there’s Bias tremolo as used in early Fender amps where the circuitry varied the bias of valves; Opto is based on the photocell circuitry seen in black-panel amps from around 1963 onwards ...
Fender equipped amps including the Deluxe Reverb, Twin Reverb, Vibroverb, Tremolux, Pro, and others, with a new tremolo ...