Little Jim

Named after Jim Marshall, the Little Jim goes after the bright, crunchy, raucous sound of a cranked Marshall, deceptively complex and rich in harmonics.

The circuit. Three cascaded common-source gain stages on BS170 MOSFETs, after a J112 JFET input buffer: the BS170's high input capacitance would otherwise load the guitar and dull its tone. MOSFETs sound much like the JFETs used in most earlier designs here, but more aggressive and crunchy, at the cost of some extra hiss.

The tone control is a variable treble roll-off that also lifts the bass, making up for the bass cut in the first stage so the low end doesn't get flabby. The three MOSFETs need similar threshold voltages, ideally around 2 V; each drain should sit at two thirds to three quarters of the supply, clipping asymmetrically.