Tremolo – How it Actually Works in Your Amp
Tremolo…..it almost seemed like it had disappeared for awhile during the whole boutique “less is more” amp craze. Then Cesar Diaz released his famous Texas Tremodillo pedal, and tremolo was reborn. Soon, tremolo was popping up again in every manufacturer’s amp line. What I will unlock here is the “mystery” behind the tremolo circuit, what it is, what it does and how it works.
Before we jump in the pool, I need to clarify something here: “Tremolo” and “Vibrato” are NOT the same thing. Tremolo is an AMPLITUDE-based effect, and Vibrato is a FREQUENCY-based effect, hence Tremolo periodically varies the VOLUME of the signal, and Vibrato varies the PITCH, two quite different modes of operation. The circuit we are describing is Tremolo. Now, Fender DID produce some amps in the past with a true VIBRATO circuit, most notably the “Bandmaster”, “Concert”, “Pro”, “Vibrosonic” and “Showman”, all in the “6G” series of amps from the early-60’s, but it was not well-received and was inherently more complex, so it’s design was dropped. As a tangent to this, it also means that whammy-equipped guitars are VIBRATO-equipped, NOT tremolo-equipped. My guess here is that Fender, not wanting to paraphrase Gibson’s more appropriate “Vibrola” moniker for a similar device decided to use the term “Tremolo”, creating a semantics error that has stuck since 1954.
There are many ways to accomplish the tremolo effect in guitar amps, but we will concentrate of the two most widely-used methods: signal modulation and bias modulation.
The heart of the tremolo circuit is the “Low-Frequency Oscillator” or simply “LFO”. An oscillator is simply an amplifier with a POSITIVE feedback loop. Many guitarists are more familiar with the term “Negative Feedback”, which exists in most guitar amps. This is when the output of the amp is fed back, out-of-phase, to the input of the power amp. This cancels some of the output signal, hence reduces the gain, stabilizes the amp and reduces distortion. An oscillator works the same way, except the output signal is fed back IN-PHASE, and the amplifier then “oscillates”, or outputs a signal of a specific frequency as the input and output feed each other a constant signal. The “frequency” or pitch of the oscillation is determined by resistors and capacitors placed in the feedback loop. In the case of the LFO, the output signal is subsonic, below audible frequencies (<20Hz). You can see the signal on an oscilloscope, but you cannot hear it. Typical tremolo LFO’s work in the 3Hz to 10Hz range.
The other key component of the tremolo circuit in signal modulation-type designs is “Opto-Isolator“, also referred to as the “Optical Assembly” “Opto” and “Bug”. This component has four legs on it, two for an internal neon lamp (or also a small incandescent lamp or LED), and the other for an “LDR” or “Light-Dependent Resistor”. This is a resistor made of a special material, usually cadmium sulfide (or simply CdS) that is packaged in a tiny metal can with a lens on top to allow light to access the resistive material. When no light shines on the resistor, it’s resistance is very high. When light hits it, the resistance drops down very low, a neat and handy device. The lamp and LDR are encased in a lightproof enclosure, usually a piece of heat-shrink tubing, or sometimes in an epoxy case for the LED versions. Save this paragraph for later.
For our purposes, just think of the LFO as a “black box” with a speed control and an output, which is a sine wave. The output, at the peaks of the sine wave, has enough juice to drive a lamp (see where this is going?). The lamp turns on and off, following the peaks of the sine wave signal from the LFO. The LDR, in turn, reacts to the lamp. When the lamp lights, the resistance goes down. When it turns off, the resistance goes up. The Opto has converted the output of the LFO to a variation in resistance. Cool! NOW we can use this to our advantage. Essentially, we have a potentiometer that is controlled by light.
If you were to turn the volume pot up and down very quickly on your amp, you’d have (ta-daaaa!!!)…..TREMOLO! Well, we have our little LDR circuit, essentially a light-controlled pot, and our LFO, which does the “turning” for us, AND has a handy little speed control attached. In amplifiers that use “signal modulation” tremolo, the LDR is just attached to the signal path in the amp, generally after the second preamp stage, and works just like turning the volume pot up and down, except automatically. There is still one control left, the INTENSITY control. Basically, this is another pot in series with the LDR to raise the resistance so that it never completely mutes the signal, creating a less intense effect. To use the volume pot analogy, it would be like turning it up and down very quickly, but never all the way down to zero where it would completely chop-up the signal. This is what the INTENSITY control does, automatically. Examples of amps using signal modulation tremolo are the Fender Deluxe Reverb, Dual Showman Reverb, Pro Reverb, Twin Reverb, Vibrolux and Super Reverb.
DEFINITION: The “signal modulation” tremolo circuit design changes or “modulates” the volume of the amp by automatically varying a resistor that is connected to the preamp signal path.
The majority of amp designs employ signal modulation tremolo. There is another design that is not as widely-used called “Bias Modulation”. What this does is vary the bias voltage going to the power tubes.
Bias voltage is like the idle on a car engine. It’s a minimum set point for operation of the power tubes. Bias voltage is like fuel. Too much fuel at idle, and the engine wastes energy and runs too hot at idle. Too little, and it can stall. With bias, if the voltage is too negative, the power tubes “stall” or go into cutoff. If the voltage is too positive, the tubes run hot and can burn up in a relatively short amount of time. Bias also affects sound, but we can talk about this a little later.
When you vary the bias of an amp, you also vary the output level to a degree. It can only get so loud, which is governed by the power tubes and power supply voltage, but you CAN shut it down almost completely if you make the bias negative enough.
In a Bias Modulation design, a single triode tube, usually one-half of the ubiquitous 12AX7, is essentially in series with the bias supply voltage for the power tubes, and has the necessary cap and resistor network (called and “RC Network”) between the input (grid) and output (plate) of the tube in order to cause it to oscillate at the low frequencies we need for tremolo. This oscillation is essentially a variable bias control, turning the bias down far enough negative to nearly cut all power amp output, as if we were turning a bias control pot with a screwdriver up and down very quickly, hence causing the tremolo effect.
Each of the aforementioned circuit designs has advantages and disadvantages. The Signal Modulation design leaves the bias supply of the power tubes alone, therefore the distortion characteristics of the power tubes remains constant. The circuit, using the Optoisolator, is also more complex and costlier to implement, so it is usually seen in higher-power (under about 40W) and/or premium amp designs. You don’t REALLY want to play with the bias supply in a higher-power amp while it is working balls-to-the-wall. The downside to this circuit is that you lose some of the signal through the LDR to ground. There is a popular mod for these circuits which uses a switch to open up the connection to the LDR, increasing gain and sensitivity of the preamp. How much this means to the amp’s owner really depends on the player’s style. For jazzers ad anyone else who plays clean, it is not likely to be an issue.
The Bias Modulation design, conversely leaves the preamp gain alone, but instead plays with the power tubes gain and consequently, their clipping characteristics. The circuit also introduces a type of distortion called “crossover distortion”. It is a nasty, odd-harmonic distortion caused by the fact that the signal is not being completely reproduced. It also can’t really be used in high-power amps. Therefore, this inherently less costly, less complex circuit is usually reserved for lower-power, inexpensive amps. Good examples are the Princeton, Deluxe and Tremolux.
As I said earlier, there are other methods used to recreate tremolo, including digital modeling. But here, in our analog world, these are the most popular methods. One thing is for certain: tremolo is here to stay. Now excuse me while I strap on the ol’ gee-tar, plug in, turn it up loud and play “Born On The Bayou”.
John R. Frondelli is the Technical Services Director at dBm Pro Audio In New York. He has been a technician for 30 years and has repaired, restored and custom built all types of musical equipment. Part of his client list includes Bob Dylan, Lenny Kravitz, U2 and The Who. �