Can anyone decipher this circuit? Built for me back in the 90's by the late Sal Trentino of San Anselmo CA. I asked him how to run my Princeton Reverb into a bigger amp for more volume on stage without changing the original sound. He said sure... I'll build you one like Neil Young uses on his Deluxe. The hookup is: run speaker cable from PR speaker out to the box input and speaker out back to PR speaker. Then line cable out to 2nd amp. Lift ground on 2nd amp. Adjust output control. It works good. It looks very simple inside the engine room, only 3 doo-hickeys in shrink tubing. What is going on in there in basic terms? I searched and can't find anything else quite like it. Is there a better way to do this?