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
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
Good article.
ReplyDeleteThx!
Stefan.
https://www.lampenbakkenservies.be
Thank you for reading! J
DeleteGot one just like it, bought it in Chicago area, maybe 1978/79, still a great rock amp.
ReplyDeleteThe best!
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