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Marshall JMP-1 Service

Marshall JMP-1 in for service at Plasma Music

Here’s a Marshall JMP-1 MIDI valve pre-amp that I’ve just serviced.

I’ve changed the valves and power supply capacitors, cleaned up the metal work and implemented my ‘Live-for-Ever’ battery mod’. The battery is bang in the middle of the board so you really don't want this to leak.

The serial number indicates that it was built in early 1992 which not only makes it twenty-eight years old but one of the first JMP-1s off the Marshall production line.

An early  MIDI valve pre-amp, the Marshall JMP-1 has always been a really under rated bit of kit, despite the fact that named artists such as Billy Gibbons (ZZ Top), Phil Collen (Def Leppard), Iron Maiden, Megadeth and many more, still use them even today. In production for well over ten years, also says something about this little gem.

The JMP-1 is not a modeler like modern stuff; it’s the real thing, just with MIDI. Run into a valve power amp (or two), it just sounds amazing and the quality of the speaker emulated outputs is definitely good enough to go straight into the board or your DAW.

With most of the distortion being generated in the semi-conductor domain (some of which is highlighted in the image below), many purists might shy away from this beast.

Marshall JMP-1 semi-conductor distortion

On the other hand, classic Marshalls such as the legendary Silver Jubilee series and the monstrous JCM900 Dual Reverb, also use semiconductors to produce some of their distortion tones.

And then there's the clean channel of the JMP-1 which is based on the classic Fender design... with a few tweaks, of course. Anyway, at the end of the day, it's the sound that counts and as I've said, these things have still got it, even today.


If there's one annoying characteristic of an ageing Marshall JMP-1, then it's got to be the humming transformer. Anyone in the know, might think it strange that this box which resides in a 1U rack enclosure doesn't have a toroidal transformer. The thing is, the JMP-1 transformer chucks out several voltages including those required for the two valves and it's quite challenging to build all that into a toroidal design. So after a few years, the laminates inside the conventional transformer can become lose and start oscillating. The mechanical vibration can be annoying but does not appear on the audio signal.

If you're heavily in love with your humming JMP-1 and want to do something about it, then give me shout!

Marshall JMP-1s now go for between £400 and £700 (yes, £700). If you can pick one up nearer £400, then I personally think you've got yourself a bargin and a seriously good bit of gear.

If you want to find out more about the JMP-1, then there's loads of resources on-line and of course the Marshyall JMP-1 forum on Facebook:

If you have a JMP-1 and it's in need of a little TLC, then don't hesitate to contact me.


WARNING: Implementing this procedure will permanently delete all user-made changes to any patches.

  1. Switch off JMP-1 via the power button on the far right.
  2. Hold down the OD1 button and the CLEAN 1 button.
  3. While holding down these buttons, switch on the JMP-1.
  4. Wait a few seconds while the display flashes and then release the OD1 and CLEAN 1 buttons.

Marshall JMP-1 Factory Reset