The Difference Between Class A and Class AB Guitar Amps


There are two main types of Guitar amps, the Class A and the Class AB beside other less common amplifier designs. It is necessary to first understand some basic concepts about tubes in order to understand what is the main difference between the Class A and Class AB guitar amps.



The Basics Of Amp Classes

A guitar amplifier’s most basic tube consists of three elements: grid, plate, and cathode. The audio input to the tube is called the grid. This element of a tube has the role in controlling the electrons flow. The plate is a tube element that attracts the electrons, having a positive charge. The cathode is a tube element that is heated by the heater. In very old tube designs the cathode itself is the heater. When heated, the cathode forms a cloud of electrons that negatively charged electrons.

Amplification takes place when a signal is applied to the grid, allowing the flow of current through to the plate. This signal provides control on how much current flows through to the plate. Because the plate has a relatively high voltage and the signal has a relatively low voltage, the audio signal produces small changes at the grid that appear much larger at the plate. This process produces the desired amplification.

A positive voltage is applied to the grid in a Class A circuit, controlling the flow of electrons. The current is flowing through the tube at all times in this circuit design. A negative “bias” voltage is applied to the grid in a Class “AB” design. When the audio waveform goes below a certain point, this voltage is causing the tube to “shut off”. Before the first circuit turns off, there is another associated circuit and tube that is turning on, reproducing the rest of the waveform. The job of reproducing the full audio waveform is shared by these two tubes.


Advantages And Disadvantages Of Amp Classes

Class “A” Advantages:

– The signal is amplified instantaneously because the tune does not need to “wake up”.
– The tube is ready at all times to amplify the signal.
– A 30 watt Class “A” amp can provide louder sound than a Class “AB” amp with equal power.
– The amp will provide a smooth compression because the current is at the maximum at all times.
– Smooth compression and instantaneously amplification creates an a response that is quick to the touch.
– Because of the lower plate voltages, there is not a lot of headroom.
– The amp will emphasize high order harmonics when is combined with EL84’s in push-pull operation.


Class “A” Disadvantages:

– Even without playing, the tubes are being strained due to the maximum current at all times.
– The tubes in a Class “A” amp will have a shorter tube life.
– In order to handle the high current demands, power transformer needs to be upgraded.
– At the same configuration, a Class “A” amp will have lower power rating than a Class “AB” amp.


Class “AB” Advantages:

– More headroom
– Longer tube life
– Higher power ratings
– Less continuous demand on the power transformer
– Tighter bass response


Class “AB” Disadvantages:

– Less “responsive” than a Class “A” amp



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