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Throughout the 1960’s Fender used several types of electrical tremolo circuits in their various amp
models, including
bias-modulating tremolo, optical tremolo, and harmonic tremolo.
Bias tremolo
is created by modulating the bias voltage on a preamp tube, phase inverter tube, or
power tube in an amplifier circuit. This has the effect of partially cutting off the current going through
the tube which reduces the tubes’ capacity to amplify the signal. Bias-modulated tremolo is associated
with a smooth, sine wave oscillation.
Optical tremolo
is produced by using an optocoupler to modulate the signal in the preamp circuit of
an amp. Optocouplers contain a light dependent resistor (LDR) which is placed next to a light bulb (or
LED in modern optocouplers). The voltage to the light bulb is modulated, which in turn modulates the
resistance of the LDR, causing the signal to be variably attenuated. Optical tremolos create a pulsing
or throbbing sound, and tend to modulate the signal in a more lopsided manner, which tends to be
quite pleasing to the ear.
In a
harmonic tremolo
circuit the audio signal passes through a crossover circuit which splits the
low frequencies and high frequencies, sending them through their own circuit paths. The two frequency
ranges are modulated 180 degrees out of phase from each other: while one frequency range is on, the
other is off, and vice versa. Because the low frequencies and the high frequencies are separated from
each other, harmonic tremolo has a wonderfully subtle phasing sound that is quite mesmerizing. This
type of tremolo was found on many of Fender’s early brownface amps, but was soon replaced because
of the large number of expensive tubes and components that the circuit needed to operate. Due to the
frequency-shifting characteristics of harmonic tremolo, the effect actually comes close to true vibrato;
maybe Fender wasn’t too far off after all by calling one of their earliest tremolo circuits “vibrato”!
