I have combo amp with a 12 inch speaker that runs at 8 Ohms. I have a 1x12 cabinet that runs at 16 Ohms. I would like to use the combo amp speaker and the cabinet at the same time. The combo amp has two 4 Ohm, two 8 Ohm, and a single 16 Ohm output. I am not sure if I should connect the external cabinet to the 16 Ohm output or to the 8 Ohm output. How can I connect the cabinet so that I can use it at the same time as the combo amps built in speaker? Thanks.
I did this 3 days ago and it works a treat, BUT, the combo speaker is 8ohm and so is the external. It's ok to attach more ohms to the amp but not less. I think what you want to do will work, however the volume of the 16 ohm cab may not be as Loud as the combo. I used a Switchcraft 1/4in mono jack plug with shunt. These are also called 'switched' jack plugs.
Hopefully you can see the diagram below. The diagram on the left is the circuit with jack inserted into the plug. You can see the middle arrow (the shunt) is not touching the tip. Solder the +ve wire from the amp to the 'tip' connector. The -ve just stays where it is, don't touch it. The current flows down the +ve wire and into the 'tip' connector of the new jack plug. This connects via speaker cable (not a guitar cable) to the +ve side of the external speaker. The current flows thru the speaker and out the -ve terminal. The current comes back to the jack plug. You'll have to solder a piece of wire onto the +ve side of the combo speaker and solder it to both the 'shunt' and the 'outer' connectors on the jack.
The result is both speakers are on at the same time running in 'series' at 16ohms. A safe load. Make sure you don't wire them up in parallel unless you can run the amp at 4 ohms (in my case 8 ohm is the only option) and could damage the amp.
The smaller diagram on the right shows what happens when the jack is withdrawn from the plug. The current comes down to the plug, jumps across the shunt which is now touching the tip and straight back to the combo speaker. Combo speaker on.
Hope this helps someone out there cause I searched for ages on how to do this and in the end (after 37 sketches!) designed a circuit that would work. The speakers sound great stacked or 5 meters apart.
I'm going to assume the 16 ohm output is driven by it's own separate winding in the output transformer. This can usually be confirmed by opening the amp and tracing the wiring back to the output transformer. If the jack is circuit board mounted, then it may be more difficult to determine because the traces on the circuit board may be hidden from view. On hand wired amps, the hook-up wire to the 16 ohm output jack will probably be a different color from the hook-up wire to the other output jacks. This is an indicator that that the winding is separate and you should be able plug your 16 ohm cabinet in conjunction with the internal speakers.