Here's the deal: That jack has a switch that connects battery positive to the preamp. You can rewire the instrument to use a TRS jack by soldering the 2 red wires together (then heatshrink them, or black tape securely if you don't have heatshrink), then separate the 2 black wires. One goes to battery negative, one goes to circuit ground. Solder the one that goes to the battery to the ring contact of the TRS jack and the other one to the sleeve contact. Purple wire will go to the tip. It'll pop a little louder if you plug a hot cable into the guitar than it did before, but you'd never do that anyway, right?