Ampeg Portaflex SB-12 preset

Verified · 90/100 confidence. Complete schematic. All component values readable. Parameters derived directly from the circuit.

The Ampeg SB-12 is a Portaflex flip-top bass combo. Bright and normal inputs feed a 12AX7 stage, then a volume and a second gain stage with bass and treble controls. A second 12AX7 adds gain and a split-load (cathodyne) phase inverter for a fixed-bias pair of 6L6GCs on a -48 V bias supply, with feedback from the 16-ohm tap, a 5AR4 rectifier, hum balance and an external amp jack.

• Made in: USA
• Wattage: 25W
• Channels: 1
• Preamp tubes: 2× 12AX7
• Power tubes: 2× 6L6GC

Tags: american, vintage, bass, 6l6, mid-wattage.

Schematic-derived preset files for 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.