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Microphone Month 3

Class A vs. Class AB guitar amps.

Q: “I’ve just started playing electric guitar and am looking to buy an amp. Could somebody please tell me what the difference between a Class “A” and a Class “AB” amplifier is and how it relates to guitar?”

A: To answer that question in detail, we have Mitch Colby, senior vice president of Product Development and Support at Korg U.S.A. (Vox and Marshall’s U.S. Distributor). Here’s what he has to say about it:

Class “A” (Vox) vs. Class “AB” (Marshall & Fender)

“In order to understand the difference between these two types of amplifier designs (there are others) you must first understand a little about tubes.

The most basic tube used as an amplifier consists of three elements: Cathode, Plate and Grid. The Cathode is heated (by the heater, another element in all tubes except in very old designs where the cathode is the heater) and forms a cloud of negatively charged electrons. The Plate has a positive charge that attracts the electrons. The Grid is the audio input to the tube and usually controls the flow of electrons.

Amplification happens when a signal is applied to the grid that allows for and controls how much current flows through to the plate. Because the signal voltage is relatively low and the plate voltage is relatively high (as supplied by the power supply), the small changes produced by the audio signal at the Grid appear much larger at the Plate, hence amplification.

In a Class A circuit, a positive voltage is applied to the Grid, which controls the flow of electrons. In this circuit design current is flowing at all times through the tube. In a Class “AB” design a negative “bias” voltage is applied to the grid, which will cause the tube to “shut off” when the audio waveform is below a certain point. Meanwhile there is another tube and associated circuit that is turning on before the first one turns off and is reproducing the rest of the waveform. In short these two tubes share the job of reproducing the full audio waveform.

Each type of design has its advantages and disadvantages.”

Class “A”

Advantages:
– The tube is ready to amplify the signal at all times.
– The signal is instantaneously amplified because the tube does not have to “wake up: from a less than full operational state.
– A 30 watt Class “A” amp will sound louder than a 30 watt Class “AB” amp.
– Because current is maximum at all times, the amp will have a smooth compression.
– There is not a lot of headroom because of the lower plate voltages used in Class “A” amps.
– Instantaneous amplification and smooth compression make for an amp that is responsive to the touch: the amp feels good and playing it is a satisfying experience.
– Combined with EL84’s in push-pull operation, the amp will emphasize high order harmonics and the amp will “sing”.

Disadvantages:
– Maximum current at all times means that the tubes are being strained even without playing.
– Shorter tube life.
– Lower power rating than a Class “AB” amp with the same tube configuration.
– Power transformer needs to be upgraded in order to handle the high current demands.

Class “AB”

Advantages:
– Longer tube life because the tubes are “idling” with lower Plate Current.
– Higher power ratings with the same tube configuration.
– More headroom.
– Tighter bass response.
– Less continuous demand on the power transformer.

Disadvantages:
– Not as “responsive” as a Class “A” amp.

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