Modified JMP 50 2204 Master Volume MK2 Probs!

[markabrown]

I have a JMP MV50 Watt 2x12 combo that is all standard and sounds pretty good. However I recently bought the same model head from 1978. This head has been modified and sounds very different. It has none of the drive of the standard model and both inputs sound the same. There is a "25watt" switch on the rear panel (makes very little difference) and both the master volume and the gain act as volumes! As the volume and gain are increased there is a massive increase in volume and the tone is very fat but muddy. There is none of that Marshall roar! I need to return this amp to standard ASAP! Any advice?

 

[damienbeale]

Both inputs sound the same? Are you sure it's not a '76 or a '77? If not, it sounds as if somebody has modded it to the early Unicord schematic for a 2204, which is a pretty naff sounding amp. It's not difficult to convert it back to proper 2204 status though.

Steve Miller has an excellent guide at the bottom of this page, rather than me guide you through it...
Unsorted Pics

Examine your amp and see if it matches up with this with regards to the components. If not, post some clear internal pics and we'll see if we can figure out what is going on.


The 25W switch is a sort of triode/pentode switch, which will make only about 3dB difference at most. It also gives you a fraction more drive, but makes the amp sound a little dull in my opinion. I don't like fitting these, even when asked, as really they're not that much use.

 

[Spacehog]

I agree with Damien on the rear-panel switch allowing the switch between 50W and 25W. It makes very little difference in sound and/or volume (to me, anyway).
I have a 1980 2203 (100W MV MkII head) which someone modified by outfitting with a metal toggle switch in place of the 4th rear output jack which allowed me to switch between 100w and 50W. This was prior to 1986 which is when I bought the head. I never really noticed a difference (though I never turned it up past '4' at most anyway for playing small - mid-size clubs). That combined with me being the type that hates upsetting the integrity of things (not to mention not being the most technically seasoned on these things) is what led me to having it removed. Plus, I believe (correct me anyone if I'm wrong), I think you have to change the ohm-setting on the rear to compliment whether you have it set to 50W or 25 W (assuming it's set up like mine was - I believe the modified switched cut 2 of the 4 tubes out). When I switched mine to 50W, I had to change it from 16ohm to 8ohm, which I think would also screw things up if you're running multiple cabs like I am (typical Marshall stack with A and B cabs containing all 16ohm speakers).

I hope that helped a bit.

 

[damienbeale]

Triode/pentode switch doesn't cut any tubes, it merely changes the mode of operation on the tube. Besides, cutting out one of a push/pull pair isn't a particularly good idea. You're only amplifying half of the signal that way. Taking out a pair in a 100W amp is another thing altogether though. Adjusting the impedance is not required for this type of switch/mod.

 

[markabrown]

Triode/pentode switch doesn't cut any tubes, it merely changes the mode of operation on the tube. Besides, cutting out one of a push/pull pair isn't a particularly good idea. You're only amplifying half of the signal that way. Taking out a pair in a 100W amp is another thing altogether though. Adjusting the impedance is not required for this type of switch/mod.

Well sorry for the late reply! All you guys were absolutely right! Had a techy reverse all these mods. It now sounds superb! Thank you so much guys!