When Vox introduced its MRB (Mid-Range Boost) circuit on its new solid-state amp line in the mid-1960s, guitarists wanted that sound, but most couldn’t afford to purchase a brand-new amplifier just to get it. Fortunately, at about the same time, Vox introduced the Clyde McCoy wah-wah pedal. By this time, the Thomas Organ Company had a controlling interest in Vox UK, and these circuits were actually designed by their audio engineers here in the U.S. Guitarists quickly figured out that a wah set to a certain position between its two extremes (toe and heel) could deliver pretty much the same sound. Needless to say, Vox wahs sold like hotcakes.
The concept is still valid today. Aside from using a wah rocked back and forth (think of Cream’s “Tales of Brave Ulysses”), the wah can be set to a specific position and left there to obtain an entirely new tonal palette. By experimenting, guitarists can actually hone in on the exact resonant frequency that will cause certain notes to produce feedback, thus sustaining almost indefinitely. Supposedly, this was actually the way Carlos Santana developed his early sound, back when he was playing Gibson SG or Les Paul model guitars. Today there are many different wah circuits, either recreations of the early Vox wah or brand-new designs that have their own distinctive sound. For instance, the Morley Steve Vai Bad Horsie 2 Contour Wah has a 2-knob contour mode for altering the frequency range and level.