JMP 50 Watt Bass hacked to a lead needs redo

[bnwitt]

I had a customer in today who dropped off his (two channel pre master volume)1975 JMP 50 watt amp today complaining that it doesn't have the bite and crunch he wants out of it and it sounds too muddy.  It is a bass model that someone did a hack job conversion on to change it to lead model specs.  Whoever did it, did a really crappy soldering job and used incorrect value resistors and caps in series to obtain proper values.  Also the type of caps used aren't what I would use in a Marshall.  There is a plug in the back panel that was put in to fill a hole that was poorly drilled at one time.  Bottom line is this amp needs some work.  Now I had him play my DSL401 and he said "Yeah.  Make mine sound like that."  

So I'm trying to decide what service to offer him.  Should I just pull all of the poor quality work and make this amp a JMP Lead the right way or do something else.  He wants trebly bite and crunch not warm plexi sound.  Because I'm not a Marshall guy I don't know if the JMP lead will give him what he wants.  I know the 1987 Plexi is probably too warm for him.  It would be good to have a two channel solution rather than the single channel 2204 MV circuit.

[DummyLoad]

build the 2204. 

[alerich]

I have to confess (although I am not Catholic) that I have used two capacitors in series before when I didn't have the right value. I ordered one too few .0022uf Mallory caps when I ordered my parts for my Studio Clone. Wanting to get it wired up and fired up ASAP I made a trip to the local Radio Shack. All they had was a two pack of .0047uf caps. So I bought them and wired them in series. Worse yet... they are ceramic caps. They're still in there and the amp sounds great. I ordered an extra .0022uf Mallory on my last order but I can't find the motivation to install it. I would never do that on a customer's amp (if I worked on customers' amps) but on my home brew amps I do what I have to do sometimes.  I don't think Tron really cares.

[bigdaddy]

Put it back to original as in what an original MKII model would be. I would use the handmade SOZO caps for this amp.

See how it sounds and go from there. It's a loud roaring beast in it's stock MKII form. I have owned a few early 70's 50 watt Marshall heads. One of the best sounding amps ever made IMO. Many guys who did not really crank it would use the channels jumpered, one channel had the bigger cap and 820 ohm resistor and the other the .68 cap and 2.7K resistor. I would just use the channel with the .68 cap and crank it...LOL.

If he wants a master volume he's better off selling it and buying a JCM800.

[bnwitt]

I am leaning to putting it back to original.  This is not a master volume nor is it a MKII.  Does anyone know what circuit number this amp is?  I'm thinking it is a 1986 Bass JMP and it needs to be a 1987.  However, it has a 4 diode rectifier unlike the 1987 so I'm not sure.  It looks most like the ceriatone plexi 50 lead layout but of course it is a pcb.

[bnwitt]

Not too many folks with strong opinions on Marshall amps in here I guess.  Ok, how about any suggested mods for this amp to make it crunchier?  Maybe a treble boost too.  Is the 1975 non MV JMP a really great amp that doesn't need mods or is it one of Marshall's not so great circuits that needs a tweak or two?

[phsyconoodler]

The 1987 is a great sounding circuit but doesn't really crunch that much unless you crank it up a bit.
   You could add a post PI master in that unused hole and that would kick it up a notch.
Use the original 1987 values and some decent caps.Looks like the previous guy changed the drive line caps to Fender brown blobs.
  At least he could have lifted the board and did it properly.
Listen to some Alman Bros for some plexi sounds.

[jerrydyer]

I would make a dead on 1987 and if he NEEDS a MV then do the PPIMV.  then if he needs it how rodded from there, I can think of about a half a dozen little mods for bigger crunchier tones. 

[bigdaddy]

I just used to plug my Les Paul into those early 70's heads and I tried to use the 120 watt bass cabs as they called them instead of the 100 watt greenbacks cabs.

I would weld it as they called it then and play. Never had a better sound for blues/rock although a good BF Super ran a close second as did the early transitional black bassman with the white knobs, like butter.

The gain was amazing and the sounds from it were just indescribable. The things that would happen with each note made you want to slow down and work each note hard and not riff out too much. More like Paul Kossoff type playing. There was very little of that modern compressed sound that makes everything generic and the same sounding. For some reason each note had it's own character. Listen to those early Allman brothers recordings at the Filmore and Live Cream. That will always be what a Marshall sounds like in my head, even the Beano Clapton sound. It's a live clear sizzling but raunchy sound and each note had such complexity. Unless you are playing music that needs more of a certain specific sound to that genre I can't think of a better rock or blues rock tone/sound. IMO if you need an MV you need a smaller amp and that's why a lot of people went to the bassman modded and other modded Fender amps because those 50 and 100 watters were just too loud......but the sound.....they sounded the best right before they blew up...LOL.

[The_Gaz]

It's been said a lot of that 'crunch' comes from the cold biased preamp stage in the 2203/4 series, as it adds more odd order harmonics. The fact it's cascaded with the previous stage and the 'treble peaking' caps add to that as well.

My opinion: Don't deface it, or ruin the tone with a MV of any sort - they just sound bad unless one only cares about playing at home...alone. That circuit does not have enough preamp gain to use a pre-PI master, and these coveted post-PI MVs break down the feedback loop, fizzing up the tone, and rendering  the presence useless, which would definitely confuse some players.

My advice would be to convert it to the lead circuit, because original probably has an unusable about of bass (see: mud) reponse, and if it's still too "warm" recommend a decent treble boost like they used to use, or even a *gasp* Tubescreamer or Distortion +

Have fun making it look pretty again!

[bnwitt]

thanks for the feedback guys.