Marshall 1959/2203 hum problem

[JPA]

Hi guys, my amp is humming really bad. It's a 1959 reissue. I reversed all the mods done by the previous owner and converted it to a standard 2203 circuit. All tubes (Svetlana) and filter caps (TAD) are new. With master volume at 0 the amp is quiet. As I turn up the master volume above 5 hum gets really loud. Guitar connected/unconnected makes no difference.

- all filter caps new
- resoldered and checked all connections and wires
- pots are ok
- pulling only V2, hum is gone and amp is quiet
- pulling only V1, less hum, but still pretty loud
- tried different/new preamp tubes, no change
http://i410.photobucket.com/albums/pp18 ... g~original
http://i410.photobucket.com/albums/pp18 ... g~original
http://i410.photobucket.com/albums/pp18 ... g~original

I don't know what to do next. Any ideas?
Thanks!

[Doug H]

ouch...

I see a lot of beautiful amps on this and other boards. Sometimes being too organized can cause problems, but usually I guess it helps. That didn't stop a lot of old amps with spaghetti wiring from sounding great, and running more or less quiet too. My amps are far from pretty.

A lot of the time simply having wires fed under the tag board makes things look tidy and right, but even with that, that amp is scary is some places, I know you didn't build it so I don't mind being blunt.

The green grid wires on v2 and v3 could really stand to be moved away from the plate and other wires. It shouldn't be too hard. But your problem is hum not PO so you could try cleaning up the heater wiring around v1 and v2, see if you can just man handle it into place a little better than it is, the pairs closer together and no green grid wires running parallel.

Still probably not the issue, looking at those pics, I don't know if they are before or after you reversed some mods, but I'd be suspicious of a ground issue not knowing what might have been done to that amp. I'm not that familiar with the 2203 so hopefully someone who's intimate with these can spot stuff that looks out of place for you.

wtf is up with the purple wire, did they have to run the feedback all the way back so it was draped across the PI wiring? Is that a resonance control mod on the back? It probably changed the original lead dress for the feedback wire.

[JPA]

The pics are after reversing the mods. Hard to believe, but the amp was modded at a well known "professional" custom amp & modding company. They've used different ugly kinds of glue all over the housing and pcb
I double checked every connection with the original Marshall schematics.

The green grid wires are already lifted up away from the plate and cathode wires, they only look flat on the photo. Actually, most of the wiring including power amp and heater is stock Marshall. And you're right, the potentiometer on the back is a depth control mod. I would also say the hum is ground loop related, but I just can't see any kind of grounding issue in this amp...

Should I try adding this sort of virtual ground with two 100 ohm resistors in the heater wiring?

[Doug H]

I've seen a lot of pics from that era, the stock wiring got scary bad. Truth is I' not sure how much of a difference it really makes as long as you stay away from the big no nos.

I'm really suspicious of that purple wire. In the pics I saw it's usually routed under the board. Makes me wonder if you pulled that mod what would change.

Most marshalls already have a center tap for the heaters, not sure if the pair of 100 ohm resistor will help anything.

I see what you mean about the grid wires, but the green wire to v2 still makes me nervous. It;s floating in a nest of looped wires it seems, even if it's some vertical distance away. I"d shove half of it under the board so it crosses the heaters at a right angle and doesn't reach out to where the heater wires are loose, just to be sure. I'd tuck in that blue wire from v1 too so it doesn't coil all the way into the v2 grid area, pulled away or not.

Wish I could offer more.

[Doug H]

and is that a diode clipping circuit after the master volume?

[JPA]

I rewired the grounding, seperated between the presence pot and bass pot, no change. I tried different positions for the grid / plate / cathode wires, no change. Rewired the heaters, no change. Purple feedback wire under the board, no change.
But as I said, no problems with the power amp section, it's perfectly quiet. I'm sure the hum must originate somewhere in the preamp section, as the amp is perfectly quiet with V1 and V2 pulled.

Here's what it sounds like: https://soundcloud.com/nuclearraccoon/marshall-hum
That's 50 Hz hum, right? Just to be sure...

I'm rather desperate at the moment... What's next? Throwing all pots, tube sockets, wires and the pcb into the trash bin and basically building a new amp with PTP wiring?

No diode clipping, that's just for the relay switching, which is completely disconnected right now. The only difference between a normal 2203 circuit would be the depth control, 1n cap in the treble peaker and the two anode caps to damp the highs.

[Doug H]

is the amp unplayable? Maybe the input jacks are messed up?

hopefully someone with more exp than me will hear what the issue is in that clip.

[Doug H]

also, try jiggling the shielded input cable around and see if that makes a diff. I don't know what it is but I whenever I put shielded cable anywhere sooner or later it eventually ends up giving me some kind of ground buzz.

[Haze13]

Use A Hot shield

[JPA]

One of the tube socket pins broke while soldering. I'll put some new ones in and report within the next days. Maybe that solves the hum problem, too.

Thanks for your help guys!

[Doug H]

If that doesn't work try pulling the mica off the plate resistor, you have to be careful with those, and with such short leads, then looking at the rest of the amp, I'd be suspicious.

[JPA]

I changed all preamp tube sockets, threw out the caps over the plate resistors and fiddled around with the shielded cable. All of that didn't change the hum problem at all.

Finally I gave some other tubes a try again, JJ ECC83S. And with these tubes the hum is completely gone! Problem solved I guess. So apparently you have to be careful using SED / Svetlana 12AX7 in the preamp section. As I said before I've already tried other new Svetlana / SED 12AX7 tubes, but they all were humming equally bad.
Anyway the 12AX7 in the PI is perfectly quiet, maybe because it's a matched / selected one?

[Haze13]

I love SED 12ax7! I use one in V1 of JCM800 and no humm issue, but in the GTO clone that I've built, with SMPS for b+ and dc for heaters - this tube is superb! May be some old tubes will sound better, but for the price it's just a wonderful tube!

[Doug H]

Wow, well at least solved the problem and your amp is all cleaned up now.

I chased a hum in a friends amp for days once before discovering the problem was the rectifier tube. Experience doesn't always come cheap .