Types of Tremolo in Tube Amplifiers

Revised 3/9/2017

Lots of folks ask me about what the difference is between types of tremolos in amplifiers. There many opinions about the different types and what sounds best. In this article I will briefly cover the basic types of tremolos and how they affect the sound. I will also have a list of on the bottom of the page of some of the amps that use each type of tremolo.

What is Tremolo?

Tremolo is the effect that changes the volume of the sound as different speeds and intensities. It gives the sound nice shimmering effect.  Fender wrongly the term vibrato for tremolo in their amps. Vibrato is actually bending the pitch of a note.    There are a few vintage amps that have real vibrato. I worked on a Magnatone amp that did. It’s a neat piece of electronics and sounds really strange. However, it’s not really all that usable of a sound.

The Three Major Types of Tremolo in Amplifiers:

Bias Shifting Tremolos: 

How it Works-

These tremolos work by changing the bias to either the power tubes or a pre-amp tube in a amp.   By changing the operating parameters of the tube the signal can be cut off momentarily and the brought back up. This creates the tremolo effect. These tend to have a very rich and soft pulsating sound.  The signal does not pass through the tremolo circuitry so they do not effect the fidelity of the signal. 

Types of bias shifting Tremolos-

There are two basic types of bias shifting tremolos: one that work on the bias of the power tubes and ones that work on the bias of a pre-amp tube.  Power tube shifting trems are generally more common and found largely in fixed bias amps.  Most small cathode biased use pre-amp tube bias shifting trems but there is version that modulates the bias indirectly through the grid leak resistors of the power tubes.

Design Concerns-

Bias shifting trem in fixed bias amps that vary power tube bias have several issues:

1. The bias must be set correctly on the power tubes for the effect to work.  The problem with this that sometimes you must bias the amp cooler than what may sound best or the trem will not function well. Another problem that occurs is beating sounds that can be very annoying.  You can minimize them but not always eliminate them. 

2. Bias shifting trems that work on power tubes are hard on the power tubes and power supply which can lead to failures.  Since the trem works by affecting the bias of power tube it can affect the stability of the power section. It is not advisable to run the trem with an amp cranked up with this type of tremolo. The strain on the tubes can lead to failure. In worst case scenario a tube failure could even damage a transformer. Over the years I have come to conclude that power tube bias shifting tremolos are not as wise to install as the other types. The sound is excellent, but power tube failure (which can cascade to transformer failure) is something you want to avoid. The pre-amp bias wiggle and other tremolo types are simply better for reliability.  The power tube bias wiggle arrange works well for players who are not driving their amps too hard.

Signal Shunting Tremolos 

 These work by cutting on and off the actual signal within the amp. There are a wide variety of kinds of signal tremolos but the most common uses a tube circuit with an opt coupler which is basically a bulb (neon, LED, or incandescent) paired with a light dependent resistor. Less common are circuits that modulate a JFet or bipolar transistor. All these types of sound different because the device that is oscillated responds differently. For instance, the 60’s Fender neon LDR tends to be rather choppy sounding while an incandescent Gibson circuit is much smoother.

You really don't need to use a tube for this type of tremolo.  Solid state circuitry can work just as well. Since the oscillator is not in the signal path there is no tone penalty for doing this. In fact, solid-state oscillator circuits tend to have a wider range. A number of amps to today use solid-state tremolo circuits and have excellent sound.

The advantage with this type of trem are:

1.  It does not affect the stability of power section and no special bias considerations are necessary. They are low maintenance. 

Harmonic Tremolos:

Very few amps have harmonic tremolos, but they are among the wildest and most hypnotic sounding tremolos.   They are found in a handful of Brownface and Blonde Fender Amps. The main trouble with harmonic tremolos is that they take three 12AX7s to run! Many amps do not even have that many pre-amp tubes. If trem is your thing than these are really awesome! 

So which is best?

This is mostly a matter of personal opinion. However, I have found that most people find the Harmonic and Bias Shifting Tremolos sound the best and have the most depth. However, both of these types have technical draw backs.  The signal affecting Trems are the easiest to maintain and went built properly come close in richness to the others. 

 Some Common Amps by Tremolo Type:

Power Tube Bias Shifting:

 Fender Blackface, and Silverface Princetons

Fender Black and Brownface Vibro-verb

Fender Brownface Deluxe

Fender 5G9 Tremolux

Pre-amp Tube Bias Shifting:

Fender Blackface and Silverface Vibro Champs

 5E9 Tweed Tremolux

Signal oscillating tremolo Amps:

Most Fender Blackface and Silverface Amps with tremolo (except the Princeton and Vibro Champ)

Vintage Vox Amps

Most 60’s Vintage Gibson Amps with Tremolo

Harmonic Tremolo Amps:

Fender Brownface and Blonde Pro, Bandmaster, Super, Showman