This effect is a ring modulator and sounds fantastic with a fuzz pushing it. The original unit used 2N5172 transistors (which have BCE pinout), but I've laid this out for more common 2N3904 or 2N5088 transistors (that have CBE pinout). I've also added pulldown resistors, polarity protection, and power filtering.
Here's the schematic if anyone needs it. Most of the ones online have the tone section wrong.