The Electro-Harmonix Holy Grail ranks as one of the most popular and recognizable reverb pedals of all time. There have been many different versions of this pedal over the years, and this specific example uses a larger enclosure than the more recent designs. Beyond its form factor, it is essentially the same pedal with a single knob to control the extent of the reverb and selectable "spring," "hall," and "flerb" reverb modes.
