Hammond organ serial number dating
It's a Leslie speaker cabinet -- an effects cabinet, really -- made by Fender from 1967-1972.It adds a phasing or chorusing effect to your amplifier's output.On the other hand, the tremelo effect is a fast, warbling effect that is immediately recognizable and unmistakably organ-like, though again miking will make it more or less prominent.But there are some differences that make the Vibratone a unique variety of Leslie.The chorale speed, also called chorus, is about 40 revolutions per minute, while the tremelo is about 340 rpm.
You may or may not even notice it unless you're listening for it.
The history of rock music is full of the Vibratone sound, but while there's plenty published about the Leslie cabinets designed for organs, there's not much written about the Vibratone, and definitely nothing in-depth.
I own an early model Vibratone (s/n 1061) and have also built a "Vibratone clone" from an internal Leslie unit out of a junked organ.
This driver feeds a spinning plastic horn which rotates in the opposite direction to the drum, adding to the density of the phasing effects.
Additionally, the treble horn and bass rotor turn at slightly different speeds, so they don't produce the same sound every revolution.