Lumpy's Tone Shop Lemon Drop/ZII/Series 7

Here's a simple little overdrive designed to emulate the old Vox 4 and 7 series amps (used by the likes of Jimmy Page and the Beatles). Pots are board mounted and it should fit in a 1590B easily. I've added a trimmer to bias the JFET at little easier. Schematic and voltages can be found here on FSB. From the manufacturer:

The 7 Series Overdrive captures the aggressive qualities of the solid state preamp found in the 4&7 series amps by using a pseudo hybrid setup consisting of a specifically selected silicon transistor for the first stage which adds a distinct bite and growl.  That pushes an FET stage into clipping adding some warmth, sustain and amp like dynamics to the tone and feel.  The end result is a highly effective emulation of the amp and a unique sonic blend that could be characterized as a "fuzzy overdrive."