Boss TR-2 Tremolo Pedal
Tremolo is a real classic effect found on many vintage amps. For those who don't know , tremolo is a an effect that periodically changes only the volume so your tone is not effected.
The pedal has 3 knobs to adjust the sound. The rate knob changes the rate of tremolo from a very slow 2/3 second cycle to a super fast machine gun speed. The depth control is the depth of the effect and goes from no effect all the way to 100%. The wave knob blends from sawtooth smooth tremolo like a Fender amp where the volume gradually falls and rises to square wave which is like switching the sound on and off.
By combining these controls you can get the classic vintage tremolo , a rotary speaker style sound, and the fast abrupt switching on/off sound which works well with distortion. Think REM, Smiths or Rage Against the Machine and you'll know what I mean.