Rosie

“Though she be but little, she is fierce.” A second JFET mini guitar amp: small, simple and easy to build, low power, but with a wide range from clean through crunch to high gain and brown sounds. Tonally close to the Boba FET, with more gain, more tonal variety and less hiss.

The circuit. The power amplifier uses a JFET and runs open loop, so it clips gracefully when pushed: not a valve amp, but nicer in clipping than most single-chip amplifiers, and simple and cheap. Emitter resistors in the output stage, even 0.22 Ω, changed the sound too much, so the bias transistors sit side by side, thermally coupled with heatsink compound, and the output transistors on small clip-on heatsinks barely get warm.

Idle current is about 60 mA from an 18 V, 1 A supply. A trimmer sets the JFET bias, which varies from device to device.