In 1964, the cream tolex was replaced with black tolex with silver grillecloth, and the circuit was changed to AB763, which used an optoisolator to couple the tremolo effect, replacing the earlier direct bias-modulation circuits.
In 1964, the cream tolex was replaced by black tolex featuring a silver grillecloth, and the circuit switched to AB763. The use of an optoisolator in this new design allowed for tremolo effect coupling, replacing earlier direct bias-modulation circuits.