Ampeg SB-12 Portaflex (7868) preset

Verified · 91/100 confidence. Complete schematic. All component values readable. Parameters derived directly from the circuit.

The Ampeg SB-12 (7868 version) is a single-channel Portaflex bass amp. Instrument and Bass inputs feed a 12AX7 gain stage, a Volume control and a second stage driving passive Bass and Treble controls. A second 12AX7 works as a floating-paraphase phase inverter with negative feedback from the 16-ohm tap, driving a push-pull pair of 7868 output valves with adjustable fixed bias and a balance control, rectified by a 5AR4 into an 8-ohm speaker.

• Made in: USA
• Wattage: 22W
• Channels: 1
• Preamp tubes: 2× 12AX7
• Power tubes: 2× 7868

Tags: american, vintage, bass, 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.