Modifying a JMP1c to sound brighter. Noob questions

[bruzanhd]

So I have a JMP1c, I love the amp and would look to tweak it to be a little more bright. The amp was voiced by the designer to sound like it is a channel jumped superlead amp. Based on the schematics I've seen for superleads, I would assume this means that in my JMP1c the cathode resistor and bypass caps are set at values that are meant to act like the regular two superlead channels in parallel to simulate actual channel jumping. I'm not that good at reading the layout of a PCB although I am fine with modifying parts on these types of assemblies. I have pictures of the amp and can get more if necessary. I'm wondering if anyone can guide me toward finding which resistor and cap I have to get more towards the superlead bright channel sound? Thanks for any advice or guidance
Link posted for the imgur thread. Can't figure out how to embed my images.

[bruzanhd]

So I’ve made some progress, I’ve traced the PCB far enough to know that the values I’m concerned with are R2, C1 and C4. C4 is already correct so I can leave that one. However in a 1959SLP, the equivalent values to my R2 and C1 are 2.7k ohms and 0.68 microF respectively. My values are 1.5k ohm and 1 microF. I just need to find out where I can source parts now.

I’ll include what I was able to hack together as far as signal flow:

[The Dude]

FWIW: If you're trying to make the amp brighter, there's a good cathode cap calculator here: There are lots of other useful calculators on that site, also.

https://www.ampbooks.com/mobile/ampl...or/calculator/

[Helmholtz]

Schematic?

[bruzanhd]

FWIW: If you're trying to make the amp brighter, there's a good cathode cap calculator here: There are lots of other useful calculators on that site, also.

https://www.ampbooks.com/mobile/ampl...or/calculator/

Thank you, I will dig in over there.

[bruzanhd]

Schematic?

It’s not available anywhere. Been scouring the web for it. Resorted to making my own trace of the signal path around the first gain stage.

[Chuck H]

I don't know how different the various models were but Dr. Tube has a couple of schematics here:

https://www.drtube.com/schematics/ma...jmp1-61-04.pdf

I had one of these for a little while. I rember thinking it was sort of bland sounding. To be fair I never attempted to put it in front of an EL34 power amp and crank it up. But as a preamp it certainly didn't step up to the "Marshall" sound as head owners know it.

[Helmholtz]

Oh, it's a preamp only (containing lots of silicon).
Had expected a full mini-amp like a JTM 1.
Or this: https://reverb.com/de/p/marshall-jmp...0-guitar-combo

[The Dude]

Great sounding preamps, the JMP-1. I have one in the shop right now waiting for a transformer. The "C" might be the actual amp, though.

[g1]

Had expected a full mini-amp like a JTM 1.
Or this: https://reverb.com/de/p/marshall-jmp...0-guitar-combo

You are correct.
JMP-1 was a rack preamp.
This is about the newer JMP-1C which is one of the low wattage series. It's a 1 watt amp.

[bruzanhd]

You are correct.
JMP-1 was a rack preamp.
This is about the newer JMP-1C which is one of the low wattage series. It's a 1 watt amp.

Correct, I'm talking about the little anniversary edition 1 watt amp.

[bruzanhd]

deleted for bad link format

[bruzanhd]

I was able to find exact replacement caps and resistors on mouser. Took a bit of datasheet referencing but I installed the new cap and it sounds great. Just a bit tighter sounding than stock. The images I linked show the original 1uf cap (light gray) and the new .68uf cap (black).

[g1]

Did you also change the 1K5 to 2K7 like you mentioned in post #2 ? Any other changes? Is the plate resistor for that stage the same as the 1959SLP ?

[bruzanhd]

Did you also change the 1K5 to 2K7 like you mentioned in post #2 ? Any other changes? Is the plate resistor for that stage the same as the 1959SLP ?

I also removed a capacitor that was in parallel with the grid leak resistor. It basically rolled of highs as you rolled back the guitar volume. And I haven't changed the cathode resistor. I ordered both 820ohm and 2k7ohm but I cant decide which way to go so I stayed with the 1k5 which is in the middle anyway. Basically I want to approximate the EVH sound at low volume. I've come to understand his 12xxx series marshall was using the 820ohm value but most other superlead users that I love to listen to have the 2k7. I feel the for now the 1k5 is a reasonable compromise. The main difference moving up to the 2k7 would be slightly less low end gain. Right now the amp is feeling a lot tighter with the .68uf cap change. I'll spend some time getting a feel for the cap change and see if I still feel the need to modify more.

And yes the plate resistor is the common 100k value from the SLP. Also the 0.0022 coupling cap is already there so I left that alone since I like that sound. Really the main differences at this point seem to be the lack of a mid and presence control. Also the amp doesn't have a bright cap. I may add one later but again, I haven't felt the need for that yet.

Really the only complaint I had with the amp was that it lacked clarity with the guitar volume rolled back and felt just slightly too loose when it was dimed. My two minor cap mods seem to have alleviated both my small complaints. Overall very pleased with my work. Never did a project like this before so it was also a great learning experience.