For the 1959 Vibrasonic, Fender designed a new tremolo circuit that required two twin-triode tubes, which worked on the Vibrasonic's Vibrato channel's preamp signal, post-volume, and post-tone. This signal was then sent on to a passive crossover network. The highs were sent to half of a 7025 twin triode, and the lows were sent to the other half. Part of the second 7025 was used as the oscillator, with the output feeding the "highs" tube as well as the second half of its own tube, which inverted the pulsating signal. The output of this tube then fed the bass tube. What you ended up with was a tremolo circuit that made the highs pulsate against the lows for a very deep, complex sound. The pulsations and inverted oscillator pulsations cancelled at a point where the highs and lows were summed. This is the Fender circuit that first carried the "vibrato" designation, although it was still just amplitude modulation.