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 ...

The amp was so ubiquitous that Marshall used ... It’s worth noting that for an extra $600/£640 (and an extra 9.54kg!) Fender’s all-valve version is available, but you won’t have access ...