Tube amp bias typically refers to the amount of electrons passing through the grid of a vacuum tube. This phenomenon also changes as vacuum tubes warm up.
In the context of the Fender Mustang Amps, Fender Fuse Software, and Remuda App, it refers to an emulation of the amp model's tube being colder or hotter state.
- LOW: emulates colder tubes for a cleaner sound
- HIGH: emulates hotter tubes for a more tube-distorted sound
In the case of the amps without power tube saturation emulation using the Master Volume FUSE control, increasing the BIAS parameter to the max will help obtain a somewhat similar effect.