Here's a simple one that should fit nicely in a 1590a. EQD's take on the Electra Distortion circuit. Instead of having a pot coming off the end of the circuit, the volume is controlled by a pot coming off the emitter of the 2N3904. It's really more of a gain control than volume and if you wanted to, you could add a master volume control coming off the output. Who doesn't love more knobs, right?
This is a library of perfboard and single-sided PCB effect layouts for guitar and bass. I'm not an electrical engineer by any stretch of the imagination, just a DIY'er who likes drawing layouts. It is meant for the hobbyist (so commercial use of any of these layout is not allowed without permission) and as a way to give back to the online DIY community.