Help with modded 76' JMP 50

David Rivers

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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.

maybe keep it as it is..and use a pedal to get that extra gain stage your looking for,..although with all the upgrades you mentioned i'm surprised its not doing it for you at the low volumes..sometimes tubes can make a difference but tube replacing can be an added cost as good tubes are not cheap to replace..dont feel you got to stay with that particular marshall if your not happy with the sound..maybe you can try amps locally to find something more suited..but like everything else the older equipment cost are going thru the roof and buying new is not cheap either..i have a marshall yjm signature 100/50 watt amp..and i also have an Orange AD30..tube amp..which is also a great amp ,best luck i hope you can find your perfect amp
 

Pete Farrington

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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
Just to note that KF had a specific design preference for his Trainwreck amps not to have master volume controls.
The confusion understandably arises as he documented and provided a nomenclature for master volume types 1 to 4 in The Trainwreck Pages p32, under the heading ‘Master Volume Mods for Marshalls’.

Interestingly, in regard of the Type 4 master volume fiited to the OP’s amp, KF advises it should be used when adding an additional gain stage.

Of the 3 main Trainwreck models, the 2 with a nfb loop (Express and Liverpool) have a presence control. The Rocket, being heavily based on an AC30 top boost channel, can’t have a typical nfb based presence control. Rather it has the AC15/30 type tone ‘cut’ control.
 

Norfolk Martin

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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?

I know that the 1976 Marshall 2204s were different from the 77 and later The later ones use two stages cascaded for the high input, giving a lot more crunch, but making it impossible to plug into both the high and low inputs at the same time. The 76 version didn't used the cascaded stages for the high, but used one half of a 12ax7 for each input If you can put a plug in the high input and the low input still works, yours probably has a similar circuit. These do not exhibit the same front end gain and overdrive.
 

NewReligion

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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?
I see there has been clarification to my Ken Fischer data which is good to know. However, if ever using a Lar-Mar type PPIMV the Tone Stack is next to useless as is the Presence.

Thank you @Pete Farrington for the Ken Fischer clarifications. I use some of Ken’s practices. He was brilliant. I especially make use of the non coplanar layout options to reduce unwanted capacitance in high gain circuits which often can lead to unwanted mid frequencies. This is where many of us differ on opinions. But back to the OP.

@RWMusic As for your questions to me:

1.) The pot on the back still remains ambiguous to me as depending on how it is wired which we can not definitively know at this point it could be a effect loop return as the white wires are in a configuration to make it so. Perhaps it is a master volume BUT you stated if one of what you referred to as a volume is turned all the way down you get a signal. That is not a good design and lead my to the possibility confirmation it may be a loop return instead of a master volume.

2.) Green wire as shielding. No it is not harmful but a lazy way to attempt reducing signal to noise (grounding). Think if you did have a very high gain circuit. There are better ways to lay out ground schemes.

3.) If moving forward with the amp as is I would address the shielding wire from V1 pin 7 that has its shielding connected to V1 pin 6 (High B+ Voltage). I would try lifting the shielding wire off of the V1 pin 6 first just to see if it affects the signal to noise ratio. If not clip it off. If so, then unsolder that grid feed wire from pins 6 & 7, turn the insulated wire around 180 degrees & solder the shielding to a known ground such as the ground side of a cathode.

Sorry for any confusion on Ken Fischer’s work as I am half asleep at 4am. I too believe there were three models but each were individually tuned to his ear using different types and values of components as I have been told which makes it hearsay. I indeed was referring to the Rocket. Other than what is in the Trainwreck Pages it is all hearsay to me as I have never been inside of a Fischer Trainwreck.

And I should have specifically noted the model (Rocket) with no Presence but again the point being that most Post PIMV’s will limit your ability to adjust Treble, Middle, Bass & negate the Presence/NFB.

NOTE: 1976 is referred to as being the first year Master Volume & cascaded gain year. However by and large only the 100 watt 2203 fit this profile as it wasn’t until 1977 that the 2204 with a Master Volume was correctly cascaded producing more gain for overdriven tones.

Hope this clarification on my attribute helps.

David
 
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RWMusic

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@NewReligion @neikeel @Pete Farrington Ok, final questions from me

1. With the amp as is, which seems to be stock 1987 with added tube buffered effects loop and master volume, can any of these additions have a negative effect on the true sound of the amp? Can the effects loop be possibly sucking tone, or is it only in effect when something is plugged into it? For the master volume, if it’s on full it should be out of the circuit right? Finally, if the added preamp tube were to fail, does the amp get damaged, or does the effects loop just stop working?

2. If I decided to restore the amp and remove these mods, other than the additional holes for the tube, master, and loop, does it look like these mods are reversible and non intrusive to the circuit? From what I understand, all if the loop’s wiring is done on the breadboard which could just be taken out without disturbing the original ST1 board. Does it seem like a straight forward job to restore the amp if the tech is competent and familiar with Marshalls?
 

NewReligion

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@NewReligion @neikeel @Pete Farrington Ok, final questions from me

1. With the amp as is, which seems to be stock 1987 with added tube buffered effects loop and master volume, can any of these additions have a negative effect on the true sound of the amp? Can the effects loop be possibly sucking tone, or is it only in effect when something is plugged into it? For the master volume, if it’s on full it should be out of the circuit right? Finally, if the added preamp tube were to fail, does the amp get damaged, or does the effects loop just stop working?

2. If I decided to restore the amp and remove these mods, other than the additional holes for the tube, master, and loop, does it look like these mods are reversible and non intrusive to the circuit? From what I understand, all if the loop’s wiring is done on the breadboard which could just be taken out without disturbing the original ST1 board. Does it seem like a straight forward job to restore the amp if the tech is competent and familiar with Marshalls?

1.) Sound of the Amp: Yes. The loop can affect (color) the tone depending on how the circuit is crafted. None of us know.

2.) Loop Tube: It is unlikely the preamp tube would fail. If so I wager it will not damage the amp. (Test it by using the loop then pulling the tube. Once it goes silent then shut down the amp).

I’m still not confident the pot on back is a master volume.

3.) Restoration: Any competent Tech can return it to a stock 1987 circuit.

David
 

paul-e-mann

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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.

Get David Hopkins to fix it for you. :yesway:
 
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