While the Vox AC-30 gets most of the acclaim, the real root of the classic British Invasion sound is none other than Dick Denney's AC-15 design (originally, though very briefly known as the AC-1/15). With "small bottle" EL84 power tubes in a unique self-biasing configuration without negative feedback to remove distortion, the AC-15 produces an incredibly musical overdrive and makes use of the "Vibravox" effect—a combination of tremolo and vibrato—to push out some of the most recognizable guitar sounds in history.