Re: Turning A 4X12 in 2 2X12's
I did that to a Marshall 4-12 once, since I had the mono version and needed the stereo version. If I did it again, I'd order the stereo jack that Marshall sells aftermarket. I decided to just add another 1/4" female jack underneath the stock jack and wire the two sides independent of each other. I wired each side in parallel, so it would bring two 16ohm speakers to 8 ohms per side. The only drawback was that I couldn't combine them. My purpose for doing it was to run two heads on 1
4-12. If I wanted to run 1 head, I just used 2 speaker cables.
Another option is to just sell your mono 4-12 and buy another used stereo one for a few bucks more. That way you eliminate all the modding hassles, expense, and get a proper stereo cab. If I ever did that again, I'd go that route, or buy the Marshall stereo jack.