This seems to be one of those effects nobody can agree on. Not really knowing one version from another I’ve given a schematic by ringworm of FSB a try. If it’s not the right version it should be an easy task changing component values.
The layout makes it possible to use on-board pots, and my idea is to fit this in a 1590B with two right angled PCB pots in a diagonal fashion.
Update (13.12.2011): Built this one last weekend. Verified working.
Update (08.01.2012): Finished building this last night. I added a bit of hot glue to Q1 because the socket wasn’t binding as much as I would like it to.