POWER AMPLIFIER DESIGN

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.

Simple tube theory part 1 - 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.   There is another part of the tube called a heater which heats the Cathode

Amplification happens when a signal is applied to the grid and flows through the plate. Because the signal voltage is relatively low and the plate current is relatively high, the small changes produced by the audio signal at the Grid becomes 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 voltage is applied to the grid which controls the flow of electrons.

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 and less continuous demand on the power transformer.

Disadvantages - Not as "responsive" as a Class "A" amp.