Amplifier classes


Created on 25.09.2019 15:30.

Updated 01.18.2020 22:40.

Author: Duncan Munro.

You’ve probably already heard about the classes of amplifiers, they are usually called Latin letters or their combinations – A, AB1, AB2, B, C, etc. What do they all mean?

Let’s forget about the numbers for a while – we’ll come back to them later. Now we will take a closer look at the options for classes A, B and C, and, so that everything is clear, we will once again look at how electronic vacuum tubes (radio tubes) actually work.

Simple amplifier circuit with a radio tube

If you know how tubes work, you can skip straight to the Classes of Operation section.

In the image, we have connected a triode to a pair of batteries and a pair of measuring instruments.

Starting from the left side of the image, we note that the following connection is made in the figure – the battery (battery) is connected between the radio tube grid and the common conductor – “ground”. A voltmeter is connected in parallel with the battery to find out what voltage the battery is sending to the grid.

Note that the battery is connected upside down, so the voltage across the grid will be negative with respect to ground.

On the right side of the picture, we have a much larger battery with a milliammeter to measure the amount of current delivered by the anode of our lamp.

Typically the grid should be one to two volts more negative for the preamp tube and 40 volts or more for the tube used in the amplifier’s output stage. We call this fact the lamp bias voltage.


By changing the voltage on the grid, we can change the current passing through the anode – this is a way to achieve gain.

Look at the diagram (it is also called the current-voltage characteristic, author’s note) – it shows what happens to our lamp when the voltage on the grid changes. With magnification
voltage on it, respectively, and the anode current increases. It is very important to note that the lamp converts the input voltage to the output current.

Whether you are using a preamp tube or an output tube, the above principles remain the same. Now that we have remembered how radio tubes work, we can consider the different classes of their operation in the output stages of amplifiers.

Classes of amplifiers

Take a look at the diagram below. The current-voltage curve of the lamp is taken from the previous graph, and the red dot indicates the set bias voltage, roughly in the middle of the curve.

If we now superimpose the signal on the grid voltage (In), the plate current begins to change, increasing and decreasing in accordance with the change in the grid voltage, forming a graph of the output signal (Out).

As the name suggests, this is class A. The main advantage of class A is that the lamp produces an output current all the time, i.e. constantly. Note that the output current (in the “Out” graph) never drops to zero.

Some of the amplifier output stages are Class A (such as the VOX AC-30), and all of the preamp stages are also Class A.

Okay, we’ve figured out Class A, but what is Class B? In the graph on the right, we have set the offset point at the point where the lamp practically stops working, i.e. conduct current.

Note that the input signal now has a much higher amplitude in order to make the tube work. In this case, the lamp generates an output current, which is only one half of the output signal waveform.

In order to somehow make such a solution work, we will have to apply a push-pull output stage, which consists of two lamps (or two sets of lamps) so that each side amplified its half of the signal half-wave. While the first output tube gives the output current as shown on the right, the second tube fills the gap in the first.

At this stage, you have probably already guessed what class AB is – it is somewhere in the middle between A and B. And where exactly – depends on your imagination!

In our graph of class AB lamps, a small amount of bias current flows through the lamp.

For the output tubes in a typical, standard Class AB guitar amp, the amount ranges from 30-40 milliamps, with peaks around 250-300 milliamps.

There is little mutual overlap in the push-pull output stage, as each lamp helps its neighbor during a short transition or transition period.

Many large guitar amps are class AB, and we’ll look at why below.

Transient distortion

Class B suffers from one fundamental problem that is only found in push-pull amplifiers. The problem is that in practice, a waveform has to transition smoothly from one half-wave to another.

When the output currents of both lamps are added in the output transformer, then on the graphs depicting the operation of lamps in class B, one can observe a “step” – the figure below. Class AB amplifiers
likewise can suffer from this effect if the bias current is too small.

In summary, we can say the following:

  • Class B amplifiers generally have low crosstalk.
  • Class AB amplifiers may have low crosstalk
  • Class A amplifiers have no junction distortion since both tubes are synchronous.>

A seemingly solution would be to use Class A tube mode for guitar amps. Then you don’t have to worry about crossover distortions at all, But …


Classes AB and B exist for certain reasons, which we talked about above. If we return to the diagram depicting the operation of the lamp in class A, we will see that it constantly outputs an output current, without interruption. To get high output power from a Class A amplifier, it is usually driven into a mode where the bias current sets the maximum wattage of the lamp based on how much it can handle.

It is not as efficient as class B, which allows the current to be delivered in the amount that is needed, or as class AB, which can be called a hybrid of both.

Less efficiency in class A means that more heat is dissipated as a result of lamp operation to achieve the same output power as other classes. Let’s make a comparison between Class A on a pair of EL34 lamps and Class B on the same lamps.

So, the information from the manufacturer of the EL34 lamp speaks about the method of its use in single-cycle class A, built on the basis of limiting the maximum dissipation power of the anode, equal to 25 watts. 11 watts of power output will have 1 0% of the total harmonic distortion (THD).

For a Class A push-pull amplifier, the output figures are doubled to 22 watts as two tubes are used. Next, we look at the description – the parameters for class B are listed there.The declared output power is 1 00 watts at …