From his website:
"During the 1960s, Jack plugged his EB-3 into a stack of Marshall amps (Marshall 4x12s with Marshall 100 heads). His desire for greater volume induced him to crank up the volume control, unwittingly producing the heavy, distorted tone that became so popular in the ensuing years. Later, Jack had instrument technician Dan Armstrong install a diode into the EB-3's wiring to produce the same effect without overdriving his amplifiers."