Marshall JMP-1 Preamp mods?

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Re: Marshall JMP-1 Preamp mods?

Postby tschrama » 06 May 2013, 18:24

edit..I try in PM...

let's talk jmp-1 mods.
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Re: Marshall JMP-1 Preamp mods?

Postby Tubetinkerer » 07 May 2013, 12:40

Jooshtin wrote:
Cheers Tubetinkerer for sharing your mods and ideas, they have got me thinking, and made me want to have a look at the clean channel sometime (you weren't wrong about the biasing etc earlier either… :wink: ). I've just noticed you said your jmp-1 has a lack of range in the tone controls and sparkle in the cleans, usually these have loads of eq available, mine can get way too bright at extreme settings. What amp/speakers are you running it through? Are you using the line outs and not the emulated speaker outs into the amp?

I may try tweaking R142 to reduce the overall gain slightly. However, I find mine sounds superb and gets used loads in the studio, and bizarrely often sounds more "hot-tubey" than some expensive rigs - for clients who want that Marshall sound. Don't underestimate the effect of the power amp/speakers/cabs on the tone. Mine is run through a 9005 all tube power amp (recapped, microphonic ceramic caps in signal path replaced, and biased nicely) into a couple of custom closed back 1x12 cabs (Eminence Wizard & Governor speakers). It gets put in the live room and cranked up loud :D Also gets used a lot for bi-amping bass too.

IMHO the jmp-1 is the natural progression from the DRP-1, 9001, 9004 preamps - alongside the JCM900. Remember too that when the jmp-1s were designed a lot of UK manufacturers were having a hard time getting decent tubes, so it's no surprise that the OD channel doesn't rely on the tubes for clipping - it does of course add to the tone/mojo and the cathode follower sets the impedance into the OD1 preset marshall tonestack etc.


Well Jooshtin, fact is that I have never been able to compare gear before. Neither am I a huge grail-seeker where it comes to tone, or did I try copying sounds from my favourit gitarists (Vai, SRV, Buckethead, Frusciante).
I still hold the guitar and picking style most important for sound.
So for years all I owned were JMP-1 and a GX-700. Thing that struck me about the GX-700 was that it's preamp section was so much more versatile than the JMP.
Off coarse it didn't have the classic Marshall sounds like the JMP, but still. I just wanted a bit more versatility from it.

Anyway I always ran it (line-outs) into a Mesa 50/50 (6L6GC) to either a Mesa Thiele EV or a Marshal 1960AV, and the overal sound always struck me as dull and flat (loud as hell, though).
Recently I got a Koch Classicitone 2*10 which I used for comparison, and it's power amp is a lot brighter, lively and open sounding than the Mesa.
The 50/50 (to me) is much boomier and less transparant. So I performed kind of a reverse deep-mod on it, and that brought the JMP-1 as well as the E530, more to life.
But I still think though, the tonecontrol mods and the treble-pot-mod are worthwhile.

BTW, just for the record, I never claimed the JMP-1 was rubbish or bad in any way. But over the years, taste changes, playing improves, more gear is stacking, or maybe it was just the boredom of owning it that made me want to change it. ;)
I might have to go hunting for a stock one now.
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Re: Marshall JMP-1 Preamp mods?

Postby Jooshtin » 07 May 2013, 16:23

Yeah, lets get back to the jmp-1, this thread has gone a bit like this!


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Re: Marshall JMP-1 Preamp mods?

Postby Jooshtin » 07 May 2013, 16:38

Totally agree with you Tubetinkerer, and thanks again for your mods and info, they've made me want to have another look at my jmp-1. Agree about Mesa's too, I had a Mk3 for a very long time (too long!) and finally realised how dull/flat the sound was compared to other amps.

Apologies, in my earlier post by preset tonestack I meant the resistor/cap network between IC12 pins 1 & 3 not the eq later in the chain. This preset tone network is the only difference I can see between OD1 and 2. It looks like a tweaked Marshall tonestack and preset level. It would be interesting to sim the eq shape, also to check the actual values in the unit. :hmmm: If this is the only change it certainly shows the difference a tonestack makes.

I'm not the biggest fan of the active eq in the jmp-1, I'm probably just too used to passive eq in amps, but IMHO when teamed up with a Marshall voiced amp and and speakers the eq doesn't need much to get it sounding good. For comparison, my basic go-to classic rock rhythm setting that cleans up well with the guitar volume, for LP and PAFs are:
OD1, Presence 1, Treble 1, Mid -1, Bass 4 (with my 1x12 speakers, these are flatter response than most cabs), Gain 3 to 4+, and Volume feels best not at max, but around 12 to 15ish.

MoonWatcher - we both have excellent taste in music! :) I'd also love to know what the mods are in Janick Ger's jmp-1. Although not a jmp-1 mod, I see on Pete Cornish's site there's a pic of his B rig, it has a mid boost control, and there's a graphic set to boost mids on the 2010 tour A rack pic.

http://www.petecornish.co.uk/jgers.html

http://www.premierguitar.com/Magazine/Issue/2010/Nov/GALLERY_Iron_Maidens_2010_Touring_Gear.aspx?Page=32&

You've probably seen this already, but it's a nice read - are Dave Murray's settings really 6 6 6! :twisted:

http://www.marshallamps.com/images/mars_law_issue_pdfs/ml%2007.pdf
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Re: Marshall JMP-1 Preamp mods?

Postby Tubetinkerer » 08 May 2013, 09:47

Jooshtin wrote:Apologies, in my earlier post by preset tonestack I meant the resistor/cap network between IC12 pins 1 & 3 not the eq later in the chain. This preset tone network is the only difference I can see between OD1 and 2. It looks like a tweaked Marshall tonestack and preset level. It would be interesting to sim the eq shape, also to check the actual values in the unit. :hmmm: If this is the only change it certainly shows the difference a tonestack makes.


See graph. Made the simulation in Duncan's Tonestack Calculator with compensation for Bass-LogB-Taper. Didn't check the actual values though, but I didn't find any discrepancies between schematics and actual values yet.
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Re: Marshall JMP-1 Preamp mods?

Postby Jooshtin » 09 May 2013, 20:58

Nice one Tubetinkerer - you beat me to it! I've done an approximation (in green) and a typical Marshall tonestack with all controls on 5 (orange). They look pretty close, Increasing the slope resistor would shift the mid-scoop down in frequency, and a trimmer instead of R98 & R99 could add some tweakability?
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Re: Marshall JMP-1 Preamp mods?

Postby okgb » 21 Jul 2013, 01:39

revisiting this thread , My JMP-1 doesn't get much use because I have a couple other amps that are all more satisfying soundwise
my biggest grip is the distortion being a little " abrupt " [ like the s.s. diodes to ground pre it is ? ] and un-smooth until the overdrive
is heavy , then it's too much . louder in a good power amp is better . Haven't bothered yet but was going to play with the diode config ,
try using the Jubilee ver. I know very unscientific , partly why I haven't yet
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Re: Marshall JMP-1 Preamp mods?

Postby Ice-9 » 17 Jan 2014, 20:17

Just re-discovered this thread as I have also just rediscovered my JMP1 which is now in a recording rack. AnyWhoo, does anyone have any details on the Voodoo mods, it would be interesting to see what they have modded Mine sounds like it need a new transformer (again) which marshall will sell me at a very reasonable price but are there any better replacements out there that are available?
It's fairly straight forward, if you want to start it , press start. You can work out the rest of the controls for yourself !

No silicon heaven ? preposterous ! Where would all the calculators go ?
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Re: Marshall JMP-1 Preamp mods?

Postby Ice-9 » 18 Jan 2014, 19:07

Jus out of interest, I did a search for JMP1 mods and came up with the guy who designed JMP1 for Marshall, Mike Scuffham. It seems he has designed a software VST and standalone guitar amp sim called 'S-Gear' so I downloaded the trial and found to my ears it is the best software amp I have tried. Well worth checking out.

http://www.scuffhamamps.com/
It's fairly straight forward, if you want to start it , press start. You can work out the rest of the controls for yourself !

No silicon heaven ? preposterous ! Where would all the calculators go ?
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Re: Marshall JMP-1 Preamp mods?

Postby okgb » 15 Sep 2015, 21:03

Yeah, coming back to this [ How to make my JMP-1 more usable ]
no consencous, but if some of the overdrive is derived from the I.C.'a and S.S. where and how ?
I have a few tube preamps, but the the top three get used and the others like the jmp-1 sit
top for me , the CAE 3+ [ desert island only pre, classic not so modern, always usable ]
Mesa Formula pre , great clean & semi grit , worth it for that alone. the od channels in my opinion
don't cut it , just not complex enough with two tubes. Bogner/ Hafler triple giant , voiced different enough
to be not a " me too " often these days running through the latest ADA speaker sim box.

I don't want to sell the Marshall , but it's not cutting it , 10 hundred dollars or best offer , ha !
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Re: Marshall JMP-1 Preamp mods?

Postby Ice-9 » 14 Oct 2015, 21:34

okgb wrote:Yeah, coming back to this [ How to make my JMP-1 more usable ]
no consencous, but if some of the overdrive is derived from the I.C.'a and S.S. where and how ?
I have a few tube preamps, but the the top three get used and the others like the jmp-1 sit
top for me , the CAE 3+ [ desert island only pre, classic not so modern, always usable ]
Mesa Formula pre , great clean & semi grit , worth it for that alone. the od channels in my opinion
don't cut it , just not complex enough with two tubes. Bogner/ Hafler triple giant , voiced different enough
to be not a " me too " often these days running through the latest ADA speaker sim box.

I don't want to sell the Marshall , but it's not cutting it , 10 hundred dollars or best offer , ha !


all of the drive is from diodes around the IC's, The valves are just for marketing purposes. The Speaker emulator circuit is very good. I have two JMP1's if anyone Is interested 1 of them is for sale. :)
It's fairly straight forward, if you want to start it , press start. You can work out the rest of the controls for yourself !

No silicon heaven ? preposterous ! Where would all the calculators go ?
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Re: Marshall JMP-1 Preamp mods?

Postby okgb » 15 Oct 2015, 16:09

Yeah , I've thought about making the diode bridge switchable , what else make the slash silver jubilee sound good
[ other then a couple of standard tube & passive tine stack ] it's another diode bridge in there for lead ch.
My main gripe is a sort of hardness or lack of smoothness in the decay or half distorted sounds, fired it up the other
and the pwr xfmr was humming so much it could be played T.V. jamming level.
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Re: Marshall JMP-1 Preamp mods?

Postby Funkystrings » 19 Aug 2016, 17:55

Just an update. It's been about 3 years sense I re-visited this thread. I think the last thing I posted was that I had my JMP-1 on the bench and was going to try some of the suggestions offered here and then post my results but sense I didn't get back in time, my last post was probably removed because it was inconclusive. Anyway, all I really ended up trying was playing with the tone stack's individual resistor values that effect how much or how little the Bass, Middle, Treble and Presence are boosted or cut, utilizing their -6 to +6 ranges. I ended up putting them all back to stock except for R158. (for the midrange). Stock is 6k8 and I raised it to the next value up which is 8k2. (remember, Raising the value slightly reduces the amount of boost or cut, without messing with the classic marshall frequencies so you're not bastardizing your preamp per say). I wanted that mid control to behave a little more like an old marshall, where you don't hear as much of a difference fully cranked or set to zero. I also did this because my three least favorite factory presets are "Filth" (-6 MID), "Mid-lands" (+6 MID), and "Sunshine" (+6 MID). Those three presets have always been unusable for me at their extream factory midrange settings of -6 and +6. In fact, to me, they are laugh out loud ridiculous. Now the midrange behaves in a more usable range and sense midrange is the most hearable frequencies to the human ear, I now find the midrange to sound a little better on those three presets that I never used to like although the change is subtle. All the other stock presets sill sound great as usual. It's a very subtle mod that makes the JMP-1 less capable of producing a bad sound and it makes it a little easier to perfectly dial mids in or out because now midrange settings between say -2 and -3 are slightly less exaggerated. So for me, what could be wrong with that? Especially when I'm trying to create perfect presets for many different guitars to achieve the same basic sound I may be going for and you still have plenty of scoop if you need it.

As for me wanting to hear the 12ax7s bite a little more to warm up the over all sound of the factory presets and especially the clean sounds, I found a simple solution: I special ordered the hand picked, high gain JJ ECC 83s offered at Tube Depot for an extra $2 per tube, (also available at Eurotube). (I replaced both! 12ax7s with high gain tubes by the way) and instantly noticed a better sounding preamp. Try it. I don't think you'll be disappointed. It simply sounds more organic and tubey. Not flabby at all ...all the JMP-1 tightness is still there in spades for the high gain settings with fuller and spankier cleans as well.

I also drilled and installed a small two way toggle switch right above the input jack to switch between the stock Bass Shift cap and one that is exactly half the stock cap's value which is useful on high gain presets that utilize the Bass Shift On setting, making them slightly less flabby yet they still retaining the nice open sound the Bass shift offers. And for the lower gain presets that utilize Bass shift On, I set it back to stock, again getting that nice open sound that the JMP-1 has always offered.

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Re: Marshall JMP-1 Preamp mods?

Postby Tube2stomp » 16 Sep 2018, 14:04

Well, guess I'm late by, what, 25 years? to the JMP-1 party.
Just got my first one :applause:

And ah.... yea... not what I expected... on its own it sounds like a solid state pedal :hmmm:
If I had it 25 years ago I would've probably been super happy youngster metal guitar player playing with distortion levels on 20!
But I'm older now, wiser, more experienced in the hands (playing) and the ears (knowing what I want to hear) and yea back then I didn't know a thing about electronics.
I bought it knowing its not the end all be all, but I thought "hey its tubes, give me anything tubes and I'll make it sound F*** great".
I started modding little things, like obvious stuff (taking diode bridge out) but it kept sounding like a pedal.
That's when I knew that "well, guess its not *tubes*".

Thanks to THIS thread I was totally schooled on things I had no clue about, how all those ICs "contribute" to the sound of this preamp.
Are there still listeners to this thread?
I dived in hard into the JMP-1 and modded it and its still in open heart surgery mode on my bench and I still need input on things I can't figure.
Don't want to write to the ether, so who ever still subscribed to this thread please give a shout :horsey:
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Re: Marshall JMP-1 Preamp mods?

Postby Tube2stomp » 16 Sep 2018, 16:34

I'm at work so will give the headlines of what I have so far, all done as I was reading this thread:

Took D12 out, with it the whole bridge rectifier.
Adding diode clipping with a pedal is something else... it's my choice... I'm not married to it all the time.
So diodes out = good.
Kept reading...
What?? IC12a has diode protection which clamps grid voltages? hell no!
Started de-soldering it thinking I'll just pull it out.
So de-soldering trying to pull it out and still keep reading... oh fuck... this guy broke the IC doing that! stop!!!
re-soldering... plugging the guitar, maybe it won't be bad? crap I know what I need to hear and I don't hear it.
Also changed R91/R120/R121 with pots to lower voltage swings on IC12b and IC12c... still no bueno.

OCD thinks "that 13 pin, why 13? bad luck, must go"... dremel in hand... sharp diamond tip...I'll cut that little fucker :twisted:
Well, didn't work well, saw pieces of solder everywhere breaking from the vibrations and I barely dinged the 13 pin :hmmm:
Resale value gone is one thing, but ruining the preamp totally? nope. re-soldered pin 13 back and fixed all places I saw damaged.
Test the JMP-1, all good, go to bed.

Next day, lowered R124 to 1.8k, added bypass cap (820n if I remember correctly).
Changed R142 with a 1M pot, testing how much - if any - clipping should come from IC19b. right now set to 500k.
IC12a pin 13 still bugs me, this is the key to having any hope for the JMP-1... still not sure how to tackle this.
So... IC12b... why do I need it all all? just to take out the tone stack so OD2 will sound not marshally and clip IC12c even more?
It doesn't make sense, what's more because of it we need R92+R91 "volume pot".
The thing is that our 2-3k CF zout is now actually 330k zout (even higher now with the mods)... and that drives the tone stack. why? :scratch:
So if I bypass IC12b all together, I don't even need R91/R92 and C51... I'll have it like a true marshall amp, drive the tone stack *direct* from CF! :applause:
Sure now OD2 will not sound as before, modern, un-marshally... because it will also be going the thru the MARSHALL TONE STACK. yea, very bad :lol:

So IC12b is totally out. what else?
R121/R122 not needed as well.
IC12c still there but I'll come back to it, because I figured how to bypass IC12a easily!!!!!
Xacto Knife, on the tubes riser board simply cut the trace to pin 2 (grid) of VB2a.
All is needed now is to solder a 1M resistor from pin 2 to ground, lift the "out" side of C86 and connect it direct to pin 2... DONE!
R125 can stay put, it and IC12a pin 13 are out of the picture :horsey:
Now you can drive VB2a's grid as much as you like, it will also now drive the CF EXACTLY like in a marshall amp, which the CF it self will drive the tone stack EXACTLY like in a marshall head.

So what we got?
No IC clipping of driver stage IC19b (unless you want it to).
No diode clipping of bridge rectifier.
No diode clipping of IC12a protection clamps.
No diode clipping of IC12b protection clamps.
VB2a clips like it should, VB2b also distorts the signal an unevenly compresses it as it should.
Now, if you dial R120 low enough (which is a pot, remember?) you can take IC12c protection clamps out of the picture as well, giving you 100% tube overdrive/clipping.

Sure, as is the JMP-1 no longer has those CRAZY amounts of clipping, it's back to classic rock gain area.
But it sounds sweet and natural :-)
Next stage will be to figure a way to use (at least) VB1a in the OD channel....

*** need to go home ****

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Re: Marshall JMP-1 Preamp mods?

Postby Tube2stomp » 21 Sep 2018, 21:51

Changed all resistors of the Marshall side preset tone stack to pots.
Getting much better sounds.
Will see over time if I'll settle on a one setting, if so, I'll preset it.

p.s. if you'll do as I did and remove IC12a completely, change C86 to lower value (22n is a classic) AND a must is to add a grid stopper (i'm using 220k) after the 1M grid leak, else the JMP will fart cause of grid conduction.

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Re: Marshall JMP-1 Preamp mods?

Postby Cub » 25 Sep 2018, 15:06

I somehow missed the most recent posts. Thanks a for sharing these mods with the gang. I think I did more gigs with the JMP-1 than with any other piece of equipment and it only moved from being the heart of my entire setup after I got myself a TriAxis.

It is what it as and that was awesome back in those days for the metal heads. My rack paired up really well with the big stereo Valvestate combos and heads of the other guitarist, probably because they both get their sound in similar ways.

Its sound is outdated and its tonal palette too limited to compete with the gear of today, either modelling equipment or traditional amps. Mods like these bring that wonderful preamp back from obsolescence!
I wish I were a chestnut tree, nourished by the sun.
With twigs and leaves and branches and conkers by the ton.
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Re: Marshall JMP-1 Preamp mods?

Postby Tube2stomp » 10 Oct 2018, 10:28

Added VB1a to the OD channel :secret:
Still needs tweaking but happy times indeed :thumbsup

Current signal path is:
IC34a > Gain Control > IC19a > IC19b > VB1a > VB2a > VB2b > IC12c > IC18a

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