Switching a cab for series/parallel/stereo

RMosack

Silver Supporting Member
Messages
7,898
I have a 2x12 cab with a couple of 8 Ohm speakers wired in stereo. In other words, one speaker for each of the two jacks.

I've seen Marshall cabs that have a little switch in the back that allows you to choose between 4 Ohm mono, 16 Ohm mono and 8 Ohm stereo.

How the heck do they do that?

I'm not adverse to installing a switch or two into the back of my cab. Switching jacks if need be too. I'm also open to slapping together a little switching box to do it between the amp and cab. I just don't know how to do it.

Anybody have a source for a wiring diagram for this?

Or, if possible, anybody know a source to simply buy one of those Marshall cab jack plates?
 

RMosack

Silver Supporting Member
Messages
7,898
Thanks for the link.

Very interesting.

Are those guys still in business. The website says "under construction" for a few of the pages. A question in the FAQ section asks when the site will be done. The response says it'll be done some time in 2006.

They have a link to dealers, but that part is also under construction. Their direct price seems a bit steep for a few jacks and some switches. Maybe I'll give them an email anyway.


FWIW, that CabDeuce seems to be exactly what I need and then some! It does even more. My 2x12 cab with a pair of 8 ohm speakers can be run:
1) normal stereo operation where one jack is assigned to each 8 ohm speaker
2) 4 ohm parallel where either of two jacks can be used while the other is wired in parallel to that for connection to another cab entirely.
3) 16 ohm series using the middle jack; this allows eithr of the two outside jacks to be used to connect another cab in parallel to either of my speakers BEFORE their series connection.

Very cool.
 

Wakarusa

Member
Messages
1,458
Or you can just roll your own with a pair of Cliff jacks and a DPDT switch:


Speaker_Wiring2.jpg
 

RMosack

Silver Supporting Member
Messages
7,898
Wow, I've never heard of Cliff jacks. What are they?

I actually sat down and figured out a wiring scheme yesterday afternoon that involved a pair of DPST (Edit because I think the switches I drew out were DPST, not DPDT) switches and regular jacks (two leads). It was kind of ugly and one of the four switch combinations wasn't usable.

It there's a way to do it with just one switch, I'm all ears.
 

Wakarusa

Member
Messages
1,458
Cliff is just a brand name. They're the jacks Marshall and others have been using since dirt was rocks. The way you get away with a single switch is by taking advantage of the switching built into the jacks. The jacks cost a couple of bucks each. The dirty secret is that if one of the jacks fail, you could unknowingly get an open circuit in the cab, so it pays to take care of the rig (and test it with a DMM occasionally) to prevent this.

For a way to pull this off with a single switch and "regular" (i.e. switchcraft 11 or 12A) jacks, look for a thread started by leftys-bbq asking about dummy loads. I posted a design that uses a single rotary switch to choose between different impedances. The up side is that you're not using the jacks for switching so the design is probably more robust. The down side is you have to find room on the jack plate for the rotary switch. You'll also have to change the design slightly to remove the 2 ohm option and allow for stereo operation.
 

SouthernShred

Gold Supporting Member
Messages
3,550
Along the same lines Todd, how about a schematic for wiring up my Super Reverb (4 ohm load expected obviously) to use all 4 speakers or switch to using only two and having the proper impedence match. Assume four 8 ohm speakers (new ones, not the existing CTS hodgepodge) with power ratings able to handle it.
 

Wakarusa

Member
Messages
1,458
Along the same lines Todd, how about a schematic for wiring up my Super Reverb (4 ohm load expected obviously) to use all 4 speakers or switch to using only two and having the proper impedence match. Assume four 8 ohm speakers (new ones, not the existing CTS hodgepodge) with power ratings able to handle it.

Actually, your super reverb expects to see a 2 ohm load, which you only get by putting all four 8 ohm speakers in parallel. Any other arrangement gets you a higher impedance (i.e. a pair of speakers in parallel will produce twice the expected impedance.. not such a great idea).
 

RMosack

Silver Supporting Member
Messages
7,898
Thanks Todd.

I don't mind using one or two switches, I just want to know I did it right.
I'll have to look for that thread.
 

RMosack

Silver Supporting Member
Messages
7,898
Or you can just roll your own with a pair of Cliff jacks and a DPDT switch:


Speaker_Wiring2.jpg

I checked the Cliff website and I'm not certain which jacks those are.
Anybody know the part numbers or some other way of ID'ing them?

Is this the diagram that Marshall uses for all of their switchable cabs?
 

SouthernShred

Gold Supporting Member
Messages
3,550
Actually, your super reverb expects to see a 2 ohm load, which you only get by putting all four 8 ohm speakers in parallel. Any other arrangement gets you a higher impedance (i.e. a pair of speakers in parallel will produce twice the expected impedance.. not such a great idea).

*smack* duh...why did I say 4...
 

Wakarusa

Member
Messages
1,458
*smack* duh...why did I say 4...

I'll play your silly game.. why? :D

Anyhow, you still have the same problem of all combinations other than all 4 in parallel presenting a higher than expected impedance. Might help to understand why you want to do this.
 

SouthernShred

Gold Supporting Member
Messages
3,550
I'll play your silly game.. why? :D

Anyhow, you still have the same problem of all combinations other than all 4 in parallel presenting a higher than expected impedance. Might help to understand why you want to do this.

why? late night in the workshop? solder fumes going to my head? lots of beer consumption? :D

reason? silly idea I had about disabling 2 of the speakers for playing smaller rooms.
 

Wakarusa

Member
Messages
1,458
reason? silly idea I had about disabling 2 of the speakers for playing smaller rooms.

You could try turning the knob marked "volume" counter-clockwise. The one on the amp OR the one on your guitar. :)

You could rig up something that drops a dummy load in place of one or more of the speakers. It'll change the amp response a bit, but probably not enough to make much difference in your situation. For more on this, R. Aiken did a nice write up on his web site on the effects of resistive loads in parallel with reactive (e.g. speaker) loads. There's plenty of room in the back of a Super Reverb to mount some power resistors and a heat sink.

For that matter, you could throw any of the commercial attenuators in between the amp and speakers, long as you're willing to accpet the risks that come with using one.
 

RMosack

Silver Supporting Member
Messages
7,898
RMosack,

I also have a 2 x 12 stereo cab and have always wanted to wire it up the way you are describing.
I've been plan planning to do it for the longest time, but just haven't gotten around to it...

I was planning to get this:
http://www.mojomusicalsupply.com/item.asp?pid=18076&pg=44990&id=9131200

To the wiring gurus, would this do the trick?


Thanks!

I'm not a wiring guru, but that would most certainly do the trick. I've actually seen those on Marshalls and that is EXACTLY what I was hoping to figure out how to do myself. I figured you couldn't buy those anywhere, but I guess you can.
 

erikm5150

Member
Messages
565
RMosack,
I was also pleasantly surprised to find that these jack plates were available for sale.

Wakarusa,
Thanks for the advice. I'm glad to see that it's available for cheaper.
 




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