jmp input mods

[allwheelz]

So I recently picked up a vintage 1978 jmp 50 watt mkII 1987 nmv head. Its the 4 hole model....my question is that is has some kind of mod done to it that does away with two of the inputs so you don't have to jumper anything...am I loosing a lot of tone with this mod? Not only that is has an effects loop mod installed on the rear along with a volume pot for the loop...that also controls main volume level of the amp. I assume the mod wasn't done properly. should I try to unmod it or just play it and enjoy it like it is...this is my first marshall so I really have nothing to go by to compare it to..hope somebody can help...thanks!

 

[iron broadsword]

You can hardwire the jumpering on the back of the input jacks, but i don't see why it'd be worth doing.. what's wrong with jumpering? (I am a noob with these amps)

Could the knob on the back just be a master volume? That was a common mod for these if it didn't have one, and he might not have wanted to mess up the front panel.

 

[matttornado]

I have my inputs internally jumpered & it sound awesome. No loss of anything. BUT, it depends on where the inputs are jumpered. At V1, it didn't sound so good but internally at the input jacks, its sounds great - just like a cord from Volume 1 low to vol. 2 high.

 

[danman]

They could be internally jumpered (parallel) or wired in series (one wire mod) like the 2204/2203 preamps. Do you still have four inputs on the front of the amp...two for bright channel/two for normal channel? Pics of the board and wiring will help to decipher it.

 

[allwheelz]

yea....all four inputs are still there..the two left ones are live yet, two right ones are all but dead. seems like volume I acts like a distortion level, and volume II is like a main volume. and the fx loop on the back acts like a main volume also, but overrides all levels..lol. thanks for all your responses!!! once I calmed down a bit and really got a feel for it and played around a bit im pretty sure im in love!! now to stretch her legs a bit for a length of time to really dial in the tone..seems like its happy place is with presence around 3, bass around 8 or 9, mid around 6, and holy crap keep the treble down around 2!! lmao..I'm using a 2014 les paul classic with 59's and wow that thing can get bright!! Chassis pics coming tomorrow...had it apart a couple times already nosing around..im in it for 1300 and it came with a jcm 900 straight cab..both are pretty clean. Rock on!!

 

[iron broadsword]

Must've misread your OP... Don't mid my last post.

 

[danman]

It sounds like the preamp has been cascaded like the 2204/2203 models. Each volume control will add gain and can be used to fine tune the sound and distortion. It shouldn't take much to return it back to factory spec if you ever decided too.

 

[Blacque Jacque]

From your description it could have been modded in several ways.

If you can pull the amp chassis & get good clear photos of the wiring & board, it should be fairly easy to decipher what's been done.