 The addition of the reverb circuit requires an additional "gain" stage in the preamp, and as such, the Vibrato channel is capable of a bit more distortion when the volume is set high.Vibrato (tremolo) is accomplished with what is known as a "vibrato bug" by Fender amplifier repairmen.The vibrato bug is a four-wire device consisting of a neon tube and light-dependent variable resistor, packaged in a short section of black tubing.