Spring reverb
Reverberation from a couple of coil springs in a metal tank, with an unmistakably boingy character.
A transducer at one end sets the springs vibrating and a pickup at the other catches what arrives, delayed and smeared. Hammond developed the idea for its organs around 1940, because living rooms and small churches lacked the acoustics organists were used to. In the 1960s Fender put it into guitar amps, and the wet "drip" of surf music is essentially a spring tank turned all the way up.
The tank is cheap and small enough to fit inside an amp, but it doesn't sound like any room — which is rather the point. It also hates being knocked: kick the amp and the springs crash like thunder, a sound dub producers used on purpose. Compare the heavier, denser plate reverb.