Marshall JMP1 preamp - buzzing sound but not through speaker

thecolourbox

Hi all

Bit of a weird one, I bought a Marshall JMP1 preamp on ebay a while back and on top of it sounding very underwhelming compared to what I remember from one I had before, I noticed a hum problem. I started thinking...ah I'm using single coils, it's on the same extension lead as my speakers so could be that, there's an electric power line near my house etc. But look like it's not anything like that.

I plugged it in tonight with no input, no output, in a plug on its own and it was still humming/buzzing. I felt it. it's vibrating in the middle. Plugged in my mandocaster and some headphones, only usual single coil noises through headphones. So it's this buzzing/vibrating, feels mostly in the middle of the unit so the other thought about it being the transformer is also seeming illogical as that appears to be on the right hand side. Here's a picture off Google of the insides, feels like it's above the light green bit in the middle where I can feel it:



Anybody got any clues? I wanted to sell it but now am looking at a hefty loss I presume. Gahhhhh hate buying second hand tat

ICBM

Try changing the valves. It sounds like the filaments could be vibrating at 50Hz from the heater current, which I'm pretty sure is AC in these (I can check the schematic later).

I would just sell it and not worry about it. Most people are unlikely to mind as long as it doesn't come out through the speakers.

If you think it's a real problem, stick some foam rubber draught proofing strip to the inside of the top cover so it presses on the edge of the valve PCB - not on the valves. I thought there was some already but I may be remembering wrong, or it may have been removed.