1971 Marshall JMP Mods & MasterVolume installation
A friend recently asked me to give his 1971 100Watt Marshall JMP ( 1959 circuit ) a facelift and add a Post PI MV. It had never been serviced before and hadn’t been played for a while ( say 20 years or so :D )Everything from tubes to caps were stock and it didn’t power up. Continue reading to see what I did to the amp and how the MV was installed.
When I first unbolted the head’s back panel I got quite scared. The chassis, transformers and tubes were so dirty and covered with dust I seriously doubt you could see the power tubes glowing at all. Spent quite a long time cleaning the thing up and flipped the chassis to see its guts. Unfortunately, the board was already PCB in the early seventies!! I hate working on PCBs, it’s so hard to perform quick modifications without unbolting pots and jacks, desoldering wires and turning the board upside down.
The chassis after a little cleaning, still quite filthy.
The amp didn’t power up and I found the H.T secondary fuse blown. Prior to replacing it I tried to find a cause for this, and discovered that at some point two power tube pins had been shorted and the socket itself was kinda burned. It wasn’t necessary to replace it and maybe all the dirt in the socket allowed this to happen, so I cleaned all sockets up and replaced the fuse. The amp did power up but had a couple of cold solder joints and other minor issues that were fixed, including a bias adjust. After that the amp was ready to play!!
The owner liked the sound of the amp as is, but found the normal channel to be quite dark sounding so I wanted to fix that. Besides, I found the Bright channel way too Bright, and though the owner liked it, I wanted to give it a subtle change. To accomplish both things I finally swapped the bright cap on the bright channel’s preamp potentiometer to an Erie Ceramic 5nF, didn’t change the original value but a subtle difference could be heard, and high frequencies where attenuated a little at low volume settings.
As per the Normal channel, after some tweaking I also decided to place a bright cap on the preamp volume pot, and found the ceramic cap that had been taken off the Bright pot to be a great choice. It gives the Normal channel a more vivid feel, keeping the distances with the other channel.
This post is still in working progress….. Will update it with details of the MV installation soon!!