Bogen CHA-20 Blues Special preset

verified · 85/100 confidence. Complete schematic. All component values readable. Parameters derived directly from the circuit.

The 'Blues Special' is a Bogen CHA-20 PA amplifier converted into a guitar amp. Two inputs feed a two-stage 12AX7 preamp with a tweed-style bass and treble network and a Tweed switch, then volume and a further gain stage into a self-balancing paraphase inverter. A 12AX7 drives spring reverb with its own recovery stage, another runs phase-shift vibrato, and two cathode-biased 6V6s have a pentode/triode switch, on a 6AX5 rectifier.

• Made in: USA
• Wattage: 20W
• Channels: 1
• Preamp tubes: 4× 12AX7
• Power tubes: 2× 6V6

Tags: american, 6v6, reverb, vibrato, diy, mid-wattage.

Schematic-derived preset files for the Neural DSP Quad Cortex and 16 Fractal and Line 6 modelers: Axe-Fx II/XL/XL+, Axe-Fx III, FM3, FM9, and the full Helix family (Floor, LT, Rack, HX Stomp, HX Stomp XL, Native).

Gridleak extracts amp parameters from original circuit schematics using AI vision — component values, gain staging, tone stack topology, tube types — then generates native preset files for every supported modeler. Every extraction is scored 0–100 and the score is published.