Tremolo

I like tremolo.  I like the way it sounds.  It sounds great on a Rhodes piano and it sounds great on a Fender guitar.  It’s a great effect, and used on lots more songs that just Crimson and Clover.

Leo Fender added a tremolo effect to his guitar tube amplifiers in the 1950s with the introduction of the Tremolux and Vibrolux amplifiers.  For some reason, marketing perhaps or a misunderstanding, Leo often called his tremolo effect a vibrato, and he refereed to his vibrato arms on his guitars as a tremolo.

Just to set the record straight, tremolo is the varying of the amplitude, or volume of a sound, while vibrato is variance in pitch.  The “Fender Synchronized Tremolo” on a Stratocaster is a vibrato effect, while the “Vibrato” on Fender blackfaced amps is, in actuality, a tremolo effect.
 
The first guitar tube amp tremolos used what’s variously knows as “bias wiggle”, “bias vary”, or “grid bias” tremolo.  This is what my amps use.  Later amplifiers used different, more complex, or more advanced tremolo circuits
 

Grid Bias Tremolo

A 12AX7 tube triode high gain stage (one triode half of a dual triode 12AX7) is normally used as a low frequency oscillator (LFO), as high gain is a requirement for the LFO to work. (The other half of the 12AX7 is often used as the phase splitter for the push-pull output section.)  To produce the oscillation, the output of the tube is fed back to the input after being processed by a series of capacitor/resistor (RC) taps- three capacitors in series, with a resistor to ground after each one.   Each RC filter produces a phase shift, and cascading them in series causes the gain stage to go into an oscillation.  There is often a footswitch to lift the ground of the circuit, which turns it off.
 
By carefully selecting the value of the resistors and capacitors, the designer can set the speed of the oscillation.  Typically, tube amp LFOs have a base rate of anywhere from 1 to 12 cycles per second.  (A tremolo rate of between 4 and 6 cycles per second seems to be the most pleasing to the ear.)  One of the resistors is usually a potentiometer that is used to vary the speed of the oscillation.  When playing guitar through a tremolo, 1 cps is really slow, whereas anything above 3 cps is pretty fast.  Sometimes you want that really slow, relaxed sound, and sometimes you want that fast warbly sound.  It depends on the style and the song.
 
Once the LFO is built, we have to find a way to use it to vary the volume of the amp.  The “bias wiggle” version takes a tap off the LFO output, runs it through a DC blocking capacitor, and then to a potentiometer that is used as a Depth or Intensity control.  The output of this is fed to the grid bias of the output tubes.  The effect of this oscillation on the output tubes bias voltage is to cause the gain of the tube to fluctuate, and, voila, we have a volume-altering tremolo, where we can control the speed and depth, and click it on or off with the step of a footswitch.
 
This tremolo is lush, warm, and pulsing, a “hypnotic slam effect”. And everything was fine.

Photo-Cell Tremolo

However, Leo was a tinkerer, and always striving to make things better.  If the coupling capacitor were to fail, and high voltage DC was introduced into the grid circuit of the output tubes… well, bad things could happen, like blowing out the tubes or even transformers.  Leo wanted to isolate the LFO from the grid circuit.  He did this by using a lamp and an optocoupler (light-dependent resistor).  The LFO voltage was used to turn a lamp on and off, and the optocoupler then effected the grid bias of the preamp circuit.  This had the desired result of isolating the high voltage from the grid, but it changed the sound of the tremolo.  The optocoupler was more of an on/off switch than a variance, taking away some of the lushness.  Still, many like the deep sound of this tremolo. The parts count and cost was still low, and while it was great for a Princeton or Deluxe, but Leo wanted something better on his higher end professional amplifiers.

Harmonic Vibrato

His solution was a completely new design.  In a 1958 design R. H Dorf patented “a combined tremolo-vibrato system for use in an electronic musical instrument.  Mr. Dorf used an LFO to control the input of his vibrato triode, and the bias of his tremolo triode, and used high- and low- pass filters to prevent sub-audible tones from reaching the power amp, and to keep his effect of the bass notes. 

imageThis design, that “divided signals into components of equal magnitude and opposite phase,” was one influence on what Fender called the “Harmonic Vibrato”. 

This multi-tube circuit split the high frequencies from the lows, and then separate out of phase LFOs for the highs and lows.  The highs were becoming louder while the lows were getting quieter, and vice-versa (see drawing below).  This produced a very rich and lush tremolo that people still argue about- whether or not this effect produces a phase-shifting pitch change, a vibrato. My best answer at this time is that the swirling effect of the highs and lows isn’t a true vibrato, but when it reaches our ears the changes in intensity of the highs versus the lows can be interpreted as an apparent pitch change, when it really isn’t a pitch changing effect.

image

The Digital Age

I have a DigiTech RP-150 multi-effects pedal that I like very much. It uses the modern digital technology – effects on a chip –that’s used in most modeling amps.  I use it with headphones for practicing, often with the built in drum machine.  I connect it via USB to my computer, and play along with music recordings, or I use it to for recording tracks.  I’ve also used it live on numerous occasions.  One of its many effects is tremolo.  I used this to add a tremolo guitar track to a song that we used at my oldest daughter’s wedding for the father-daughter dance.  I bought the mp3 from Amazon, and then used my RP-150 and Audacity to add a tremolo guitar track.  The results were very pleasing.

I also built a transistor tremolo foot pedal to add tremolo to my Palomino V-16 amp for playing live and recording.  It sounds pretty good, and and gives a nice pulsating tremolo tone.  I used it every time we played Bob Dylan’s Everything is Broken.

Sometime after I built my first tube amp, with tremolo, I decided to do an A-B-C comparison.  I had been told by professional guitarists that my old-fashioned grid bias tremolo sounded very good, and wanted to put it up against my other tremolo effects. So, I did.

I hooked everything up, and played them one against each other.  Third place? The digital tremolo.  What sounds great on a PC or in headphones or in a recording environment, is not necessarily what works live.  See my post on why you can’t sound like your favorite guitarist. Second place?  The tremolo effects pedal that uses transistors.  The sound is more organic, more rich.  The clear-cut winner though, was the real tube tremolo.  Oh, my nothing beats the real thing, baby!

There is a richness, a depth, a feel, to tube tremolo that just can’t be gotten any other way.  Just as a good tube amp will be rich with a dynamic touch responsiveness to go from clean to crunch just by varying your right hand on your guitar, tubes give a rich tremolo sound that approaches the swirling sound of a rotating speaker.

This is why I build my amps the old way.

Conclusion

While tremolo was the first on-board guitar amp effect, and went out of favor in the late ‘70s, to be replaced by distortion, it’s still a widely-used effect, that can have a very pleasing effect to the ear on the right song.  Whether a slow ballad or a jazzy up-tempo number, a little tremolo can add a nice touch.

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