Help with modded 76' JMP 50

RWMusic

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Picked this up the other day. Has 3 mods. Master volume on the back, High Gain mod on channel 1, and an effects loop. Had a couple techs confirm through email that the circuit looks mostly original and that the mods seem to be well done, but no one could really give me any details.

I am hoping the experts here (especially Niekeel, as he seems to know his stuff) can shed some light on what exactly is going on here. I am not sure what type of master volume this is. I am also not sure what the extra preamp tube is for? I'm assuming for the high gain mod?

What exactly is this high gain mod? That extra piece of white board stuck to the side of the circuit board is something I've never seen before. Has a bunch of caps attached to it, but what are they for, and what are they doing? I'm assuming again that this is for the high gain mod?

I was told by a previous owner of this amp that it can get all era's of Marshall tones, from AC/DC to EVH and more, and that of 40 years of owning vintage Marshalls, this one is killer. But he also had no more details on the mods. He bought it with the mods in place, and the amp came from a studio in Canada that shut down.

However to be honest, I can only play it at a lower home volume (night time playing) and with the master way down and treble volume on 10, it seems all I can get is typical plexi gain level. Up to AC/DC but maybe slightly lower then that. Hardly a high gain mod that this claims to be.

My JMP 2203 at similar settings is way more aggressive and more gain at a reasonable home level that's not too loud.

I'm guessing I might have had unrealistic expectations, as I was hoping I could get the hot rodded tones I heard on youtube from modded old marshalls. EVH, Lynch, DiMartini type of stuff. Even in Pete Thorn's video on Vai's "Jose" Marshall, when he's talking to Dave Friedman, they are able to get a really saturated tone as speaking level. I guess that was my unrealistic expectation

Seems like you have to dime the master still, but at that point the amp is unplayable to me as it's too loud.

Maybe I'm not using it as intended and need to invest in an attenuator, even though I really don't want to have to go that route.

One weird thing that I haven't seen before is that when the master is set high or on full, and all the volume controls on the front of the amp are at 0. There is still a clean signal. Not loud, but maybe speaking/TV level. Is that supposed to happen?

Anyways, here are the pics. I'm hoping I can get educated on what exactly I have here.
 
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RWMusic

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Looks, from a considerable distance, like a butchered amplifier.
I would return it to original and if you don't like it, sell it on and buy something you do.

Any details on what was done based on these pics? I'm looking for details on these mods and what they are doing if possible.
 

neikeel

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The amp is broadly original but has had some mods.
The MV appears to be inserted between the tone stack and pi so a conventional pre pi MV. On my phone I can’t make out the mf resistor bands but likely a 150-220k divider to ground because the signal from the extra tube stage is too ‘hot’.
Looking at the board it looks like an extra triode stage (maybe even two) inserted between v1 and cathode follower. I would need more and better pics to be sure
How does it sound?
 

RWMusic

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The amp is broadly original but has had some mods.
The MV appears to be inserted between the tone stack and pi so a conventional pre pi MV. On my phone I can’t make out the mf resistor bands but likely a 150-220k divider to ground because the signal from the extra tube stage is too ‘hot’.
Looking at the board it looks like an extra triode stage (maybe even two) inserted between v1 and cathode follower. I would need more and better pics to be sure
How does it sound?

Well, I can only play with the master down very low, and with that setting all I can say is 'meh'. My 2203 sounds way better at low volumes. Can I email you these pictures in a larger form if you want. What would this extra triode stage be doing? Is that the "high gain mod"?

I was under the impression that I could get more saturated tones at lower volumes, but that doesn't seem to be the case. I wanted something more than my 2203. Not sure if I want to get an attenuator, or sinking more money into this.

I know nothing about how amps work or anything about electronics, so I apologize.

Also, why would there still be signal coming through with the master on full and volume knobs on 0?
 

RWMusic

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Maybe the extra valve handles the fx loop send and return circuits?
:shrug:
More, less blurry photos needed.
Assuming original photos are good, imgbb seems to allow better image quality that imgur.

here are the images on imgbb. Hopefully they are better.

 

neikeel

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I think Pete is correct now I can see better.
The extra tube and bread board looks to be to the loop which is (as is typical) between the tone stack, before the pre-PI MV.
Personally I would prefer NMV without master and use a good attenuator. The loop will depend on design and implementation.
The black wires with red shrink wrap are just screened grid wires on V1 and can be left alone.
 

RWMusic

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I think Pete is correct now I can see better.
The extra tube and bread board looks to be to the loop which is (as is typical) between the tone stack, before the pre-PI MV.
Personally I would prefer NMV without master and use a good attenuator. The loop will depend on design and implementation.
The black wires with red shrink wrap are just screened grid wires on V1 and can be left alone.

Sorry for all the questions. Does it look like any gain mods were done? Also is it normal for the Master on full and volume knobs on 0 to still produce a clean tone? Maybe this is what the gain mod is. start with signal, even on 0 and then turning up the amp adds on top of that to break it up quicker?
 

neikeel

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I cannot tell for sure but from the new pics and on my lap top I can’t see any gain mods, just a loop and pre pi MV.
I had a late 76 JMP50 (stock) and it was a beast!!
 

Pete Farrington

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is it normal for the Master on full and volume knobs on 0 to still produce a clean tone?
This is a powerful stage amp in a domestic setting.
The real question is ‘would it cause a problem in a stage, rather than domestic, scenario’?
The pots used in amps are cheap with wide tolerances. A bit of signal getting through on zero is not going to cause a problem when the amp is used for its intended application.
It’s neither a fault or a design feature per se, rather just one of those things.
If it’s really bugging you, then try replacing the pots.
 

playloud

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Have you tried cranking it on full, even for a couple of chords?

That might help you decide whether an attenuator is worthwhile :)
 

RWMusic

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Have you tried cranking it on full, even for a couple of chords?

That might help you decide whether an attenuator is worthwhile :)

I generally know how these amps sound because I used to own a Metropoulos 10,000 series about 8 years ago, with a THD HotPlate both sold a long time ago when I went on my failed journey with Modeling technology. It's just a matter of me assessing whether I really want to continue to spend more money. I spend a lot of money on this amp, with the thinking that I might not need an attenuator because of the master volume and high gain mod. If I got one I'd want a Fryette PS2 which is another $800 that I honestly don't want to spend.

Looks like assessment from multiple people says that there is no high gain mod here, Just MV and effects loop.

I have 3 options basically:

1. Leave it as is, and try to enjoy it for what it is. Then in a few years it will probably rise more in value even with the mods and I can sell if I want. I can maybe goose it with a pedal to get the saturation I was looking for. Although I really hoped that I didn't have to do that. Basically I was expecting 80's "Jose" hot rodded marshall, but it seems like its a stock 4 hole with an added MV. Which is not bad, it's the classic Marshall tone. Just I was hoping for something more.

2. Sell it as is. Hopefully break even.

3. Spend more money and get the MV and effects loop removed, holes plugged and returned to stock. Amp would basically be unusable to me without an attenuator. Either spend additional money on an attenuator, or hold on to it in stock state for a few years where maybe it will be worth even more being stock and I can sell it then.

Don't get me wrong. I'm not saying it sounds bad. It sounds good with the master low. Think early AC/DC, or Free. But based on how the amp was presented to me, saying it had a high gain mod, and claiming it can do most era's of Marshall gain. I haven't found that to be the case so far.
 

playloud

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I generally know how these amps sound because I used to own a Metropoulos 10,000 series about 8 years ago, with a THD HotPlate both sold a long time ago when I went on my failed journey with Modeling technology. It's just a matter of me assessing whether I really want to continue to spend more money. I spend a lot of money on this amp, with the thinking that I might not need an attenuator because of the master volume and high gain mod. If I got one I'd want a Fryette PS2 which is another $800 that I honestly don't want to spend.

Looks like assessment from multiple people says that there is no high gain mod here, Just MV and effects loop.

I have 3 options basically:

1. Leave it as is, and try to enjoy it for what it is. Then in a few years it will probably rise more in value even with the mods and I can sell if I want. I can maybe goose it with a pedal to get the saturation I was looking for. Although I really hoped that I didn't have to do that. Basically I was expecting 80's "Jose" hot rodded marshall, but it seems like its a stock 4 hole with an added MV. Which is not bad, it's the classic Marshall tone. Just I was hoping for something more.

2. Sell it as is. Hopefully break even.

3. Spend more money and get the MV and effects loop removed, holes plugged and returned to stock. Amp would basically be unusable to me without an attenuator. Either spend additional money on an attenuator, or hold on to it in stock state for a few years where maybe it will be worth even more being stock and I can sell it then.

Don't get me wrong. I'm not saying it sounds bad. It sounds good with the master low. Think early AC/DC, or Free. But based on how the amp was presented to me, saying it had a high gain mod, and claiming it can do most era's of Marshall gain. I haven't found that to be the case so far.

Ah, now I see.

If it were me, I'd rule out option 1 (life being too short and so on).

3 sounds like a lot of money to spend for something that would ultimately be a compromise.

What about getting the amp "remodded" along the lines you're looking for (high gain etc)? You already have the extra tube socket installed. There are plenty of people out there who could turn this platform into something really special (great bones!) Even a couple of members here in fact...
 

ApathyForum

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What about getting the amp "remodded" along the lines you're looking for (high gain etc)? You already have the extra tube socket installed. There are plenty of people out there who could turn this platform into something really special (great bones!) Even a couple of members here in fact...

I’ll bet forum member David Hopkins (New Religion), who mods Marshalls, would give you some good advice in terms of options, etc.
 

RWMusic

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Ah, now I see.

If it were me, I'd rule out option 1 (life being too short and so on).

3 sounds like a lot of money to spend for something that would ultimately be a compromise.

What about getting the amp "remodded" along the lines you're looking for (high gain etc)? You already have the extra tube socket installed. There are plenty of people out there who could turn this platform into something really special (great bones!) Even a couple of members here in fact...
Hmm, that's a possibility. I'll think that over. Any recomendations on people that get me what I'm looking for? I would want something that's reversable in case I want to revert back in the future.
 

NewReligion

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Hello @RWMusic & my other friends here at
The Marshall Forum.

I have read through all of the posts. I have exploded the photos as well.

1.) I see only stock value components on the PCB where they should be. That is to say I see no cascaded gain circuit to include no added triode for gain in the circuit. So, you are only getting 2 gain stages at a time regardless of plugging in to Loudness 1 (Volume 1) Hi / Low or Loudness 2 (Volume 2).

This is why your other amp has more gain as it is using 3 gain stage triodes cascaded with a stock post tone stack master volume.

2.) My money is on the extra tube buffering the effects loop as the gentleman here have already assessed.

3.) What you are referring to on the rear plate of the chassis as a possible post phase inverter master volume I will suggest not being intended as a master volume but rather a effect loop Return potentiometer.

4.) Look at V1 pin 7 Grid feed connection using shielded wire. The shielding terminates dangerously on pin 6 which is the anode (plate) which caries the B+ a.k.a. The DC voltage which at times may approach 500VDC however is likely dropped to around 200VDC or lower at this point in the circuit.

My observations of use and possibilities are as follows.

A.) You have a fine amp which can be used as a very loud stock 1987 model with an effect loop with a return level control as it is though I can not support the Lee Jackson style of wrapping the green wire around other connected leads to be used as noise canceling shielding.

In which case you could use an Alesis Quadraverb or some similar effect processor for verb/delays/pitch etc.. but mor importantly attenuate the volume level using the effect processor level send or return (as the Quadraverb has each on the front panel) as a master volume as I have many times. But you will need to add a over drive pedal for Lynch type gain levels.

B.) You also could easily cascade the preamp by doing the one wire mod which cascaded the gain stages giving you 3 stages turning Loudness 1 into the master volume & Loudness 2 into a preamp Gain control. You still will wish to have a over drive pedal. And still have the option of using the effect loop assuming it functions properly enough to not add negative anomalies.

C.) Send it to someone for a proper restoration which defeats your intended wishes. I will flat out tell you if this is as I see it the sell greatly exaggerated the versatility of the circuit.

D.) Or send it to someone to modify the circuit to your liking. There are a few guys to choose from. I myself…(I will post a couple examples with guitar plugged straight in to include a at whisper level master volume demo).

Shea at Monomyth Amplification or Jeremy at Ground Zero Ampworx for the Fortin Cali type mods. I can do them but I stay out of that wheel house and focus on post tone stack master volume analog circuitry though Cameron gave me the lead channel schematic to his CCV. I also have done Marks version of the Jose & similar Atomica mods.

Michael Audette a.k.a. Michael R/T for what I consider comparable to the Mod 5 which PPIMV in conjunction with the one wire mod. This mod negates the Presence/NFB & severally limits the use of the TMB Treble Middle Bass Tone Stack. You will often see the pots on the front full up as the Dual Gang 250k or 500k post phase inverter master volume is often located in the rear. In essence there are about 4 types of these PPIMV’s as outlined in Ken Fischer’s Trainwreck pages. These PPIMV tap into the bias feed resistor circuit (usually two each 220k or 150k resistors) this is how overall volume is reduced.

When Ken built his famous Trainwreck amps he did not bother with a Presence pot as it was useless due to the PPIMV’s used.

Of course you can reach out to any other members here as there are many who do killer work.

I believe Dave Friedman still takes mod work.

Voodoo Amplification.

There are quite a few more I believe that are available which I apologize for not mentioning.

It is up to you to research & listen to the work offered by the guys which make theirselves available to work with you.

Below are a few of my short videos to get you started should you decide to go for what you thought you had.

Thank you for your interest & reaching out to me.

Best Regards, David Hopkins

ALL DEMO’s ARE GUITAR PLUGGED STRAIGHT IN:

Hopkins Pandora 3 & 4 high & low gain stages. Sounds true at a whisper.
WHISPER MASTER VOLUME DEMO:


Pandora High Gain 4th stage input:
Lead Lines:


Pandora Chords & Lead Lines mixed:
Clean to Mean:


Axis 4 & 5 Gain stages:
Playing in the style of George Lynch:


EVL (Evolved Valve Logic) My take on the Snorkler as I wished to hear it. Reference circuit contacts include Mark Cameron, Owner of the Snorkler at the time Ronnie Champagne (very nice guy) & Bogner schematics I have:

In the style of Jerry Cantrell of Alice In Chains:


Okay, enough from me on the subject. I hope I was able to help clear up what you have the your options as I see them.

Again, thanks for the interest, David
 
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RWMusic

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@NewReligion Thank you for your analysis it was really helpful. There is one part I want to make sure I'm clear on. You're saying the control on the back was probably meant as a effects loop return level, but can double as a master volume? Which is why the volume of the whole amp lowers when I use it?

Also, can you explain that green wire shielding portion again? You said you can't support it, meaning that this is something that can damage the amp? Is this something that is advised to be removed if I decide to keep the amp as is?
 


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