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Wiring a cab for switchable impedence?

Leonc

Wild Gear Hearder
Platinum Supporting Member
Messages
18,243
I used to have a Marshall 4x12 1960a cab that included a back plate with two input jacks and a 2-position switch. I'm looking for solutions (store bought or home-made) that will provide similar functionality.

Here's how that switch worked. The cab included 4 16 ohm spakers.

With the switch in one position, each input jack took an 8 ohm load (two speakers, in parallel per jack) and was intended for stereo output. Of course, you could also use it to play out of half the cab at 8 ohms.

With the switch in the other position, you got either a 4 ohm (2 parallel pairs, in parallel) or 16 ohm load off all 4 speakers (2 parallel pairs, in series).

So...can anyone tell me where to get a swtich like this? If not, what kind of swtich to buy and explain the wiring to the switch?

Now...I'd also like to have a 2-pos switch for my 2x12 (with two separate input jacks) which is loaded with 2 8 ohm speakers let me do something similar.

In position one, I get an input to each speaker.

In position two, one jack gives me both speakers, in parallel, and the other gives me both speakers but in series.

As usual...I need to know what kind of switch to get and how to wire it up.
 

John Phillips

Member
Messages
13,038
The easiest solution is to buy the Marshall switching plate - it should be available through any Marshall main dealer as a spare part (I think you may need to order the plate and the internal electrical bit separately). This will do exactly what you want - it doesn't matter that the cab is a 2x12" with 8-ohm speakers rather than a 4x12" with 16s.

If you don't want to do that (the Marshall unit uses a PCB, which may put you off), you need two sockets and a DPDT switch.

I'll see if I can find (or draw) a diagram of how to wire it up, and post it. I have it in my head but that's not much use to you right now...
 

JamesPeters

Member
Messages
1,397
Hi Leon. I whipped this up...it seems to jive in all instances:

1) using the SPDT switch in the "up" position (the pole makes contact with the lower terminal in the diagram) means each jack gives 8 ohms on each side, "stereo"--two speakers for each jack. Use either jack, or both.

2) using the switch in the "down" position means the left jack gives 16 ohms total (all four speakers)--or using the right jack gives 4 ohms total (all four speakers). Use only one jack at a time.

You'll need two mono switching jacks for this (the standard "Marshall" type jacks which have switches on both contacts), and a SPDT switch (use one side of a DPDT if you can't find a SPDT).

http://petersamps.com/series_parallel_switching_412.gif

After you wire it up, you'll probably want to check each position/jack with a meter (by inserting a speaker cable into a particular jack, and using the meter on the speaker cable to give a resistance reading). I think I got this right, but I am pretty tired at the moment. :D I'll be sure to look at it later on anyway, after I catch a nap.
 

JamesPeters

Member
Messages
1,397
Oh yeah, and you can omit the bottom two speakers from that diagram for the purposes of your 2x12. Imagine those upper two speakers are both 8 ohm speakers instead of a pair of 16 ohm speakers on each side in parallel. Same thing. :)
 

Leonc

Wild Gear Hearder
Platinum Supporting Member
Messages
18,243
John - that is clearly a good suggestion!

James - wow! That is so very nice of you to go to the trouble to draw this up. This will be a great little project for me! Actually, for me, it is project of monumental complexity...:rolleyes: heheh, but thanks, this is perfect!
 




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