The Mercury IV is a Germanium Harmonic Boost, a dynamic tone enhancer that coaxes out different harmonic qualities of both instruments and amplifiers. Featuring a whooping +35dB of Gain (!!), this one brings the thunder, while keeping your tone in tact and enhancing it simultaneously. Starting with the knobs, Boost gives you control over how much of that +35dB you want or need in the signal chain. The Harmonics knob blends in light, tube-like harmonic distortion, even adding octave-like overtones to your solos and unique textures to chords. This knob is very reactive to your playing, instrument and pickup choices, and reacts strictly to the input signal, making it a completely different animal for different guitars and playing styles.