Question about impednce from Speaker out Jack on amp

[Jerry2013]

Heres whats going on: I have a Marshall JMP 2203 head. When I bought it used , someone had disabled the Impedance selector switch and put 3 new hardwired switchcraft output jacks in for 16 , 8 , and 4 ohms.
A good idea I think to ditch the awful wobbly Marshall impedance switch, besides the originality issue, which doesn't concern me, since this amp is lifetime keeper.

I've run into a concern however, for I would like to now run a full stack- two cabinets.
There is also a 4th unmarked jack besides he marked three( 4,8,16), that I'm not sure if it is wired up , and if so, how.
Could this be a second cabinet jack that would function at the same output load resistance of that which the other jack is plugged into ( 4,8, or 16 Ohms).

Any way to check it for proper resistance?

If such a thing is possible , and it doesn't seem likely, and then if there is a way to check the resistance, I would do that.

If not, I will probably take the cabinet off and see if the other 4th jack with no markings of ohms is actually wired up, and if it is, take pics to show here.

EDIT- Actually, I just tried the unmarked speaker output jack , and there is no signal, so it must not be wired up. So then, would it be possible to wire it up to work in conjunction with the normal outputs ( 4,8,16 Ohms) like the two jacks do on a stock Marshall with Impedance Selector Switch?
If not, I wouldn't be adverse to adding two more jacks, and wiring them up if that would be an option?


Thanks for your help.

[teemuk]

If not, I will probably take the cabinet off and see if the other 4th jack with no markings of ohms is actually wired up, and if it is, take pics to show here.

I highly recommend doing this instead.

[rock_mumbles]

I agree that opening it up and looking/taking pictures would be a good idea

...

The fourth jack would have to be wired to just one of the other jacks ...

You could do the following:

With the amp off plug a guitar or speaker cord into the fourth jack ... plug another cord into the 8 ohm jack ... now check for continuity between the tip of one cord and tip of the other and sleeve of one to the other ...

If the two tips and sleeves have continuity the 4th jack is a second jack connected to the 8 ohm tap so you can run two 16 ohm cabinets in parallel as an 8 ohm load.

If the 4th jack has continuity with the 4 ohm jack the 4th jack is for running two 8 ohm cabinets as a 4 ohm load.

[Jerry2013]

I agree that opening it up and looking/taking pictures would be a good idea

...

The fourth jack would have to be wired to just one of the other jacks ...

You could do the following:

With the amp off plug a guitar or speaker cord into the fourth jack ... plug another cord into the 8 ohm jack ... now check for continuity between the tip of one cord and tip of the other and sleeve of one to the other ...

If the two tips and sleeves have continuity the 4th jack is a second jack connected to the 8 ohm tap so you can run two 16 ohm cabinets in parallel as an 8 ohm load.

If the 4th jack has continuity with the 4 ohm jack the 4th jack is for running two 8 ohm cabinets as a 4 ohm load.

I just need to have the two 8 OHM Jacks wired together? Would that work? I have two 16 Ohm Cabinets.
I'll take it apart and see what I'm dealing with.

[rock_mumbles]

I agree that opening it up and looking/taking pictures would be a good idea

...

The fourth jack would have to be wired to just one of the other jacks ...

You could do the following:

With the amp off plug a guitar or speaker cord into the fourth jack ... plug another cord into the 8 ohm jack ... now check for continuity between the tip of one cord and tip of the other and sleeve of one to the other ...

If the two tips and sleeves have continuity the 4th jack is a second jack connected to the 8 ohm tap so you can run two 16 ohm cabinets in parallel as an 8 ohm load.

If the 4th jack has continuity with the 4 ohm jack the 4th jack is for running two 8 ohm cabinets as a 4 ohm load.

I just need to have the two 8 OHM Jacks wired together? Would that work? I have two 16 Ohm Cabinets.
I'll take it apart and see what I'm dealing with.

Yes that's what you want if you have two 16 ohm cabinets ... the 4th jack would connect to the 8 ohm jack, tip to tip sleeve to sleeve (ground)