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5 Things About Amp Tremolo

April 16, 2012
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1 IT’S A BASIC EFFECT, BUT IT MOVES US

Tremolo is defined by an alternately on-and-off pulsing of the signal being amplified, although usually it’s heard as a repetitive more-to-less transition, rather than all or nothing. That being said, its different forms—though broadly doing the same thing, according to our strict definition—do vary quite a bit in their nuances, and therefore inspire fanatical devotion from different players. Some are lured by the soft, watery pulse of one type, some by the “off-balance” wave of another, some by the extreme chop-chop-chop of yet another still. So, a rudimentary effect, perhaps, but its flavors exhibit surprising variety.

2 CIRCUITS CAN BE EXTREMELY SIMPLE, OR IMPRESSIVELY COMPLEX

A tremolo circuit can be butt-simple, or mind-numbingly complex. From the rudimentary types found in many old amps, such as the Fender Princeton Reverb or the Gibson GA-19RVT—both of which use only half a preamp tube and a mere 12 components in total—to the intricate Harmonic Vibrato (see #5 below) of larger brown Fender amps of the early ’60s, which use two or two-and-a-half preamp tubes and up to three times as many components, there’s a wide variety of ways to produce the effect, and they all sound a little different.

3 GO “BIAS-WIGGLE” FOR VINTAGE WOBBLES

Many early tremolo amps—notably Fender’s tweed Tremolux, as well as several Gibson, Valco, and Ampeg models— used bias-modulated tremolo, otherwise known as “bias wiggle,” to affect the signal either at the preamp, phase inverter or output tubes. So-called because it modulates the tubes’ bias to create its effect (effectively turning the tube on and off, or partially so, at a speed and depth determined by the associated controls), this form of tremolo produces a lush, warm, and roundly pulsing version of the effect that many players are rediscovering lately. It is probably most revered in the forms that modulate output-tube bias.

4 OPT FOR “OPTO” FOR CLASSIC BLACKFACE SURF AND COUNTRY TREM

For many players, the tremolo produced by the blackface Fender amps of the ’60s and the similar silverface amps that followed is the epitome of trem for certain styles of music. Known as opto-coupled or photo-cell tremolo (opto for short), this circuit uses an optocoupler (also called a light dependent resistor) in an amplitude-modulating circuit that induces a pulsating of signal level within the preamp stage of the amplifier. Most such circuits produce a lopsided wave, rather than a steady sine wave, that is actually quite pleasing on the ear, and marks one of the distinctive characteristics of this form of tremolo.

5 TREMOLO… VIBRATO… AND WHAT’S THE DIFFERENCE?

Strictly speaking, and put as simply as possible, tremolo modulates volume, while vibrato modulates frequency. Think of the repetitive rise and fall in volume of the former, verses the repetitive fluctuation of a note to above and below pitch of the latter, as produced by finger vibrato on a guitar. Fender, though a champion of great vibrato effects, confused the issue early in the game by calling the tremolo effect on most of its amps “vibrato.” The complex Harmonic Vibrato featured on brown-Tolex Super, Pro, Twin, Showman, and Concert amps from 1960-’63 did, however, produce a form of pseudovibrato, which lands somewhere between pure tremolo and a phase-shifter effect.

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