Marshall JMP-1 Preamp mods?

Tube or solid-state, this section goes to eleven!

Re: Marshall JMP-1 Preamp mods?

Postby tschrama » 13 Apr 2013, 19:27

Tubetinkerer wrote:I am sorry, but to my knowledge, you are wrong on the fender bias. 100k plate and 1.5k cathode will result in a dead-center DC-bias voltage, whatever the plate-voltage.
This is also the recipe for largest possible headroom.....


That's a gross misinformation. Center-bias depends heavely on V+. Neither does center bias does automatically result in largest headroom (or lowest distorting, or highest gain for that matter, which do coincide).

Great review btw. The E530 is indeed nice. But mine developped a noise issue in the clean-high gain setting. A relay issue?
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Re: Marshall JMP-1 Preamp mods?

Postby Tubetinkerer » 14 Apr 2013, 20:03

tschrama wrote:That's a gross misinformation. Center-bias depends heavely on V+. Neither does center bias does automatically result in largest headroom (or lowest distorting, or highest gain for that matter, which do coincide).


We do not agree, obviously. Please provide a link to prove me wrong. I don't want to be a wiseguy, but obviously I learned by the wrong info and my knowledge needs correction, or at least suppletion.
AFAIK DC-cathode-grid-bias is dependant on Rplate and Rcathode. At infinite DC load and disregarding the AC-load, 100k plate/1k5 cathode, will result in center-bias independant of V+ (I checked this on the graphs)
This is supported by the idea that preamp valves are configured as self-biasing stages. (Maybe this is different for power-valve-stages. (That I do not know))
At infinite (or relatively large) AC-load, the DC and AC loadlines will line up. This will result in max headroom, (highest possible gain) and symmetrical distortion if any.
AC-load shifts the AC-loadline from the DC-loadline and thus determines/alters the frequency response of the stage, dependant of the filtering of the AC-load. Where am I wrong ?

Anyway, this is not a matter of opinion, but a matter of technical knowledge. Maybe we should take this to a new thread ?

tschrama wrote:Great review btw. The E530 is indeed nice. But mine developped a noise issue in the clean-high gain setting. A relay issue?


Relay issue ? Tube gone south ? C16 ? Soldering joint ? (I noticed these are quite hard and thin, so prone to cracking)
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Re: Marshall JMP-1 Preamp mods?

Postby Jooshtin » 14 Apr 2013, 21:17

Check out http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

Interesting ideas, especially for the clean channel which I hardly use, it would be good to make it more useful. I find mine cleans up pretty well on OD1 with v low gain settings, even with my overwound PAFs, just by backing off the guitar volume etc. The gain is quite fierce though! The JMP-1 has that 90s jcm800/900/Jubilee Marshall sound which use diodes for distortion, and it does it very well, I'd recommend anyone who doesn't want that sound not to get one!
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Re: Marshall JMP-1 Preamp mods?

Postby tschrama » 15 Apr 2013, 15:39

Jooshtin wrote:Check out http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

Interesting ideas, especially for the clean channel which I hardly use, it would be good to make it more useful. I find mine cleans up pretty well on OD1 with v low gain settings, even with my overwound PAFs, just by backing off the guitar volume etc. The gain is quite fierce though! The JMP-1 has that 90s jcm800/900/Jubilee Marshall sound which use diodes for distortion, and it does it very well, I'd recommend anyone who doesn't want that sound not to get one!



Again, in the jmp-1 the distorion does not come for the diodes ...... :?
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Re: Marshall JMP-1 Preamp mods?

Postby tschrama » 15 Apr 2013, 18:15

You a wiseguy in a nice way, with good ideas about mods.

no link...I don't know how you come to those conclusions about biasing ; just beware that the internet forums are full of bullshit. Just use your scope and learn more that than the internet can ever teach you.

I'll attach graphs with plate curves, lines for center biasing values for three differen V+ volatges, and one set of lines for symmetrical clipping value (which is not the same a maximum gain or lowest distortion).

center bias cathode Ecc83s JJ.jpg
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Re: Marshall JMP-1 Preamp mods?

Postby Tubetinkerer » 15 Apr 2013, 19:04

Well.......dunno dude....... :scratch:
Graphs.jpg

here's mine.... looks pretty deadcentered to me..... 8)
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Re: Marshall JMP-1 Preamp mods?

Postby Jooshtin » 15 Apr 2013, 19:13

Tschrama - I didn't say whether the jmp-1 distortion comes from the diodes or not, read it again sir.... You have got me intrigued through, I'll have to get my scooe out and have a look! You're right about internet forums... I'd recommend Tubetinkerer head over to Merlin's Valvwizard site (excellent books!), Aiken Amps tech pages, Kevin O'Connors books are full of great info, and of course a must have is the Radiotron Designer's Handbook http://archive.org/details/RadioDesignersHandbook
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Re: Marshall JMP-1 Preamp mods?

Postby Jooshtin » 15 Apr 2013, 19:14

What's a scooe - I meant scope!
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Re: Marshall JMP-1 Preamp mods?

Postby Tubetinkerer » 15 Apr 2013, 19:21

Jooshtin wrote: I find mine cleans up pretty well on OD1 with v low gain settings, even with my overwound PAFs, just by backing off the guitar volume etc. .......................................
....................... I'd recommend anyone who doesn't want that sound not to get one!



Thx for your input, Jooshtin. I totally agree on OD1. I used it for 2 presets; driven cleans and rhytm. Then I had one for "as clean as possible" and one (OD2) for high gain solos, which would pretty much drown if only a very little delay
was added. And then I had another 96 presets which I couldn't find any use for. (Modding the feedback resistors in the EQ makes it much more flexibel and useful)
Nowadays I doubt I would have gotten one, but at the time of introduction (when I got it), there wasn't that much of choice.
Maybe Triaxis (expensive) or ADA (unknown to me). So the JMP-1 seemed a safe bet at that time.
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Re: Marshall JMP-1 Preamp mods?

Postby Tubetinkerer » 15 Apr 2013, 19:27

Jooshtin wrote:........... I'd recommend Tubetinkerer head over to Merlin's Valvwizard site (excellent books!), Aiken Amps tech pages, Kevin O'Connors books are full of great info, and of course a must have is the Radiotron Designer's Handbook http://archive.org/details/RadioDesignersHandbook


Thanks... will check it out....... and learn. :idea: Still pretty much a noob on the subject.
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Re: Marshall JMP-1 Preamp mods?

Postby tschrama » 15 Apr 2013, 20:26

Tubetinkerer wrote:Well.......dunno dude....... :scratch:
Graphs.jpg

here's mine.... looks pretty deadcentered to me..... 8)


For what V+ is it actually deadcentre then?

I see V+150, biased at 90V, 250, biased at 160, and 360 biased at 235volt.
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Re: Marshall JMP-1 Preamp mods?

Postby tschrama » 15 Apr 2013, 20:36

Right.. it is finaly getting to my that we used different meaning for "centre bias".


For centre-biasing, concerning availeble grid volatges,, roughly about 2/3rds HT, 1K5 gives roughly good results for a widerange of HT.
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Re: Marshall JMP-1 Preamp mods?

Postby Tubetinkerer » 20 Apr 2013, 09:49

tschrama wrote:
Great review btw. The E530 is indeed nice. But mine developped a noise issue in the clean-high gain setting. A relay issue?


I was changing tubes on the E530 and I also suddenly had a noise issue in clean high-gain. Turned out that the pins of the older tubes i had laying around, were corroded.
Maybe worth checking.
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Re: Marshall JMP-1 Preamp mods?

Postby tschrama » 24 Apr 2013, 11:34

Opened my E530 this morning.. poked around.. turns out the Omron relay at the right (when facing the preamps front) has an issue causing lots of noise on the Clean-High and Lead-Low channels. A few firm tabs on the relay fixed it for now.

It's a damm fine sounding preamp if you ask me 8)
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Re: Marshall JMP-1 Preamp mods?

Postby Tubetinkerer » 25 Apr 2013, 09:22

tschrama wrote:It's a damm fine sounding preamp if you ask me 8)


Still stock ?
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Re: Marshall JMP-1 Preamp mods?

Postby Funkystrings » 26 Apr 2013, 19:17

Tubetinkerer wrote: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. :-)


I just now registered in order to speak to tubetinkerer and tschrama. I agree that the French mod post on this thread is pretty useless and that tubetinkerer's approach is very impressive and addresses more of the JMP-1 shortcomings which are; not enough eq range, needs a better clean sound, and the tubes should be hot enough to produce some clipping while the diodes produce less. I've been googling for a thread like this for 15 years now. I'm begging you. Please don't abandon this thread. I only wish you both had you're preamps open, and on the bench and were coming to more of a consensus on getting the Fender like clean sound more up to unity volume against the OD channels while keeping it as fenderish as possible as it's already a bit low volume stock. And yes, I would so love to hear clips!, I've got a shit load of resister and cap values and I'm ready to tackle this but please keep in mind that I'm a little limited when I hear some of the amp design terminology you guys use. Reading a schematic is fairly new to me as well. I'm just a tone fanatic w/ a soldering iron. for example, I did the so called Cameron mod to my version 1, 5150 head because it's so easily reversible and I ended up only keeping the lifted cap that reduces bass in the clean channel and that alone made it all worth wild because now I can finally actually use the channel switching on it live sense the amp has shared eq for both channels and the clean was always too bassy. The problem for me was that the 5150 doesn't have the component numbers printed on the circuit board so it wasn't until they got the pics up that I was able to make sense of the mod. I find it interesting you both have E530s and JMP-1s. Would you both agree when comparing the two, that the JMP-1 is a bit too bright and lacks bass w/ the bass shift off? I don't have an E530 but I find the bass shift on the JMP-1 a bit too mushy, even though it does add bass at mid gain settings and when I disengage it, it sounds more open but a +6 setting is not enough. If you watch the Premierguitar Rig Rundown youtube for Billy Gibbons. you see that he sets it at bass +6, mid+3, pres -6 and treb-6. That's ridicules!!!! that he has to struggle to those extremes to get more bass and less highs. I do too. If I run it through my bright Vintage 30s, it's even worse. I'm going to change that bass resistor value first in order to get the bass up. Tubetinkerer's mod, that runs the tubes hotter and diodes less at the expense of some OD gain is fine w/me also as long as it ends up sounding more like the all tube Marshalls of yesteryear (even though it's always going to be a tube/ diode hybrid). I find that the JMP-1 has tons of gain on tap to start with. I just so! want to hear some clips from both tcshrama and tubetinkerer, switching between an OD and clean patch to show there's no loss of volume happening w/ the clean channel. Or that two patches can at least be programed in such a way to make up for the clean channels drop in volume, and sound good doing it.

So my initial questions are to tubetinkerer; Question 1, which one of the "feedback resistors (R177/R158/R178/R181)", effects the bass? Oh, I figured it out on the schematic: R177= Bass, R158= Middle, R178= Presence, and R181= Treble. Upping their values increase their ranges. I may just lower! the Middle value by 2 to make it's range less and more tweekable because I find that I never scoop or add a lot of mids with it stock.

Quesion 2, when you say, "Clip 1 side of D12, this takes the rectifier from the circuit. Decouple pin 13 of IC12a from R125" That when right over my head. could you post a photo of this for us lay people?

Question 3, when you say, "Use a vactrol to take over IC12a´s function. This requires some fiddling with the circuit around TR9". Could you explain what a vactrol is? and it's component value, if any, are? and component the values at TR9?

Question 4, when you say, "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". To what safe values did you change them to?
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Re: Marshall JMP-1 Preamp mods?

Postby deltafred » 27 Apr 2013, 11:57

Welcome to FSB Funkystrings.

If I might make an observation, your first paragraph takes a bit of reading, any paragraph that length will put a lot of members off reading it. It would really benefit from a few carriage returns as unlike paper screen space costs nothing.

Funkystrings wrote:I just now registered in order to speak to tubetinkerer and tschrama....


.... Or that two patches can at least be programed in such a way to make up for the clean channels drop in volume, and sound good doing it.
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Re: Marshall JMP-1 Preamp mods?

Postby Tubetinkerer » 28 Apr 2013, 15:23

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. :-)
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Re: Marshall JMP-1 Preamp mods?

Postby Funkystrings » 29 Apr 2013, 18:59

Sorry deltafred. It didn't look too bad in it's draft state, but when I hit post, it crunched it up even more. I totally agree. some carriage returns are needed.

Thanks for your thorough response Tubetinkerer. I so appreciate it. Your knowledge of this pre is mind-blowing. I opened mine up on the bench last night after reading your response and played it for the first time in months. I tuned my ears first by playing through the 5150 alone and got the massive sound they're known for. and then through the JMP-1 into the 5150s return which leaves only the resonance and presence controls available for tweaking on the 5150, and as usual, I instantly had to boost the resonance (for more lows), and reduce presence quit a bit to get the JMP-1s factory presets to sound balanced so I know I want to do that bass boost mod first.

I like to use the Marshall factory presets as a starting point and then create my own patches but I always find myself wanting some "in between sounds" from the midrange. For example, -3.5 as opposed to -4 would be so much more desirable. Or on factory patch 05, in which Marshall only boosted the mids to +1 and ended up naming it "Nasal Tone". I could actually see myself wanting to use that patch in certain applications if the mids were set to +1/2. It would make the whole preamp more like turning an analog knob to get the mids perfect. I know it's all subjective, but personally, I would think that if stories popping up on the net say that you need hours and hours of tweaking to get it "just right", it would be because of the mids, sense those are the most humanly audible frequencies. I don't think a good tone stack should even produce an overly Nasal or scooped sound. But again, that's subjective.

Lowering the midrange's sweep range just makes more sense to me because factory preset 08 "Filth", which uses -6 on the mids is lol thin sounding, (it's also my least favorite patch). Although when I boost it to -4 mid, it begins to sounds right to me. That's always the case, no matter what speaker I use. I have a WGS Green Beret, an ET65 and a Celestion Vintage 30 loaded Marshal 4x12 to test with. So that's my goal.... to make -6 sound like -4 and make patches like 15 "Mid-lands", and 16 "Sunshine", (two more presets I don't really like), which use +6 mid settings sound better. I think they sound better at +4 middle after adjusting the highs accordingly. I just think the JMP-1 boosts and scoops too much and I happen to LIKE scooping mids out to get good high gain sounds so I might as well change it's midrange to; -4 through +4, and have 12 settings to choose from for mid tweaking.

Thanks for giving us the values of your eq mod and also for clarifying that the values need to be lowered to increase range and upped to decrease. I believe i'll start by decreasing R177, as you did, from 2K7 to 2K2 for the Bass increase. And then increasing R158 from 6K8 to 8K2 for the Middle and see what happens. I'll let the forum know after I try it.

Also, regarding the clean sounds, (as my preamp sits on the bench completely stock at this point); The cleans surprised me last night. Patches 22 "Grit Clean" and 17 " Britaevious" with their higher gain settings for example. Even to the point where I got a nice Nashville sound when plugging in a Tele after tweaking them a little, and then it dawned on me.... it reminded me of my first amp which was a Musicman 112 Sixty Five, which had a Solid state preamp and was designed by Leo Fender himself. So i'm wondering if that's as good as the JMP-1 is ever going to get with clean sounds. A lot of people still like old Musicman amps although I prefer Fenders with all tube circuitry.

Does the clean section of the JMP-1 use 100% of that 12ax7? or is it also a hybrid?

My approach to modding anything audio related is; "if it ain't broke, don't fix it, and if it's broke just a little, fix it just enough" and also be sure it's reversible. So I'd really like to just do little things at first to get the ball rolling and hear if I like it after A/B ing it because I don't hear that the JMP-1 has huge problems with it's sound. I just don't use it anymore because my 5150 and Boogie Studio Preamp sounds better. I'm just wondering if I can get this thing to change my mind.

Also, as you can probably tell, I'm a fan of the original 5150 and apparently, there's two opinions on the net regarding it's sound, and that is, if you don't like the way it handles high end, you call it "fizz" and try to modify it, but if you do like the way it handles high end, you call it "that great 5150 sizzle" and leave it alone. I'm of the later group. I don't know if you know the story, but apparently Eddy insisted on adding a fifth 12ax7 at the end of the preamp just prior to it's release because he wanted the gain to be "out of control" beyond 6 and 10, (in his words). And trust me, those settings are unusable. You get nothing but hiss and over compression in that region! This adds some kind of mojo to settings between 4 and 6. He used to set it between 5 and 6. Maybe the JMP-1 could use a little of that. If it's OD tube was a little hotter.

So my question now is this; Could I first just try boosting that 12ax7 slightly for the OD sounds? and then just lower all the gain settings in my presets? Or would that blow something out? I'd like to make the JMP-1's overdrive a little more "out of control" like the 5150, and utilize it's OD 12ax7 tube the way the infamous "fifth tube" of the 5150 did sense that's why it was put there in the first place in both amps. I just think the tube in the JMP-1 should be a bit hotter so that if the gain was maxed at 20, it would then be a little "out of control" and start hissing like a snake the way the 5150 does when maxed. thus forcing one to simply turn the gain down on the JMP-1 so that say, maybe 10 would now have the same signal to noise ratio of the stock maxed 20 setting. Just to see if some mojo happens in that 10 region. Would that be simple enough? how would you go about this?
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Re: Marshall JMP-1 Preamp mods?

Postby tschrama » 29 Apr 2013, 20:57

sorry ... TLDNR 2x

but I did right someone trying to drive the OD tube from the clean tube.... dammm I envy such creative minds. Nice idea 8) ! They should have just done so in the first place. 4 tube stages can get you any where.

I propose not to change the whole preamp, but go step by step, adressing your issues. The Bass-shift, work pre-clipping. So it doesn't really at bass to you output, just to your input. It makes humbuckers mushy. For singlecoils it might be usefull.

Q1: more bass.. sure it can be done, but the EQ focusses the bass around (dunno) 100Hz. So upping this band, makes you loose some < 100Hz. So here the trick: keep the bass at 0 to keep the most sub-bass. Adjust mid, trbel and pres from there. Got this trick from the ADA MP-1. For a more sparkling clean: follow tubetinkerers idea of putting a 250K pot in the fixed tonestack. It's adjustable and elegant. I think BG uses B6, M3, T0, P0. (dunno) .. and it all depends on the power amp and speaker. But at the end of the day, and GEQ just doesn't repond like a Marshall tonestack. e.g. upping the Marshall tone stack bass controll. increases the bass (duhh!), but also allows more sub-bass. Typical GEQ, do the opposite:, they relatively limmit the sub-bass when you dial them up! The best aaproximation of a marshall tonestack (B6M4T6), with the jmp-1 is with B0, M-6, T0, P0, and adjust from there.

Q2: Just to het the message home: the dioded do NothinG for the clipping in the jmp-1. It might prevent some bias drifting due to grid current. But essentially, the diodes harly effect the sound. Go ahead , snipe them out, and play again... much change?

Q3: IC12 is the fisrt element in the signal chain that clips. Not the diodes, not the tubes, it's IC12. If you want to prevent that, you might ne able to use a vactrol (like the ADA MP-1) (LCR/LED combo). But how are you gonna drive the vactrol... I know! by an IC like the IC12 (Like the AD MP1)!

Q4: most IC's are diode protected. Unless lots of current flow, they'll be ok. IC12 has protective diodes, making them clip 0.6V above Vdig.

Let's just get you through one issue at a time, and suggets one mod at a time. ....would love to compare clips via PM.
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Funkystrings (30 Apr 2013, 20:33)
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