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Positive Grid Bias Head

Modelling Guitar Amp
Published March 2017
By Bob Thomas

Positive Grid Bias Head

Insanely tweakable, and promising the ability to mimic your own amp, could this be the last word in amp modelling?

For any guitarist, there’s a reassuring air of familiarity and class about the Bias Head’s black, grey and silver enclosure, but its front and rear grilles are purely cosmetic — inside lurks a 25-preset, deeply programmable combination of Positive Grid’s highly regarded Bias Amp software, the DSP to run it, and a 600W Class-D power amp. And the result might appeal equally to the incessant tweakers and those who just want to plug and play.


Other than the global output level, the front-panel controls are all programmable. The single input jack sits on the left of the lower of two rows, and this hosts familiar gain, bass, middle, treble, presence and master controls, as well as a Modern/Vintage voicing switch. The upper row is less familiar territory: the first two knobs are five-position rotary switches, and these select between the five preset banks (Clean, Glassy, Blues, Crunch and Metal), and the five presets within each bank. Adjacent to these is a second pair of controls, the first of which switches in up to five cascading preamp tube stages. The other modifies the overall level of distortion in the selected stages. A Bright/Normal preamp voicing toggle-switch sits between them.

Next comes a four-position switch that sets the power-amp topology. You can choose from: Single-Ended (tube Class A); Split Load (tube Class A/B with a split-load ‘concertina’ phase splitter); Push/Pull (tube Class A/B with a long-tailed pair phase splitter); and Solid State (transistor). A rotary Power Amp control governs the chosen power-amp stage’s distortion. The Custom knob can be programmed to carry out one of a range of functions, and, finally,...

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Published March 2017