‘78 JMP 2203 Impedance Selector

[rekelso]

Hi, recently picked up a 1978 JMP 2203 and I’m confused about the impedance selector. It’s not an original and doesn’t have the little window to say which impedance it’s running at, just an arrow pointing away from any of the markings. The previous owner had it connected to a 4x12 in mono and said the shop that fixed his selectors set it to run with a 16ohm cab but the way he had it looks like it should be running at 8ohms (can be done as far as i understand) and hasnt had any issues for the years he’s had it. I’m getting a 2x12 which is 8ohm mono but don’t know which impedance the amp is currently at or what I’d be changing to. Can anyone help please?

 

[Ramhead]

That looks like a Hiwatt/Sound City/Orange plug. You need this type of selector:

Impedance selector plug window switch voltage valve amp marshall output vintage | eBay

These are often missing, broken or crusted up. Two in the pic for angles, but quantity is the amount you put in the box.

www.ebay.com

 

[StingRay85]

If its hardwired to 16 ohms it (edit: the impedance selector) might not be functional. Check the internals

 

[Pete Farrington]

a 16ohm cab but the way he had it looks like it should be running at 8ohms (can be done as far as i understand)

There should be no problem at domestic friendly, low output levels. But at medium and high power outputs it’s pretty important to get the load impedance selector set correctly on a EL34 (suppressor grid type pentode) amp, especially a 100W model, as the higher HT supply voltage puts everything under more stress.

 

[Seanxk]

Your mains selector is correct and if you use that it will tell you where you are via the window, BUT only if you have as others have mentioned the correct internal wiring, pics will help there.

Your second pic shows you are running at 8 ohms with the incorrect selector.

 

[neikeel]

To use that selector if your amp is on the cab in correct position to play the arrow needs to be pointing up and to the left to be on the 16ohm option.
You can get the correct window type but you will need to take care as the 'Hiwatt' plug has fatter pins than the Marshall so the Marshall one might be too loose. You can re-tension the socket but if you are going to take it out and gig it and only use 16ohm cabs it would be safer to hard wire it internally. If you use a variety of cabs then a suitably rated rotary switch might be the answer (please don't hog the chassis out for a modern coin selector) use a blanking plate with a hole in it and use the original screws, fit the 3-way rotary switch in the hole. If you sell the amp you can give the new owner the old selector (just my 2c).

 

[rekelso]

Your mains selector is correct and if you use that it will tell you where you are via the window, BUT only if you have as others have mentioned the correct internal wiring, pics will help there.

Your second pic shows you are running at 8 ohms with the incorrect selector.

What would I be looking for on the internals? I’m not terrible with circuits but I’m not brilliant with them either, not great with the circuit diagrams

 

[Matthews Guitars]

Open up the amp, take photos of the selector wiring, post them here.

 

[Gutch220]

based on your 3rd photo (as I see it, upside-down)......connecting the center to the 12:00 input is 8 ohms..........center to 8:00 is 4-ohms.......center to 4:00 is 16-ohms. It makes more sense with a window on the knob to see the number.
Of course, this all assumes the internal wiring is correct.

 

[Dblgun]

As Neil said unless you're going to to be very diligent about monitoring the stock selector it is better to remove it and store it while utilizing a modified version. It only requires bolting in a plate and then affixing a more available and reliable rotary switch. The plates are easy to make out of a number of materials but look to be commercially available as well.

BLACK ALUMINUM PLATE IMPEDANCE SELECTOR SWITCH IN OLDER MARSHALL

Placa o plate de aluminio negro del conmutador de selector de impedancia para viejos amplificadores Marshall Plexis y JMPs up hasta 1979.

www.electronicabaile.com

 

[rekelso]

Open up the amp, take photos of the selector wiring, post them here.

Hi, these are the internals, do they look stock on the selector?

 

[Matthews Guitars]

Here's a normal looking example.

 

[rekelso]

Looks like the same wiring configuration but different colour coding? I have green+purple on one pin where that is green and black, yellow on mine where that has green and brown where it has yellow. Does this mean it’s standard and marshall just weren’t consistent with colour coding?

Here's a normal looking example.

View attachment 116199

 

[StingRay85]

Yes they are stock. Follow neikeels advice a few posts earlier. The amp was not hardwired on a certain impedance, so don't run the amp without the selector

In principle you also just run the amp the way it is now. Just make sure the center is connected to the impedance of choice by placement of the two pins of the selector, and neglect the direction of the arrow.

 

[Seanxk]

Is the OT original, looks like it's been joined to existing cable, nicely placed over the filter caps and running over to the PT. Personally I would like to know if all is correct by testing the OT first.

 

[rekelso]

Is the OT original, looks like it's been joined to existing cable, nicely placed over the filter caps and running over to the PT. Personally I would like to know if all is correct by testing the OT first.

As far as I’m aware, all the parts (bar valves etc) are original and the only mod the previous owner said was that the power selector was hard wired at 240V

 

[Dblgun]

Something looks odd there. The yellow, brown and green are all spliced to twisted pairs running under the board. They would seemingly be going to the O/T but that does look like anything I've seen before. Can you post some photos of the transformers?

 

[rekelso]

Something looks odd there. The yellow, brown and green are all spliced to twisted pairs running under the board. They would seemingly be going to the O/T but that does look like anything I've seen before. Can you post some photos of the transformers?

Like this?

 

[StingRay85]

Your power tubes are toast. Bring this amp to a tech

 

[neikeel]

Those spliced wires look to be the OT secondaries to me.
Does look to have been run hard and hot!