Tung-Sol.com:: FAQs What is the difference between Class A and Class AB amplifiers?

FAQs

Question:
What is the difference between Class A and Class AB amplifiers?
Answer:
Class A and Class AB are terms that describe how the power tubes work within the power section of a guitar amplifier. To properly explain the technical differences between these classes of operation would require a lengthy discussion.

In Class A circuits the output tubes are conducting and drawing current all of the time even when no signal is present. Class A amplifiers sound more vintage, are very touch responsive and have a somewhat spongy feel with a singing response when overdriven. They also have an interesting harmonic content with asymmetrical clipping that is highly desired by many musicians. Since the output tubes are drawing current all of the time, they run much hotter than Class AB amplifiers. They run at lower voltage and have less output. All single ended (one output tube) guitar amplifiers, such as the Fender Champ, are Class A. Some push-pull amplifiers are Class A, the most notable being the Vox AC30.

All guitar amplifiers that are Class AB are push-pull. In this circuit one output tube conducts plate current and amplifies during the positive cycle of the input signal. The other output tube conducts plate current and amplifies during the negative cycle. When one tube is conducting and amplifying the signal the other tube is basically "resting" because it is drawing little or no plate current. In this case, the amplifier runs much cooler. Another advantage of Class AB operation is the plate voltage can be higher and the bias can be much deeper allowing for more efficient operation and higher power output. A typical Class A amplifier using a pair of 6L6 tubes will have a maximum power of about 20 watts, while operating the same pair of tubes in Class AB will easily yield 50 watts. All Fender and Marshall amplifiers with an output of more than 40 watts are Class AB. Class AB amplifiers tend to have greater dynamics, sound punchier, are cleaner, and have cooler running tubes.

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