79 JMP 2203 Impedance Selector Wiring Incorrect?

[TommyVonVoigt]

I'm trying to figure out if my 79 JMP 2203 impedance selector switch is wired incorrectly.

The OT is stock, as far as I can tell. It's a Dagnall C2668. I might be misunderstanding how this thing is supposed to be wired, but isn't the black (and purple feedback) wire supposed to go to Pin 1, for 4ohms (it's a 6550 head)? Yellow would go to Pin 2 for 8ohms, and green to Pin 3 for 16ohms?

16ohms in written in sharpie on the bottom center, and before I started cleaning it, it said 4ohms with an arrow pointing to 4ohms, and 8ohms over where the 16ohms is on the selector.

I'm happy to test this, but I'm not sure how to test it.

 

[Gunner64]

Go to the Dr. Tube website and download the schematic for your amp. Then you will have an idea what has been monkeyed with.

The purple neg feedback wire can be moved to other taps. Maybe your buddies amp has that neg feedback wire at a different tap than yours.

Seriously though, get the schematic.

 

[TommyVonVoigt]

Go to the Dr. Tube website and download the schematic for your amp. Then you will have an idea what has been monkeyed with.

Off the top of my head that wiring should be correct. The purple neg feedback wire can be moved to other taps. Maybe your buddies amp has that neg feedback wire at a different tap than yours.

Seriously though, get the schematic.

I found the schematic, but I can't make any sense of the OT & Imp. selector wiring from it. And it doesn't give me any indication of the color codes.

 

[Gunner64]

The info I have on the Dagnall C2668 says,

Green- 16 ohms
Yellow- 8 ohms
Black- 4 ohms, and the neg. Feedback wire.
Brown- common

 

[TommyVonVoigt]

The info I have on the Dagnall C2668 says,

Green- 16 ohms
Yellow- 8 ohms
Black- 4 ohms, and the neg. Feedback wire.
Brown- common

That's what I was able to find, too. And I was certain that it would go Black to Pin1, Yellow to Pin 2, and Green to Pin 3. If that's correct, this is all messed up. That would mean that switching it to what I thought was 4ohms has actually been switching it to 16 ohms. 8ohms has actually been 4ohms, and 16ohms was 8ohms. Very, very screwy. Looks like the negative feedback wire was at least tied to the 4ohm wire off of the OT, but yet attached to the wrong lug on the switch. And it's probably been this way for decades, too.

 

[Gene Ballzz]

I suggest that you remove all but the brown wires to the switch and use a meter to confirm which lugs are which, in each switch position, and then wire it correctly! Likely easiest if you unbolt the switch from the chassis. It looks like enough wire to do so.
Simply Switchin'
Gene

 

[TommyVonVoigt]

I suggest that you remove all but the brown wires to the switch and use a meter to confirm which lugs are which, in each switch position, and then wire it correctly! Likely easiest if you unbolt the switch from the chassis. It looks like enough wire to do so.
Simply Switchin'
Gene

Just got done doing that, actually. Turns out, I was right about the pin 1 being 4ohm, pin 2, being 8ohm, and pin 3 being 16ohm.

And everything I can find is saying that this OT should be:

Brown = Common
Black = 4ohm
Yellow = 8ohm
Green = 16ohm

All signs point to this thing having originally had the window style selector. Since there were sharpie marks on it, I'm guessing that at some point, somebody lost that part of the switch, and was just using a jumper. At some point after that, they had the entire selector replaced with a newer style rotary switch. And at that point, whoever did it wired it up wrong. They still had the negative feedback wire tied to the 4ohm tap on the OT, but they had them both going to the 8ohm lug on the switch. They the yellow 8ohm tap going to the 16ohm lug, and the green 16ohm tap going to the 4ohm lug. And I bet it was like that for decades, too.

That's if I'm right about all of this, of course.

This also means that when I ran it a few times here at home, at super low volumes, I had a bad mismatch. I ran both of my 8ohm Mesa cabs, so I switched the head to 4ohm. Unbeknownst to me, I was apparently actually switching the head to 16ohm. And when I took this thing to rehearsal and cranked it up for a few minutes, I was running it into a 16ohm Marshall cab, but didn't realize I was setting the head to 8ohms (not as bad of a mismatch, as I understand it).

 

[rocker68]

Good catch.

 

[Gutch220]

If this were me, I'd get a window selector like the original and rewire it to stock, then clean off the sharpie writing.