on the OD channels. The clean channels don't seem too noisy to me as is.

That translation makes for an amusing read.


I wonder how close some of these mods are to the voodoo mods.
I know those sound great.

Thanks for posting Jooshtin.

As far as I can read from the rtf file this mod is purely a hi-fi mod one would do to studio gear, or to a better stereo for less noise and distortion. As far as I can see there isn't really any tone shaping suggested in that text. Of course there will be some minimal changes in the sound do to value differences and mojo from the caps in the signal path. Not sure how much impact a cleaner power supply will have on the overall tone though.

But the text suggest that he has posted odder mods for the JMP-1?

Oops, didn't realize the edges didn't show up... here's the links to view the article:
http://www.latentlemon.co.uk/LLimage/jm ... p118hs.jpg
http://www.latentlemon.co.uk/LLimage/jm ... p121ds.jpg
http://www.latentlemon.co.uk/LLimage/jm ... p132dj.jpg
http://www.latentlemon.co.uk/LLimage/jm ... p145wu.jpg

Yeah, I think this mod is more about improving what's already there rather than changing the whole tone of the thing, and so the power supply can handle the OPA 2604s. I do like my JMP1 as it is though, and so long as I keep it at least 1u from the power amp and don't have the preset volumes too high it's very quiet even on high gain. It is a Marshall, so it wouldn't be right if there wasn't some noise! Like all manufacturers (especially Marshall!) they use the cheapest parts and construction they can, so are susceptible to ground loops etc. The eq is ok, but I wish they'd used a proper amp tonestack between IC12b and c rather than the preset network. What I've heard of the voodoo mods, they don't really do it for me, but I bet those mods are pretty much all in this thread! Like Ice-9 said, it's a standard circuit without all the midi switching stuff

"operate the beast with open heart"

Loving the JMP-1! Its cleans are much warmer than the ones in the Mesa/Boogie TriAxis. I only used OD1 with the Bass Shift on for my dirty sounds, OD2 sounded a bit too "modern" for my taste.
Do you think it's possible to make a stompbox with just OD2 in it?

Yeah, should be straightforward, I think there's a thread somewhere about building a jmp-1 in a pedal.....? The only difference between the two OD channels is the preset tonestack between IC12b and c. I used a MkIII Boogie for years, it was ok for the time, but it always sounded lifeless and 2 dimensional compared to other tube amps (Fender Bassman, vintage Marshall etc), a real tonesucker! The jmp-1 rocks! Here's the jmp-1 schematic:

http://www.drtube.com/schematics/marsha ... -61-04.pdf

Jooshtin wrote:Yeah, should be straightforward, I think there's a thread somewhere about building a jmp-1 in a pedal.....? The only difference between the two OD channels is the preset tonestack between IC12b and c. I used a MkIII Boogie for years, it was ok for the time, but it always sounded lifeless and 2 dimensional compared to other tube amps (Fender Bassman, vintage Marshall etc), a real tonesucker! The jmp-1 rocks! Here's the jmp-1 schematic:

http://www.drtube.com/schematics/marsha ... -61-04.pdf

Aye, The IC12b and c just switch out part of a tone shaping circuit allowind the OD2 signal to basically go straight through that section. It could be an interesting project to do a JMP1 mini preamp with proper tone controls for clean and dirty chls in a reasonable size enclosure. The valves in JMP1 are just marketing bullshit, they do pretty much nothing for the distortion of the unit as its mostly diode clipping thats responsible for the drive.

There is some gain from the valves unlike unity gain devices or starved plate designs

There's an interesting bit in Merlin Blencowe's preamp book about cathode followers and their effect on tone etc, funny how its become the Marshall circuit signature, but was just what happened to be in the version of the bassman they copied They've been using diodes in their amps since the JCM800s I think, a cheap way to get more distortion, the JMP-1 is v similar to the JCM900s. The supply and quality of tubes was pretty uncertain when these were first built built so it was a neat way of making a tube-ish circuit that folks would happily buy that pretty much any tubes would work in. Thankfully at least the tubes don't need LEDs behind them to make them look like they're working like Behringer mic pres!

Hi all. My first post here. (topic still allive ?)

I've owned a Jmp-1 for ages. I never really liked it that much, due to its lack of dynamics, small-range tone controls and lack of sparkle in the cleans.
Only OD-1 always gave me a great low-drive crunch that I loved. When I learned that the tubes don't have any part in distortion in the OD-channels,
I decided to try and change some stuff on it. In the end I did a lot of mods to the thing, some more satisfying then others.

To tackle the dull cleans, I replaced R78-R80 by a 250k trimpot. Since this RC-network is nothing but a preset classic Fender(!) tonestack,
this basically changes R78-R80 into a treble control. I just set it to my taste, (tone controls to 0) and left it there. This mod is a must.

For improving the range of the tone controls (a 4 channel active EQ, instead of a classic guitar amp tonestack), I replaced the feedback resistors (R177/R158/R178/R181)
by bigger ones (don't know exactly, but can check if anyone's interested). This amplifies the negative or positive feedback, giving improved spread of the controls
(with less resolution, cos the number of steps doesn't change of coarse). I prefer this over the hours of tinkering to get the sound just right.

Furthermore I don't like how Marshall claims tube distortion, while no distortion comes from the tubes at all.
The way the circuit is set up, the tubes (V2a/V2b) CAN NOT contribute to the distortion. The input voltage at VB2a-grid is 3.6Vtt max.
If the cathode follower were set to high gain it would generate some distortion, but that's not how it is. For high gain, you'd expect
a bypass capacitor on R124. All the cathode follower does, is warming up the harsh distortion fed in by IC19b and the bridge rectifier.
I know; this is how all high gain JCM's work, but I don't like it.
So I tried to change the stage to full tube distortion, but one cathode-follower just will not cut it, because there's no distortion-upon-distortion-chain
like a Mesa or Soldano. Even if set to max gain.
So I ended up with te following :
Clip 1 side of D12, this takes the rectifier from the circuit. Decouple pin 13 of IC12a from
R125 (or leave it as it is if you are happy with the sound. Max. grid voltage will be 10Vtt because pin 13 of IC12a, will now clip the signal at 10Vtt through it's overload protection ).
Use a vactrol to take over IC12a´s function. This requires some fiddling with the circuit around TR9.
The grid voltage will now rise from 3,6Vtt to near 30Vtt. This is where IC19b will start clipping and introduce solidstate distortion.
The clipping point can be changed by lowering R142 (later clipping e.g. 470k), or lowering R143 (earlier clipping e.g. 2.2k)
Put a 1uF cap across R124, which will increase tube gain from 43 to 75.
Changing R124 to 1.5k will change to a less symmetrical clipping, thus changing the sound.
Don't start the engine yet, because the voltage at C51 will rise quite dramatically, and probably damage, if not blow up, IC12 and IC18.
Therefore the voltage divider R91/R92 needs to be adjusted.

Although it is quite an extensive mod, which takes away the metal like distortion, I didn't change it back.
It leaves you with a dynamic distortion stage that knows HB's from SC's and responds to your guitars volume knob.
For true, dynamic, high gain tube distortion, I bought something else in the end.