Fender Harmonic Vibrato

Partial Schematic of the Fender Harmonic Vibrato as used in the early Showman amplifiers. The unlabeled resistors at the bottom are 1M. "Depth" control (not shown) adjusts LFO level applied to the LFO inputs shown.

Fender Harmonic Vibrato

The Fender "Harmonic Vibrato" was a complex effect used on several Fender amp models in the early 1960s (see list below). Leo Fender designed many tremolo circuits for his amps, usually labeling them "vibrato," even though they were volume effects, not pitch effects. After his simple volume modulation designs, he tried this complex version that requires many tube sections to operate. Early "Harmonic Vibrato" circuits use 4 triode sections, and later versions use 5 triode sections. In contrast, the simpler tremolo circuits only require 1 or 2 triode sections. "Harmonic Vibrato" was the term used in the Fender catalogs; the term does not appear on the amps. The name refers to the signal being split into high frequency and low frequency components, and the output switches back and forth between the two. The design disappeared by the mid 1960s when the Fender amp line switched to opto-coupler attenuators to achieve tremolo.

The design is complex because 1) it requires two low frequency oscillator (LFO) signals that are identical but 180º out-of-phase, and 2) it requires two separate audio amps to be modulated by the two LFO signals. The basic idea is still tremolo, but now two channels that are turning off and on out-of-phase with each other. To differentiate the two channels, one has a high pass filter (HPF) on its input, and the other has a low pass filter on its input (LPF). The HPF -3dB frequency is about 636Hz, and the LFP -3dB frequency is about 144Hz. These are gentle -6dB/oct slopes created by single stage passive RC networks (HPF = 250pF + 1M, LPF = 220k + 5nF).

The end result has a striking sonic similarity to a Uni-Vibe, which was a solid state phase shifter introduced near the end of the 1960s.

Is it a tremolo, vibrato, phase shifter, or what? Can we call it a voltage controlled filter effect? Vibrato is it most definitely not. To my knowledge, Leo Fender never created a pitch bender effect beyond the mechanical "whammy bars" fitted to his guitars (which he called "tremolo arms.") Phase shifter? There may be academic reasons to label it so, but I don't consider it one. A phase shifter stage only requires one triode section, and only 1 LFO signal. A filter effect? Yes. A very complex, very cool, tremolo-filter mash up.

List of Fender Amp Schematics with Harmonic Vibrato

Bandmaster

6G7 (4 Triodes)

6G7-A (5 Triodes)

Concert

6G12 (4 Triodes)

6G12-A (5 Triodes)

Pro

6G5 (4 Triodes)

6G5-A (4 Triodes)

Showman

6G14 (5 Triodes)

Super

6G4 (4 Triodes)

6G4-A (5 Triodes)

Twin

6G8 (5 Triodes)

6G8-A (5 Triodes)

Vibrasonic

5G13 (4 Triodes)

6G13-A (5 Triodes)