I have two main combos that I use; my self-built SE-5a in a 2×10″ combo and my Fender Twin with the standard 2×12″.
I’ve got in mind modifications to both so that I can feed the SE-5a through the Twin’s 2×12″ and vice versa. The problem with this is that of matching impedance. The Fender Twin requires an impedance of 4 ohms whereas the SE-5A requires either 8 or 16 ohms.
All of the speakers in question are 8 ohm. For the SE-5a they’re wired in series for a combined impedance of 16 ohm and for the Twin they’re wired in parallel to give 4 ohms.
I needed to be able to switch impedance. After a few minutes of googling I couldn’t find what I was after so I sat down, worked out how to do it and thought it may be worth sharing for others in the same predicament.
With a DPDT (double pole double throw) switch I can make a pair of speakers switchable between series and parallel. With 8 ohm speakers this results in each pair being switchable between 4 and 16 ohms. Here’s the schematic.
Please note: I haven’t tested this yet. Once I’ve confirmed this is working I’ll report back but, for now, please treat this with scepticism.
Dave Mac's Window on the World 
Can’t see any reason why this shouldn’t work. Make sure that the switch can take the voltage and you should be good to go. Alternatively, look at ampmakers box of matches. On May 24, 2013 2:11 PM, “Dave Mac's Window on the World” wrote: > > damacleod posted: “I have two main combos that I use; my self-built SE-5a in a 2×10″ combo and my Fender Twin with the standard 2×12″. I’ve got in mind modifications to both so that I can feed the SE-5a through the Twin’s 2×12″ and vice versa. The problem with this is th” >
I’d not thought about the Box of Matches (http://www.ampmaker.com/store/Box-of-Matches-30W.html). Cracking idea. And it has a variable line out which means I could have a go at slaving the amps together. Thanks.
kewl Idea Dave, I dont see why it wouldn’t work….. might need a beefy switch though…
I was looking at a 2×12 cab many crossroads ago and was thinking of doing a similar thing so I wouldn’t have to open it up if it ever needed impedance matching. however i was going to use a small patch panel mounted on the back of the speaker cab and just re-patch to what i wanted… your idea is much better 😉
Cheers Alfie. Yes it’ll be beefy switches. I did think about having two switching jack sockets but this seemed like the simpler approach.