Not all Fender tremolo effects were created equal. According to John Teagle and John Sprung (two Fender experts), the company actually used a number of different methods to produce the tremolo effect in their amplifiers. Despite often using vibrato and tremolo interchangeably (remember, Leo Fender did not play guitar, so the distinction was lost on him), the effect we are talking about is amplitude modulation, just as you’d create on an analog synthesizer. All of Fender’s tremolo circuits did use a tube oscillator. The 1955 Tremolux amp had a pulsating signal that was low in both level and frequency, and this was connected to the cathode of the phase inverter tube. A pair of triodes were cathode-biased as one, and as the signal from the oscillator “collided” with the bias voltage, the vacuum tube would actually be shut down, then turned back on in sync with the oscillator frequency. The speed of the oscillator and the depth (intensity) could then be controlled using a pair of control panel potentiometers. This circuit tended to produce a low-level tick when in use, but it was not audible when a guitar signal was passed through the amplifier and out via the speakers.
