Class A vs Class B Amplifiers – What’s the Difference?

Class A vs Class B Amplifiers

Let’s discuss Class A and Class B amplifiers and what makes them different. We hope that after reading this article, you’ll be able to choose the best type of amp for each kind of playing situation you come across.

We’ll start with a broader discussion of tube amps and the differences between their classes. While there are quite a few different classes, there are only a few that are traditionally used with guitars.

Amplification can get pretty scientific, with a lot of mathematics and graphs. We’ll keep it simple and discuss what’s important with guitar players in mind, so you can make the best decision on what kind of amp will suit your needs.

Class A vs Class B

Let’s take a look at the different classes.

Class A Amplifiers

Lee De Forest invented the first amplifier in 1906 when he created the triode tube. He connected the tube to a speaker, and the first guitar amplifiers became available around 1912. Amps have steadily grown in popularity and complexity since.

At first, all amplifiers were Class A. Class A means that one tube amplifies the complete signal. Additional tubes increase power output. If you take a look at Figure 1, you’ll see a plain sine wave.

Fig 1

A Class A amplifier will amplify the entire sine wave seen in Figure 1. Your guitar will produce a wave that is considerably more complex than this one, but the idea is the same.

Class A Advantages

• Class A amplifiers produce a very high gain. A 10 watt Class A amp will be louder than a 10 watt Class B.
• The Class A tube amplifies the whole signal, making it the cleanest and most accurate amplifier of all the classes.
• The tube is always on, making it very responsive to your playing. It’s ready to go and instantly reacts at all times.

Class A Disadvantages

• Class A amplifiers use a lot of power to keep the tube in an always-on state. Besides raising your electric bill, the tubes will get extremely hot and will burn out much faster than the ones in the other amplifier classes.
• When the tubes are always on, they will be working to amplify sound even when you are not playing.
• Class A is considered the least efficient of the classes at 30%.

Examples of a Class A Amplifier

• Fender Tweed
• Vox AC4TV
• Epiphone Valve Jr

Class B Amplifiers

It wasn’t long before the need to solve some of the problems inherent in Class A required a slight redesign of the amplifier circuit, which is when the Class B amplifier was born.

A Class B amplifier uses a pair of tubes to amplify the signal, but unlike class A amplifiers, each tube only amplifies half the signal.

Take another look at Figure 1.

Fig 1

In Class B, the first tube will amplify any part of the signal that is above the blue line, while the second tube amplifies any part that falls below it. When two tubes work as a team this way, it is called operating in a push-pull configuration.

Class B Advantages

• Since each tube is only required to amplify half the signal, it can spend the other half in an off state. This allows the tube to run cooler and consume less power.
• The tubes last much longer than tubes in a Class A amplifier.
• Tubes “idle” when you are not playing, saving power.
• Since the tubes run so much cooler, we can put more of them in an amplifier, which leads to higher watt amps.
• A Class B amplifier is 50% more efficient than a Class A.

Class B Disadvantages

Despite the significant advantages of the Class B amplifier, there is one notable downside to it. It takes a small amount of voltage for the tube to turn on and begin amplifying a signal. If we take a look at Figure 1 one more time, you’ll note that the blue line in the image represents zero volts.

Fig 1

What happens is, it takes a small amount of voltage to turn the tube on (.7 volts). If we look at Figure 2, we can see that the green lines represent the amount of voltage required to turn the tube on and begin amplifying the signal.

Fig 2

Half of what is between the two green lines will be lost. In the image, our wave begins by going up. It will need to reach the green line before the first tube turns on and starts amplifying the signal. Once the wave reaches zero, the first tube will turn off, but the second tube won’t turn on until the wave hits the green line below zero.

The lines in the image are an exaggeration, but the idea is to show that any lost signal will result in an unwanted and uncontrollable distortion and coloring of your sound.

Examples of a Class B Amplifier

• Music Man 112 RP

Class AB Amplifier

As a result of the Class B disadvantages, the Class AB was developed as a compromise between the two classes. The circuit is very similar to a Class B amplifier with two tubes acting as a push-pull team, but this time the tubes stay on a little longer, so there is no gap in amplification. Take a look at Figure 3.

Fig 3

Our wave begins by going up. It still needs to reach the green line before the tube turns on, but this time it will stay on after it reaches the zero until it hits the red line. Since the red line is past the green line, the second tube will have come online and can take over while the first tube shuts off with no interruption in amplification.

Class AB Advantages

• Class AB amplifier advantages and disadvantages are not all that different than those of Class B. Class AB has all of the advantages Class B has over Class A, including cooler running, longer lasting tubes. It has an advantage over Class B because there is no distortion inherent in the circuit.

Class AB Disadvantages

• The primary disadvantage over Class AB is a lower efficiency because the tubes are on longer. More on time means the tubes will burn out quicker than those in Class B, but not as fast as those in Class A.

Examples of a Class AB Amplifier

• Marshall JCM 800
• EVH 5150
• Orange Rockverb

Summary

So, in examining Class A and Class B Amplifiers, we can see that the Class A vs Class B discussion really turns into a Class A vs Class AB comparison. There are plenty of great uses for Class A amplifiers. Most DIY tube kits are Class A and sound great. They are perfect for practice and recording. Class B is what you need when you want to get loud for live performance, as well as recording certain types of music.

So, we hope that we’ve helped clear up any questions you might have about the different types of tube amps (we’ll have to tell you about the other types at another time). If you’ve learned something about Class A and Class B amplifiers, please feel free to share this article on Facebook and Twitter. And be sure to visit us at humbuckersoup.com to take a look at other articles on guitar electronics.