Optical tremolo is the effect found in many combo amps of the 1960s. This version of tremolo relies upon a neon light bulb and a light dependent resistor called an "optocoupler."

This unique effect first appeared in Fender™ “brownface” amps. The engine is a very pleasing and complex tremolo that has characteristics reminiscent of a phaser.

This approach to tremolo involves using an LFO to modulate the bias voltage of the tubes in an amplifier. The result is a smooth amplitude modulation with a mild overdrive created by the tube saturation.