Marshall 1922 2X12 Cabinet Wiring?

[Vanncad]

I just picked up a used Marshall 1922 2X12 Cabinet. It has 2, 16 ohm Celestians in it. It has 2 speaker jacks. One is used to run each speaker in stereo (16 ohm load) and the other jack is used to run in mono (8 ohm).

The previous owner re-wired it run each 16 ohm speaker individually (i.e. each jack is wired to each speaker with no connection between the two jacks).

I want to re-wire it back to original so I can run my Bandmaster head through it using both speakers in mono.

I have been searching all over but I can't find a picture of the original wiring.

I have wired the the two jacks back the way I *think* they should go but when I put an ohmeter on the "mono" jack, I still get a reading of around 14 ohms.

My amp tech told me before that 8 ohms is fine with the head, but not 16.

I just want to make sure I don't fry my head. Does anyone have a pic of the wiring of a stock Marshall 1922 cabinet?

Thanks,

 - Vann

[Geir]

If you're just gonna use it as a mono cabinet you just wire the 2 +'s together and the 2 -'s together and one wire from each to the jack input. That puts the two speakers in parallel and you get 8 ohms

I'm off to the cabin for the weekend, but if you haven't gotten any schematics by monday I'll draw you one then.

[SdC]

Simple; wire them parallel: combine the speaker's +-wires to the tip of input jack and the minus-wires of the speakers to the sleeve.

1/Rt = 1/R1 + 1/R2

1/8 = 1/16 + 1/16

So the resulting resistance is 8 Ohm.

[SdC]

..... when I put an ohmeter on the "mono" jack, I still get a reading of around 14 ohms....

This is NOT a reading that is representative for speakers! A speaker has different resistance at different voltages. To measure the true impedance of a speaker or cabinet requires a rather complex procedure involving signal generators, power amplifiers and high frequency AC voltmeters.

[64Guitars]

Does your output jack plate look like this?



I mean, are the jacks labelled Left and Right, with the left marked MONO? If so, then the right speaker jack will have more contacts than the left (unless the previous owner replaced the jack). The extra contacts of the jack form a normally-closed switch so that the speakers are connected in parallel when only the left jack is used. When both jacks are used, the switch contacts are opened so that the speakers are no longer connected to each other (for stereo).



What impedance is the output of your Bandmaster? If it's 8 ohms, then connecting two 16-ohm speakers in parallel is ideal, as the total impedance is 8 ohms. If the Bandmaster is only 4 ohms, the 8-ohm load of the parallel-wired speakers won't hurt your amp, but its output will be slightly reduced. Connecting the speakers in series would increase the total impedance to 32 ohms. Again, this probably won't hurt your amp but it will reduce its output even further. So you should definitely wire the two speakers in parallel.

When it gets dangerous is when you have the reverse situation. For example, if your amp's output impedance was 16 ohms and you connected a 4-ohm speaker cabinet to it, then the amp's output current could be driven to levels that it wasn't designed for, and you could fry the amp.

[Vanncad]

Hey thanks guys!

Geir - Thanks for info. I hope you have a good time at the cabin. 64 was nice enough to draw me a a schem so I think I'm all set.

SDC - Thanks for the info as well. I'm not exactly an electrician, so I appreciate the ohm meter explanation. Thanks for taking the time to reply.

64 - Thanks for the diagram. Did you do that in "MS Paint"? 
My jack plate is exactly like the one at the top of your image (1922 2x12). The right jack has 4 connections and the left has 2.  By examining your design, I believe I do have it wired correctly now. My Bandmaster is 4 ohm, so the  8 ohm parallel-wired speaker option is exactly what I wanted.

Thanks again for all your help guys.

This site rocks!

  -Vann

[64Guitars]

64 - Thanks for the diagram. Did you do that in "MS Paint"? 

No. I downloaded this image and modified it with the GIMP.

[Vanncad]

Cool. I had GIMP on my old PC, but I couldn't quite get used to the tools (not quite the same as Photoshop).

Thanks again for your help.

 - Vann