1977 Marshall JMP 2x12 50 watt combo model 2104, HUM!!! Easy fix.

This is a regular on my bench. The owner likes to run it good and hot so it cooked another 6550.

That part was easy. But the amp had an excessive amount of hum coming off the preamp. It was re-capped using JJ filters, but I tried replacing the one in the preamp to see if that solved it. Nope.

Here's the weird thing, the hum would change behavior. At one point it seemed like a ground loop hum. You could turn the master up and it was awful with the preamp all the way down. The preamp in the middle and it was tolerable. All the way up, unusable.

I tried replacing the preamp tubes to no avail. The client also told me the tone would get brittle then nice then back to harsh again. That was enough to remind me. It's simply the ground connections.

Marshall uses mechanical ground connections. In other words each ground point is soldered to a terminal that is connected to ground through nuts and bolts or the pots on the face of the amp. These can go bad over time. I re-soldered every ground point and it got better but I knew it could go even better than that. Then I heard this spark sound. I'm on the right path. Some ground point on the face of the amp was loose.

It's not unusual to need to pull all the pots and jacks, take a little sandpaper and sand lightly on the front of the amp, (make sure you get any metal dust out of there!) then re-connect everything good and tight. That can be a dramatic improvement.

Or.... Solder the ground rail directly to the chassis:

If you have a nice Weller iron like mine, set it as high as it will go (800 degrees). I used a scratch awl to make a rough surface where I soldered my new wire. Use rosin and heat that surface then hit it with solder. Test that solder to make sure it's not going to come off. Then solder your wire. Test that to make sure it's tight.

Amp is really quiet now. I've been letting it run all day. Tone is good, not slipping in and out of bad sound land. I'm happy.

And, I wrote this post largely to remind myself to check the grounds first. I tend to forget the simple fixes!

And yes, still one of the greatest rock n roll amps out there. These aren't terribly expensive either. Go get yourself one. You need to really know how to play to get the best out of it. These are simple devices unlike stuff with all that channel switching. What you do get is maximum tone. You get what you put into it. Play softer for clean tones, dig in for your nasty sounds!

-Jef

1978 Marshall JMP 50 watt combo

Yes. That's all I need to say.

These are fantastic amps. This one was blowing the H.T. or high voltage B+ fuse. We had hoped it was one of the 6550 tubes but unfortunately the output transformer was cooked.

When the H.T. fuse pops 9 times out of ten it's just a power tube. Once in a while it's a shorted filter can but every so often it's the output transformer. That makes for an expensive repair. And the sad part is it does tend to hurt the value whether it sounds better or not.

Here's the ultra simple control panel. Gain, Master, EQ and presence. You really don't need any more with an amp that has this much sound. These are known as rock n roll amps but in the right hands can be good with any style. Someone clipped out one of the treble caps, the one on the volume knob. I owned one and clipped out both of them. I don't need that much treble and clipped out you can simply turn up your treble and presence if you need it.

This one has the original speaker intact. A pair of lovely 30 watt "Black Back" Celestion speakers:

So what output transformers do I like these days? I'm a Mercury dealer and I also deal Mojotone as well. For the price you can't beat the Mojo. I believe they are made by Heyboer and they are first class transformers. Friendly right out of the box. The Mercury product is expensive. Very expensive. I like there iron very much but there are only a few things to me that are worth that much more. When I was building amps for people I used their ultra expensive "Radio Spares" iron in a JTM 45 I built and always used their "Radio Spares" output transformers in my 18 Watters. They simply can't be beat in that capacity. I had the pleasure of playing my JTM amp next to a real "coffin logo" '62 model and mine was so close. I sold that amp for about $1300. The '62, granted is ultra rare, but cost around $20,000. So parts makers like Mercury and Heyboer helped me lose my lust for old amps! Why spend all that money if my intent is to enjoy it?

But for general repairs and price, the Mojotone iron can't be beat. I've used many of their transformers with dynamite results. In this Marshall I can say DYN-O-MITE! It's got it where it counts and was a relatively easy install!

JB