Tremolo, in electronics, is the variation in amplitude of sound achieved through electronic means, sometimes mistakenly called vibrato, and producing a sound somewhat reminiscent of flanging, referred to as an "underwater effect".
The first self-standing electronic tremolo effects unit may have been produced by DeArmond, in which a motor shakes a canister containing a "hydro-fluid" (not mercury as some people assume), oscillating the canister containing an electrolytic fluid that sends the signal to ground.
[3] The most notable early amplifiers with built-in tremolo functions were the 1961 Fender Princeton and the Gibson Falcon.
The effect was achieved through "bias wiggle", in which the bias of a tube, in the preamp or output stage, was modulated (turned off and on, or partly off and on) in a pure sine wave.
Such circuits typically had controls for speed and depth, and produced an effect described as "lush, warm, and roundly pulsing".