Here you go:
This assumes your head wants to see an 8 ohm load. This is the text of the article, the pictures are below.
I found it online somewhere awhile ago. The important things are if you are using a tube head in mono, make sure it wants to see an 8 ohm load. I assume you already have this cab and are using it. Don't plug your head into the unused jack while in mono, or either one if you use a switch and it is in stereo. Could damage the head. And you need to isolate each jack on a separate plate, or use a plastic one.
Anyway, here is the article.
4 Musical Instrument Speakers. Impedance selectable by using a switch.
Option 1
4 - 4 ohm speakers = 8 ohms to each channel if run in Stereo, 4 ohms in Mono mode.
Option 2
4 - 8 ohm speakers = 16 ohms to each channel if run in Stereo, 8 ohms in Mono mode.
WARNING: Do not flip the switch while power amp(s) is(are) powered up
First picture goes here
The Right channel jack is completely disconnected when in Mono Mode. Do Not alter this configuration (you risk doing serious damage to your power amps if wired differently).
Mouser Electronics Sells 1/4 inch phone jacks that have a built in DPDT switch - the switch contacts are electrically isolated from the 1/4 inch Jack connection (they have to be in order for you to wire this correctly) - Marshall used this type of Jack/Switch to automatically set the speaker selection mode. They are expensive and prone to failure over time, however, using them often simplifies set-up of your rig. If you decide to use these, the Mono/Stereo switch functionality would be associated with the Right channel, where when nothing is plugged into the Right channel, the cabinet is operating in Mono Mode.
WARNING: Never run a Tube Type power amp to the Right channel if the switch is set to Mono mode - it will damage the output transformer.
WARNING: The Left and Right jacks cannot be mounted on the same metal plate (serious damage to some power amps will occur if you do this). Use a seperate jack plate for each channel, mounted on a non-conductive surface
Mono/Stereo in same cabinet
Personally, I prefer external mounted DPDT switches over the 1/4 inch Jacks that have built in DPDT switches. Part of this is because I use cabinets that have Speakon Connectors as well as 1/4 inch jacks, and there is no way to automate the switching with a Speakon connector. Another reason is that the switched jacks are not that reliable over time. Many of my cabinets are 20 years old and get used frequently - all switches malfunction over time and switched jacks are hard to find if you suddenly need to replace one (buy spares if you decide to use them).
NOTE: For all DPDT switches shown - I suggest 5 Amp (or higher), 120V rating
Second picture goes here.
Example of translating a DPDT Jack to/from DPDT Switch (Your Switched Jack may be different)
NOTE: The Ring Connection would be left open for all examples shown above
A regular DPDT switch will not include NO and NC markings - this is because either position is its normal state. The Switched jack, however, is a momentary contact switch, as as such, when nothing is plugged into it, the NC positions are what it is set to. When you plug a cable into it, the NO positions are what it is set to. Use this logic to figure out what mode you want the Default switch configuration (ie. when nothing is plugged into the jack) when you wire up the jack to the speakers. The NO and NC markings are on the drawing to give you an better cross-reference perspective.
Be aware that if you use a stereo mode switching jack along with a regular mono phone plug, you have to push the plug in all the way (as you will feel the first detent - about 1/4 of an inch (approx 6 mm) before the jack is plugged in all the way) in order for both the switched connections to end up properly switched.
Note: If you do not know how to read schematic diagrams for switches and jacks, please try to find someone who does, to assist you in this effort.
