In addition to the different classes of amplifiers, there are four types of guitar amplifiers: tube, solid-state, digital, and hybrids thereof.
Tube Amps: Tube amps are preferred by many guitarists for their great tones and very organic-sounding distortions. Tube amps usually sound louder than solid-state amps of the same wattage and have a definite "feel" that you don't get from solid-state amps. Tube amps require more upkeep and are definitely more delicate than other kinds of amps, but the sounds they get are completely worth it.
Solid-state Amps: Sold-state amps use transistors for their preamp and power sections instead of tubes. They are less expensive, very reliable, and seldom need repairs. They often have pleasing clean tones, but their distortions sound much thinner. These amps are the sound of many genres, and are recommended to anyone looking for a reliable touring amp that needs little upkeep.
Digital Amps: These amps use digital processors to simulate the sound of many different kinds of amps: tube and solid-state. Using software that models the sound of various amplifiers, cabinets, and sometimes even mics and micing positions these amps put the sound of many variables in one box and allow for some amazing experimentation. Modeling amps are programmable, and often have built-in digital signal processing such as delay, chorus, etc. Some include digital or analog outputs with speaker simulation for going direct into a recording interface or to house.
Hybrid Amps: These amps use a tube preamp section in conjunction with a solid state power section. These amps generally use the combination of tube and solid-state technology to create a tube tone without requiring the use of power tubes.
Classifications of Amplifiers
There are three main classes of operation for a guitar amplifier. These are: A, AB, and B. Each class of operation has certain applications and characteristics. Likewise, each class has its advantages and disadvantages.
Guitar amplifier classifications are determined by the amount of time, in relation to the input signal, that current flows in the output circuit. This is a function of the operating point of the amplifying device. The operating point of an amplifier is determined by how it is biased.
Class A amplifiers are generally known to have good fidelity and low efficiency. Fidelity means that the output signal is analogous to the input signal in every way, save for amplitude. 100% (360 degrees) of the signal is used throughout the entire circuit. There is a 1:1 ratio regarding energy used to energy lost, as Class A amplifiers are not efficient in the power department since the circuitry is designed for quality of fidelity.
Since the output signal is a 100% (or 360) representation of the input signal, current in the output circuit must flow for 100% of the input signal time. This is the definition of a class A amplifier: amplifier current flows for 100% of the input signal. Since every device takes power to operate, if the amplifier operates constantly when powered on, it uses more power than if it was only on when signal is present.
If the amplifier uses more power for the input, less power is available for the output signal and efficiency is lower. Since class A amplifiers operate for 360 of input signal, they are low in efficiency. This low efficiency is acceptable in class A amplifiers because they are used where efficiency is not as important as fidelity. In some cases, there may be a phase difference between the input and output signal (usually 180), but the signals are still considered to be in phase. Efficiency refers to the amount of power delivered to the output compared to the amount of power that is supplied to the circuit at the input.
If all stages of the amplifier are biased in Class A, and the amplifier operates in Class A to full output (enough current flowing at idle that could be required for full output), it is said to be a "Pure Class A" amplifier. Pure Class A designs are understandably expensive to build and are usually only found in high-end boutique amps.
Class AB is, as the name suggests, a hybrid of Class A and B operation. AB amplifiers remedy cross-over distortion (cross-over distortion is a type of distortion that occurs when one side of the output stage shuts off and another turns on) to a great degree by combining the best features of both classes. If the amplifier is biased so that the current flows in the device for 51-99% of the input signal, the amp is known to be operating in Class AB. Likewise, if an amplifier operates in Class A mode for only a portion of its output, and has to be supplemented with an additional current in the device for the remainder of its output, it is said to operate in Class AB. Class AB amplifiers are usually defined as amplifiers operating between class A and class B because class A amplifiers operate on 100% of input signal and class B amplifiers operate on 50% of the input signal. Anything between these two limits is operating in class AB.
Relying on the use of two Class B units, a Class AB system is a pair of complementary push-pull devices, each amplifying ~55% of the original signal and combining them afterward, resulting in a full signal. The push-pull drivers are carefully biased just above their fully off state so that the transition between drivers is smoother. Therefore, each driver is never completely turned off. This alleviates most of the cross-over distortion at the expense of efficiency. The reason why each device takes more than 50% of the signal is to ensure that the signals crossover and match up, and no device is completely turned off at any time. However, Class AB amplifiers are still extremely efficient. The risk of crossover distortion still exists, where the mismatched signal ends clipped when summed; even at most performance volumes, the distortion is not readily noticeable and the power efficiency of the amplifier is considered to be worth it. These amplifiers have better power efficiency but poorer fidelity than class A amplifiers.
Most amplifiers are in this category since they operate in two classes. In class AB and B, the amplifier is slower than in Class A because there is a finite time between the application of the input signal and when the devices are turned on to produce a flow of current to the speakers. However, Class AB and Class B are more energy-efficient than Class A and do not require such large power supplies.
Class B amps differ from Class A in their utilization of power. There is no current flowing in a Class B amp when the output devices are idle, and so they must turn on from a zero current state when signal becomes present. In a push-pull Class B design the output devices would each produce half of the audio waveform (one set for the positive half, another for the negative half) and would have no current flow when the other half is operating. Class B designs tend to have a slower slew rate and more crossover distortion but are less expensive and require less-intensive power supplies.
The class B amplifier not only reproduces half the input signal, but amplifies it as well. Class B amplifiers are twice as efficient as class A amplifiers since the amplifying device only conducts (and uses power) for half of the input signal.
These amplifiers utilize two drive elements operating in a push-pull configuration. On the positive excursion of the signal, the upper element supplies power to the load while the lower is turned off. During negative going signal excursions, the opposite operation occurs. This increases operating efficiency, but suffers from the nonlinear turn-on, turn-off region created where the driver elements switch from their ON state to their OFF state. This switching error creates a condition commonly called cross-over distortion.
Total Class B amps are less common among guitarists due to their unusual output signal. Instead, they are usually paired with with a matching push-pull element, resulting in a Class AB system.
I truly hope that after reading this article one can understand a little more about how amplifiers get their sound, and how to achieve results in the never-ending search for tone.