Here's a variation of the classic Tone Bender Mk II Professional from ProAnalog Devices. It uses PNP transistors, so you'll have to use either a battery or voltage inverter to daisy chain this with other pedals. It seems they've used several different transistors, but they're outlined in the schematic from Scotty himself. Easy fit in a 1590B. And if you want to make it as a negative ground effect, use NPN germanium transistors and flip all polarized components in the layout.