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.    Interestingly 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 in fixed bias amps.  Most cathode biased use pre-amp tube bias shifting trems.

Design Concerns-

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

1. The bias must be set correctly 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 usually eliminate them. 

2. Bias shifting trems are hard on power tubes.  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 as the strain on the tubes  can lead to failure. 



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 oscillation tremolos but the most common uses a tube circuit with an opt coupler which is basically a bulb (neon or incandescent) paired with a light dependent resistor. These types of tremolo give a nice fluttering effect however, in many amp are not a deep or rich sounding as the other types.... at least not in their stock forms.  A few very small circuit changes make them sound deeper, slower and more useful however they never seem to be a soft sounding.   I often modify Fender's with opto-coupler trems. It takes the trem from a rarely used effect to a usable and desirable one.

You really don't need to use a tube for this type of trem.  Solid state circuitry can work just as well. 

 An important thing to keep in mind with this type is that the signal can pass through the tremolo circuit and thus affects the fidelity of the amp when not in use.  A common thing players do with Blackface Fender Amps that do not use their trem  is remove the tube that runs the tremolo circuit. There is a small increase in fidelity with the tube out. However this problem can easily be fixed by adding a switch onto Intensity control like Weber Speaker does in their Blackface kits so there is no effect on the dry sound with the effect switched off.

With a solid-state oscillator this is not usually a problem.   

The advantages 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. 

2.You really don't need to use a tube for this type of trem.  Solid state circuitry can work just as well. Often using a solidstate circuit can eliminate the load on the signal as well.   Some Garnet amps uses a tube oscillator that turned on and off a JFet transistor. The transistor acted as switch. Very clever!  


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 techincal 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

Carl's Custom Amps Blonde Series Amps

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 Vintage Gibson Amps with Tremolo

Harmonic Tremolo Amps:

Fender Brownface and Blonde Pro, Bandmaster, Super, Showman