Amplifier class is the classification of a tube amp. The class tells you something about the way the amp is designed., but what does the amplifier class means exactly?
In a Class A amplifier the current is constantly flowing. In other words the amp uses all of the signal that you put into it. Reason for this is that the the tubes have a positive bias voltage on the grid.
This leads into a very effectively amplified guitar signal. The tubes in this amp are working very hard. Even if your not playing, a stream of electrons is flowing through the tube.
Amplifier Class A:
Class B amplifiers are using one pair of tubes that work together (or more). One tube amplifies the positive half of the signal wave, the other tube the negative half. So two tubes are used to amplify the full signal wave.
The tube in a class B amp only operates half the time, is using less power and wil last longer. A class B amp is less sensitive to your guitar signal than a class A though and wil react a little slower. It is seldom used for guitar amps .
Class AB design is most used for guitar amps. It takes the best of class A and class B so to speak.
The bias voltage on the grid is negative and set in such a way that only a small stream of current is flowing through the tube (instead of the 100% a class A will have). That means that when you are not playing there is still a small amount of current flowing through the tube, which makes it very responsive.
One pair of tubes are working together like in a class B amp. One tube amplifies the positive part of the signal wave, the other the negative part. That way it is more efficient, less power is needed and of course the tubes last longer.
Class D is used for some light weight solid state amps. in this amps transistors are switching on and of to amplify the sound wave. When there is no signal the transistor switches of.
This kind of amps can amplify a clean tone with very little power and is very efficient. You can find it in some bass amps and small solid-state guitar amps.
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