Ice Cube II Sustain Unit for all Fender Reverb Amps made 1978 and after, if you have a Pre-1978 Fender Reverb Amp that you want the ICE CUBE and NOT the ICE CUBE II. Also. It is made to plug into the reverb send and return on the amp in place of the reverb tank This turns the reverb control into an overdrive control as it sends the signal through the ICE CUBE II instead of the tank.