Hi Funkystrings, welcome to the thread.
First off, I did not come up with all the mods myself. I found a thread somewhere, few years back,
where someone made similar suggestions that ispired me to take the lid of it. I think it was the Marshall forums. (Credit where credit's due)
At that time I was where you are now. Eager to change it, but not knowing where to start.
Personally I don't think the JMP-1 is bright. I feel it's just dull and it doesn't cut thru. But that's today.
Back in the days Marshall ruled the world pretty much, and that was the sound you wanted.
I never found a good use for the Bass Shift, so never used it. It's too strong but, off coarse, tweakable.
(For cleans increase R76 or replace with 50k trimpot. For OD replace C116 by 47nF or smaller.)
Q1: It's the middle and bass that control most of the sound. So to me it sounds pointless to
lower the range of the mid-eq. I do understand what you are saying though, but you'd be limiting the control
instead of expanding it. I think that that is due to the stories around that you need hours and hours of tweeking
to get it "just right". From that perspective, it would make sense to increase the resolution of the mid-eq.
My way of thinking is this; in the end it's a balance of the four controls. So if you'd expand the range of the controls,
it should be much easier to find a useful balance. And I can tell you that it worked out that way.
Values I used Bass: Unchanged (I rem. peak distortion but maybe try 2k2) Middle: 4k7 Treble : 4k7, Presence : 4K7 (Apparantly I decreased them,
which makes sense cos the opamps are inverting)
Q2: I removed D12 altogether. On IC12; I wrecked it when I tried to lift one pin from the board. So I replaced it with an IC-socket,
giving me easy possibilities to lift IC-pins and solder to them. Not the neatest way of doing things but it suffices.
I'm just too lazy to take the PCB from the casing for every mod. "One day I will tidy it up"
Q3: Google "VTL5C2". IC12a is part of the distortion, in the way that it will clip at +/- 5V, through it's
overload protection. If you want true tube distortion, you need to take this out and use something analog, like a vactrol.
I did not though. I replaced C86 by 100k-100k resistor en connected the middle of the two, to IC12a pin13.
I also replaced R124 by 1k8 to increase max. possible gain (or "hotter", or "more symmetrical". Take your pick)
Q4: R91/47k R92/470k | R93/220k R94/680k (R93/94 I dont remember why exactly, but I think it has to do with balancing Clean and OD levels)
Also removed R120 (or bridge R119), R121/150k, R122/680k
So actually the JMP is on the bench. I was trying to switch on/off the rectifier bridge for ODII by IC12d-ODII.
Unfortunately that signal goes logic "0" on ODII, so I need other parts.
On the "fenderish" issue there's no conclusion, but there's different paths to Rome.
I prefer the channel not to be as clean as possible, while somebody else might.
However, I stick to the idea that R83 to 1.5k will clean up the channel.
Losing volume, as tschrama experienced when changing R84, can easily be adjusted by increasing R93.
And because drive to the stage decreases, that will also clean up the channel.
One day I also tried driving the OD tube from the clean tube. That worked, but the whole thing went microphonic.
I believe it can be done though, given time and patience. But if you ever get tempted to doing so, that's probably the moment to
get yourself an E530.