Marshall JMP-1 Preamp mods?

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Re: Marshall JMP-1 Preamp mods?

Postby tschrama » 30 Mar 2013, 15:06

.. all above post IMHO are worthless, including the French article. That guy is hopelessly lost about what's actually going on in the JMP1.

So congrats! Finaly someone makes some sense on modding the JMP1: 1] Clean channel POT: great suggestion, 2] increasing EQ sesitivity, great suggestion. I am not sure if you fully realized it: the JMP1's secret is that de clipping comes from IC12b (Vmax 5.6Volt). If you change the volatge divider R92-R91, to prevent clipping at IC12B pin15, you will about 19dB of overdrive.
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Re: Marshall JMP-1 Preamp mods?

Postby Tubetinkerer » 30 Mar 2013, 15:47

tschrama wrote:.. all above post IMHO are worthless, including the French article. That guy is hopelessly lost about what's actually going on in the JMP1.

So congrats! Finaly someone makes some sense on modding the JMP1: 1] Clean channel POT: great suggestion, 2] increasing EQ sesitivity, great suggestion. I am not sure if you fully realized it: the JMP1's secret is that de clipping comes from IC12b (Vmax 5.6Volt). If you change the volatge divider R92-R91, to prevent clipping at IC12B pin15, you will about 19dB of overdrive.


Thx tschrama :)

Well you are right, partially. I forgot IC12b is also part of the distortion, like IC19b (driven to clipping at gain = 200). The reason for changing R92-R91 is to bring the increased ACV from the tubes below IC12b clipping point,
to prevent introducing solidstate clipping. The 19dB loss, as you state, can be compensated by also modding dividers R119-R120 & R121-R122, which I am pretty sure I did.
It's been some time since I performed these mods so I don't remember all the details.
Like you said, there's a lot of mods for JMP-1 around the net. Like the fabeled Voodoo Mod that I doubt for it's value for money. But that's just my two cents.
The best mod for a JMP-1 would be to get in an extra tube (2 stages between gain-control and cathode-follower), but it would take an awful lot of tinkering, replacing the solid state switches with vactrols or relays.
Furthermore I doubt if the transformer can handle an extra tube, since it already has a hard time coping with increased HT (R64)
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Re: Marshall JMP-1 Preamp mods?

Postby tschrama » 30 Mar 2013, 15:52

I bet your mod is more worth than the voodoo.

IC12b clips way, wayyyy before IC19.
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Re: Marshall JMP-1 Preamp mods?

Postby Tubetinkerer » 30 Mar 2013, 16:01

"I bet your mod is more worth than the voodoo."

You think so ? And I just gave it away ... !? Dang. :slap:
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Re: Marshall JMP-1 Preamp mods?

Postby tschrama » 08 Apr 2013, 16:38

I think the designer of the jmp1 made an error in the clean Channel. R84 is 1meg but, to my ears , clearly should be 330k.

I put 470k across r84 ...Much better !
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Re: Marshall JMP-1 Preamp mods?

Postby Tubetinkerer » 08 Apr 2013, 20:57

tschrama wrote:I think the designer of the jmp1 made an error in the clean Channel. R84 is 1meg but, to my ears , clearly should be 330k.

I put 470k across r84 ...Much better !


I don't think it's a designflaw. In a classic Fender tone/volume configuration (which this seems to be. See my earlier post) R84 would be the volumepot. Usually this is a 1M pot.

By changing R84 however, you change the roll-off frequency for R84-C56 (from 1.5kHz to 4.5kHz) so that willchange the tone.
I can't decide on the change though; More or less mids. Please describe "Much better !"
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Re: Marshall JMP-1 Preamp mods?

Postby Tubetinkerer » 08 Apr 2013, 22:13

Oops I should take that back.
I was misled by the way the schematic is drawn up, but you're right!
However, changing the value to 330k is like turning the volume down to the next
stage, putting more emphasis on the bypass cap. Maybe it's "better" to change the cap
to 1nF which would boost the treble just as much without losing
the volume. Maybe worth a try.
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Re: Marshall JMP-1 Preamp mods?

Postby tschrama » 09 Apr 2013, 11:47

The 680K//100p tot 1Meg might look like a small treble boost, but in conjuction with the 12ax7 miller cap, is actually a treble cut. It makes the clean sound dull and liveless without any sparkle. Judging from other marshall design from that period, I have no doubt the marshall enginer didn't realize that at all. At that time, say early 90s, they were absolutely clueless.

Lowering the 1Meg to 330k makes a subtle treble boost and comparable to a Fender volume pot. Since the put a exact copy of a fender-type tonestack just before it, it makes no sence to cut treble after it with a flawed fender fixed volume pot. Once I fixed this to 330K, the familiar Fender sparkle is there. it only occurred to my after the nmod, that the 680K//100P+330K makes a good approximation of a 1Meg fender volume pot.

I have no doubt that at one point, a Marshall enginer designed it this way, but in the end the design way altered for what ever reason. Maybe because it causes the clean channel to be a bit low-volume compared to the OD channels.

I prefer 180pF acros a 1Meg pot for sparkling cleans. Maybe 200-250pF max, but 1nF is definitly too much as it boost the mids.

* Removed the false analysis*
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Re: Marshall JMP-1 Preamp mods?

Postby tschrama » 09 Apr 2013, 12:32

After a bit more thought..

I think the originally meant it to be 680//100pF-330k-gnd, to make the copy of fender tonestack complete. But theye mysteriously lost about 6dB of volume. That would be due to the capacitive feedback of the 100pF and the 2pF anode-grid capacitance, which, I think, they didn't realize. That's why they upped the 330K to 1Meg to make up for it, unknowlingly creating a treble cut of about 4dB at 2KHz and up. Very un-fenderish sounding btw.

BTW I still think you idea to replace the R78,R79,R80 with a pot to dial in the amount of fenderish treble is still an excelent idea.
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Re: Marshall JMP-1 Preamp mods?

Postby Tubetinkerer » 09 Apr 2013, 13:24

"I prefer 180pF acros a 1Meg pot for sparkling cleans. Maybe 200-250pF max, but 1nF is definitly too much as it boost the mids."

It seems you are misled the same way that I was. Rotate the RC-combo clockwise on the schematic and you'll see that u r left with a volume-pot of 1.68 Mohm, preset to whatever (due to log taper), bridged from feed to ouput by a bleed-cap (treble boost).
So changing the cap should have a similar effect to changing 1M to 330k. In the end it all comes down to the cornerfrequency of the RC-combo.

I came up with 1nF without doing any calculations, half asleep after a beer or two. You are right, 1nF is way too much. I think, again without calculations, that 330p would be the way to go.



"fenderish treble from a Marshall" ...... dude ? I am with u though.... 8)
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Re: Marshall JMP-1 Preamp mods?

Postby tschrama » 09 Apr 2013, 16:17

Not being mislead in anyway at all. And it's not just a simple RCtime. You have to take account of the miller cap too. It forms and feedback network with the 100pF cap. It affects the max forward gain around this stage. If you change the 100pF, you'll also change the gain around this stage.
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Re: Marshall JMP-1 Preamp mods?

Postby Tubetinkerer » 09 Apr 2013, 19:41

I can't argue on the Miller cap. It's beyond my understanding, at this point. AFAIK Miller cap only plays a role in inaudible higher (radio)frequencies, but correct me if I am wrong.
Furthermore, the entire tonestack IS definitely a fender tonestack, including all C & R values. The only value that's off is the resistors mimicing a preset pot (1.68 M vs 1M, maybe just to compensate for the attenuation).

I checked the calculation at Ampbooks, and concluding from that, (besides my 330pF gamble being wrong :-/) is that by replacing 1M by 330k, you are virtually turning the pot CCW, thus emphasizing
the higher frequencies from the bleed cap. The RC that defines the roll-off is 100pF||680k (and ||Cm if u wish).
The downside from ur mod is IMO, attenuating the max. volume (or "drive") of the base signal, to the grid.
So I would try to find a solution on the feed side of the "pot", keeping the base signal at a same level.
Maybe just replace all 3 components with standard 120 pF and 1M pot + 680k . Set it to your taste, and measure the balance of the pot.
And then put back components according these values.
I bet ya, this is how they actually engineered the tonestack... on a breadboard with pots..... and a resistance-meter :)
I guess that's how I would do it...

Anyway, thanks for your feedback.
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Re: Marshall JMP-1 Preamp mods?

Postby tschrama » 10 Apr 2013, 09:39

The loss of volume by replacing the 1Meg with 330k is anoying indeed! Allthough, offcourse it can be dealt with, I like the fact that it's such a simple mod (only bridging the 1Meg with 470k).

Replacing the 680K//100pF-1Meg with a 1Megpot//120pF is another excelent idea 8) . Together with your treble pot-mod you it would create a very versatile clean channel. mYou might even wire the pots to the back of the jmp-1 replacing the dreaded speaker-emulated outputs?
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Re: Marshall JMP-1 Preamp mods?

Postby Tubetinkerer » 10 Apr 2013, 11:08

Studied the schematics a bit more and I think I found the answer to the attenuation issue and cause for the increased Rvalue to 1.68M.
The answer, it seems, is in R83. I would expect this this to be 1k-1k5 for a true clean channel. Instead it's 2k7, which reduces gain in VB1b (but induces assymmetrical distortion).
That kind of supports my theory for attenuation compensation in grid feed.
So, if I am right, to install a 1Meg pot||120pf, you should add 680k between pot and ground, to maintain drive level.
If that doesn't give you the treble you want at required drive, change the cap to different value.
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Re: Marshall JMP-1 Preamp mods?

Postby tschrama » 10 Apr 2013, 13:00

The cathodes are not the classical fender 1K5, because of the low V+ (160volt) in the jmp1 versus 260V in a fender*. I think marshall chose 3K for stage1 and 2k7 for stage 2, to keep the bias point, (relatively) similar to the fender bias point; which in fact is assymetrical in both the jmp1 and fender.
The loss at frequencies < 3KHz is about 10dB with the 1Meg//470K mod. Transductance does go up with squareroot of quensient current, but that would only account for 1dB.** Besides: the output volume could be set to any arbitrary level by choosing r93-r94. You just have to keep the peak AC voltages below clipping of the IC18B buffer and the subsequent EQ circuitery. But it beats me, why they just didn't use a 680K/330K combo, 2k7/10uF allround the clean channel.

The jmp1 was marketed as a "all classical marshall sounds from a single midi preamp", yet they cleary copied a fender clean channel. :scratch: And while doing so, they missed every oppertunity to do it correctly. I think these the are the same people that started the Blackstar Amplification brand... :roll:

* which reminds me: intergrid capacitance is higher at low Vanode, which doens't help jmp1's dull clean sound.

**Which leads to something most people don't realize: highest voltage gain is achieved at low quensient current.
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Re: Marshall JMP-1 Preamp mods?

Postby Tubetinkerer » 12 Apr 2013, 08:31

tschrama wrote:The cathodes are not the classical fender 1K5, because of the low V+ (160volt) in the jmp1 versus 260V in a fender*. I think marshall chose 3K for stage1 and 2k7 for stage 2, to keep the bias point, (relatively) similar to the fender bias point; which in fact is assymetrical in both the jmp1 and fender.


I drew up the graphs to check but they do not agree with you. Classic fender design at 1k5 is very symmetrical as these both stages clearly are not (according the graphs)
The other thing is that actual gain is not always equal to maximum gain, because the output voltage for one side of the wave is limited by the clipping point.
If you didn't do R78-R80 mod yet, try that first and maybe your worries for 680k/330k/100p might just dissapear.

Anyways, after rebuilding my rack yesterday, I played around with my JMP-1 for the first time since a few months. I am still very pleased with the results, so I am not going to change anything on the clean channel anymore.
It's not spot on Marshall, neither Fender but it's fine as it is, transparant, clear and dynamic and very usable for SC's as well as HB's.

There is one thing left to try in the modded OD channel though,
and that is to activate the bridge rectifier (i think in conjuction with a zener-diode) for OD2 by means of a vactrol.
But I won't for a while though. Spring is here and my ENGL E530 (yaya, also modded) is doing a great OD2 for me right now.

In the end, the original JMP-1 is not a bad machine; let's not forget that it was (one of ?) the first programmable midi tube preamp(s) available.
It's just not as versatile as one would wish for a preamp with 100 presets (I only used 4 ?!). So you'd really want two of them. A great original one for true Marshall sounds and a modded one for anything else.
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Re: Marshall JMP-1 Preamp mods?

Postby devastator » 13 Apr 2013, 00:48

if a proper traduction of the french document is wanted , let me know .
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Re: Marshall JMP-1 Preamp mods?

Postby tschrama » 13 Apr 2013, 09:46

Fender stages are assymettrical... in general... Let's not forget that Fender uses 1k5 cathodes with 100K anodes in 260V deluxe reverbs (Va avbout 160V) , as well as in 400V twins (Va about 260). In neither case it's symmetrical biased.

If you want to draw load line, don't forget to include the anode load (the tonestack, being about 100K too). Then again...Fender never adjusted anything for high or low anode loads.

Whomever designed the jmp-1 tubes stages, must have thought that with lower V+ you need high Rcath. That was an error, since it limmits the current draw for the load connect to the anode (the fender tone stack) causing clipping way, way before the voltage-rails clipping. A much better solution would be to use a 1k5 Rcath with a lower, say 47K, Ranode. This way the clipping behavior would be much more similar to an real fender amp (where the clean stage was modeled after).

I have a also still have a stock jmp1, so I can compare the stock sound to the moddded jmp1. I can do some clips and would love to hear yours too! You got some cool mods up your sleave.... love to here what you did to your E530..got one too.
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Re: Marshall JMP-1 Preamp mods?

Postby Tubetinkerer » 13 Apr 2013, 15:00

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.
Yes, the AC-load will influence the behaviour of the stage, that's why you'd want to keep the Z(out) as large as possible. However, the great thing about
the tonestack is that it is an integral part of the amplification-stage and that's exactly why it will never sound the same in a ss-pre. This is one of the reasons why tubes rule. (Hell yeah :) )
The tonestack alters the AC-load as it is tweaked, thus changing amplification/attenuation for different frequencies.
(Hat tip to Mr Leo Fender once more)

Well.... I guess whoever designed it like this, did so for a reason. And I suppose carbon-copying fender amps would not have gone down well with the Marshall crowd either.
Somebody would have found out, sooner or later. Let's just stick to "a great inpiration with a marshall twist", ok ?

A rendition on my view off, and mods on E530, can be found here : http://www.ultimate-guitar.com/reviews/ ... index.html
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Re: Marshall JMP-1 Preamp mods?

Postby Tubetinkerer » 13 Apr 2013, 15:14

devastator wrote:if a proper traduction of the french document is wanted , let me know .


If you think it's useful from a technical stance (that is, not from an Audiophile POV), I would certainly be interested.
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