Metropoulos Forum

My 2203, I just unmodded it, and now its squealing.

Let me get into a lil more detail. The amp had a bunch of mods, and one mod was the two 220k resistors next to eachother that have wires connecting to the power tubes were changed to 270k. And I put that to original, and changed a couple other resistors back to stock.

Anyways, I put new tubes in it, and I haven't rebiased it yet. My electronics teacher is gonna do that for me on Tuesday. But I figured I'd test the amp out because its always nice to enjoy the fruits of your labor. The amp sounded great, but the volume, presence, and treble knobs when turned up a lil more than halfway start to squeal.

The only things I thought it could be are the tubes need rebiasing, the swamp resistors, or the presence wire is too close to the other tone controls. I moved that wire back and it improved some, but theres still lots of squealing.

The 2203/2204 amps are also a little touchy about where the wires from the input jacks/volume controls go across the board to the preamp tube grids. From the factory they started using white goop to make sure that the green grid wires didn't get too close to the blue plate wires at the first preamp tube. They also used shielded wire from (I think, if I remember right) the high input to its 68K input resistor and used goop to keep that in place as well (it makes a funny loop over to the resistor.) I'm not sure what mods had to be undone but I'd start my quest at making sure the wiring is laid out correctly ("lead dress" if you need to impress your electronics teacher )
Also be on the lookout for hidden mods like changing cathode resistors on the board for more gain, and hence more oscillation potential.
I think Clay Finley's MV information on his web site has a good pic of a '78 2204 that shows some of what I'm saying about the lead dress. Check out the sticky post somewhere on this forum about "Detailed Marshall Circuit Info" and there's a link to his site. Good reading.

you definately need that shielded cable from the 68k input resistor. or just get it off the board and run shielded cable to it. the 2203 doesn't have very much gain, but it has enough to squeal if you dont shield.
mike

You guys got any other suggestions?

how about some pics?
mike

sorry, i have no camera to take pictures

pictures would really help, there are a buttload of things that could cause the squealing, some of them very subtle.
mike

My 2203 feedsback on its own at about 8 on the Preamp Volume, is this oscillation?

Dax wrote:My 2203 feedsback on its own at about 8 on the Preamp Volume, is this oscillation?

Yes it is. Try swapping your preamp tubes around and see if there is any affect on the squealing.

George

As long as we're talking PO here......
The Super Bass whistles with a guitar plugged in and the volume dimed (no whistle when the guitar is unplugged or turned down, and it's fine up to 8 or so) so I re-did the B+ buss wire going to the first 12AX7's plate resistors, and re-routed the purple wire from the treble pot, and the green wire from the mixer resistors; no help. But before I go any further, since the head case isn't here yet I've been sitting the bare chassis on a 4x12, no shielding from the speaker magnetic fields -- do the speakers put out enough of a field to cause oscillation problems? Do the head cabs have the shielding plate like the Marshall originals? If I had free time I'd pop the chassis in my '78 head cab (which is sitting around until Bob Pavao gets the rocker switch faceplates done) and experiment, but my son doesn't need his ears ruined quite yet......

A lot of times amps will have ocillations while sitting normal on th cab, but it goes away if you turn the amp on it's side. Giive it a try.

Your white case shipped today!

George

Now that you mention it, mine is not in the head enclosure. And it makes this oscillation, and when I tilt it, the oscillation is gone. Why does it do this? And will it go away if i put it in the head enclosure?

It should if the head case has a layer of metal on the bottom inside. Which mine do not........yet.

I'll look into putting a piece of tin inside like Marshall does. That will help.

I have also seen stock Marshalls (with the tin strip) that do this same thing.

George

I may try the Fender trick of stapling screen down under the chassis. I know I've got some of that around. I still need to sort through my preamp tubes to find some vintage stuff for this amp. Maybe the JJ's will go in PI slots to get everything spread around, and I can swap tubes at that time and see if it helps.

My Mistake, the amp doesn't feedback until 10 on the preamp and 8 on the Master, which is too frickin' loud at that point anyway! Amp sounds the best with preamp at 7 and master at about 5. Its a 76' 100 watt 2203.