How Tube Amps Work
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Triodes
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Tube amps convert the AC current that comes from audio equipment to DC current used by speakers. Tube amp structure varies, but all contain a vacuum tube that functions as an electricity valve.
Tubes can be diodes, triodes, tetrodes or pentodes. The terms refer to how many elements are in the tube. Triodes and pentodes are the most common in tube amps. A triode has three elements (filament, grid, and plate) while a pentode has five (the same as a triode plus a suppression grid and a positively charged grid).
When the AC current hits the tube, electrons boil off the cathode into the vacuum; hit the grid, where the voltage is controlled; and pass to the anode, where the voltage is passed on to the speaker. The grid greatly amplifies the gain of the signal.
Circuit
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The amplifier will be designed with a type of circuit and given a class rating, typically Class A, Class B, or Class AB, based on the circuit type.
In Class A, the power tube conducts the same amount of current at all times. It is very inefficient but produces very low distortion because it is linear, always amplifying the whole input.
A Class B circuit means the amp idles at zero; they are much more efficient but produce more distortion. Class B amps are designed in push/pull configurations, where two circuits each amplify half of the incoming signal. The "join" area of the two signals is subject to distortion.
Class AB combines the two, and is the most common in guitar amps. In this circuit, one tube's circuit is driven until it cuts off, then the other takes over. Class AB circuits are usually designed in a push/pull configuration like a Class B, but will also each amplify a portion of the other signal, trying to achieve greater linearity and less distortion.
In theory, a Class A circuit can achieve 50 percent efficiency, while a Class AB and a Class B circuit can reach 78 percent. In practice, all three usually have much lower efficiency outputs.
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Output Transformer
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In most tubes, the output transformer matches the load of the speaker to the output from the tube circuit. Output transformers are designed based on the type of tubes, their power output, and the type of circuit, and then speaker loads that you want to power.
In contrast, transistor circuits are output transformerless (OTL) circuits, which usually have direct coupled output. Such circuits do not have a transformer to adjust the load impedance between the tubes and the speaker. Transistor circuits can also be capacitively coupled.
Some companies have developed OTL tube amps, but they are not as common. Output transformers can be a significant source of distortion, so the cost of making a good output transformer is often a large part of the cost of the amp.
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