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

1966 Ampeg B15, your studio requires one.

Simple job today, just re-cap the power supply. Amp was running quiet but now it just sounds better with a stronger low end. It will be mated with a beautiful P-Bass soon and ready to cut phat tracks. You have a studio? It's not a serious studio without a real B15!

Stick o' dynamite cap, 30uf@600v, replace with a C.E. Manufacturing 25uf@800v stick:

Also replace the big can with a C.E. triple 40uf@525 volt can.....

I left the audio circuit alone. I like these green .02 Cornell Dubilier capacitors. They're still good and the amp sounds great! Ampeg in those days didn't skimp on small parts. They are built to last.

My client and I bought this amp from a new shop in Brooklyn called Rock N' Roll Supplies. Great place and they have a great tech working there. Give them a visit! Some mighty nice amps hanging out there and reasonably priced!

http://www.rockandrollsupplies.com/cm/

https://reverb.com/shop/rock-and-roll-supplies-inc

JB

Gibson GA-5 Les Paul Jr amp. My 2nd favorite small amp! And about "too big to care!"

This arrived in the mail today. An early 60's Gibson GA-5 Skylark. I love these amps. It's to me, a Champ killer. The Fender stuff is built much better but some of the late 50's to early 60's Gibson amps just have such a great thing going on.

Typical 3 tube 5 watt design: Single 12AX7, 6V6, and 5Y3 rectifier. This one came loaded with a nice RCA 6V6 and 5Y3 and a great nasty sounding GE 12AX7. It was also 100% original. I changed the electrolytic caps and installed a grounded power cable for safety.

I've bought a few of these as fixer uppers and flip them like a burger. They are easy for me to sell cause they are just that good. A bit wider and more open sounding than a tweed Champ and much less dough. I may keep this one, my last one I was bummed about selling.

Now, in the pictures. Notice how filthy this amp looks. Fresh from the barn. When I turned it on it crackled and popped, the pot was filthy. Not sellable in my opinion. I would have bought it at the asking price, no problem but that's cause I can fix it good and proper. If I was just a guy who didn't have those skills I would have turned away. I bought this from the worlds largest guitar store online. Won't mention the name but you know who I am talking about. I did a stint with said company and was baffled by their treatment of vintage gear, good vintage gear like this. No effort made to clean or improve. I suppose somewhere corporate sees that extra effort hurts that bottom line. I like to make money. I like to make as much as I can. It doesn't take much effort to clean something up. People like it when they don't need to worry and don't have to do anything except negotiate a price and be done with it.

But that's the nature of some big ass companies. Too big to care. It's okay to get big, but never give up the caring part! There's money to be made there and more importantly, there are people make happy.

And here's the schematic for you tech heads. Ultra simple signal path. LESS IS MORE!

So what do I like about these over a Tweed Champ? Well, first, I ain't gonna lie. The price! Tweed Champs to me are simply overpriced. No fun for me. Second? I just got back from playing this at my practice space for 2 hours. These amps tend to not fuzz out as much. The notes articulate better for me. Pick soft and you get a lovely clean sound, punch it and it sings, nasty distortion but....clear. I can hear all of the notes in each chord. It really sounds like a cranked Marshall in that respect.

This particular amp has the original .022 famous Bumble Bee Sprague capacitors. Yeah, they are leaking. Can tell by the fact that the volume pot scratches slightly from top to bottom. A little DC on that, and also by the way the notes decay. I may change them and sell those to a rabid Les Paul fanatic. But for now I don't care to change anything else. There is something I like about the slight trashiness.

So what is the difference between a GA-5 Les Paul Jr. amp and a GA-5 Skylark? Electrically, there is no difference. Same great Jensen 8" speaker too. The cabinet is slightly different and the Skylark says "Skylark" on it. The Les Paul Jr amp is a bit earlier, the Skylark was made in the early 60's.

Here's a Skylark I sold 2 years ago:

And the earlier Les Paul Jr amp:

If these amps were mint you would see that dapper Gibson logo on the front. The Skylark had it on the grill cloth, the Les Paul Jr amp has that lip across the top of the cabinet where the logo went.


 If you are considering one of those sad GA-5 Reissues, don't bother. You may find one of these for about the same money and these are simply better amps. And if you find a dead one cheap, they are so simple it won't cost much to get it going again. Even the higher priced ones are worth it as a tool. Nothing beats one of these in the studio. Pure, simple, direct and awesome sound.


JB