BIAS Head/Rack — Amp Controls Guide – Help Center

This guide explains every amp control section on BIAS Head/Rack — from preamp tube stages through to power amp topology, EQ, and master controls.

Preamp

Tube Stages

The TUBE STAGES dial controls the number of cascading gain stages in the preamp. More stages means more gain and harmonic saturation.

1 — Glassy, clean tone
5 — Overdriven, high-gain tone

Preamp controls

Distortion

The DISTORTION knob fine-tunes the amount of drive within the selected tube stages, giving you greater resolution to dial in the exact break-up point.

Bright / Normal Switch

The BRIGHT | NORMAL switch adds extra top-end frequencies to the preamp signal before it reaches the power amp — useful for subtly shaping your overall tone.

Power Amp & Topology

The TOPOLOGY dial selects the power amp circuit type, defining when and how the power amp breaks up. Adjust the POWER AMP knob to set the amount of breakup in your preset.

Power Amp controls

Topology Options

Single Ended — Models a Class A single-ended amplifier. Produces a uniquely abrupt distortion character when pushed.
Split Load — Models a Class A/B push-pull amp with a concertina phase splitter. Delivers the smoothest, richest power amp distortion.
Push-Pull — Models a Class A/B push-pull amp with a long-tail pair phase splitter. Balanced, warm gain structure with workable headroom.
Solid State — Models a transistor-based power amp. Cleanest tone with minimal distortion — ideal for pure, uncolored sound.

EQ

The Bass, Middle, and Treble controls replicate the gain and frequency response of the selected tone stack topology. Each parameter ranges from 0 to 11.

EQ controls

Start all dials at 12 o'clock, then sweep each fully up and fully down before settling on your preferred position.

Recipes for Great Tones

American Clean
Bass 6 / Mid 4 / Treble 5

English Crunch
Bass 4 / Mid 6 / Treble 6

English Chime
Bass 6 / Mid 4 / Treble 6

Modern High-Gain
Bass 6 / Mid 4 / Treble 4

Presence & Master

Master and Presence controls

Master Knob

The MASTER dial controls the overall power amp breakup level. Use it in combination with the preamp DISTORTION dial to balance preamp and power amp distortion. Start at 6, sweep to max, then roll back to taste.

When the power amp is heavily saturated, the EQ and Presence controls will have less effect and the tone may become more "boomy".

Presence & Modern / Vintage Switch

The PRESENCE knob boosts or cuts upper frequencies in the power amp stage. Start at 0, sweep to max, then roll back to the sweet spot.

Modern — Cuts through the mix more aggressively; ideal for higher-gain amp types.
Vintage — Classic, balanced upper range. Pair with the Bright switch in the Preamp section if you need to recover lost high-end.