Guitar: Buying an Amplifier: An Open Learning Guide

Amplifiers come in a wide variety of designs and your choice of amplifier should be based on the type of music you wish to play. Your local guitar dealer will let you test the different amplifiers they stock though they may only offer a limited range due to space restrictions. Its always a good idea to visit many dealers including pro-audio outlets to test amplifiers across the entire price range before committing yourself. This chapter will explain the difference between a tube amplifier and a solid state amplifier as well as exploring the variations on these two basic designs.

Tube amplifiers produce a warm and fat tone that is popular with guitarists. New models are available from Marshall, Fender and Vox as well as other manufacturers which range in quality and price. Vintage tube amplifiers from the 1960s and 1970s are available to buy, though maintaining them can be expensive especially with regards to the cost of replacement parts. The continuing popularity of vintage amplifiers from earlier decades has resulted in a market for reissues.

Pure Class A operates by having a full continuous current flowing through the tubes. The tubes are still fully powered even when there is no signal to divert to the speaker. This makes Pure Class A tubes expensive to run; similar to a car idling in neutral with the throttle pedal right down. Pure Class A responds very fast to an input signal with a tube saturation (distortion) that many guitarists find appealing. Many Pure Class A amplifiers use tubes with a low wattage to offset their inefficient operating mode thereby increasing the tubes lifespan and reducing running costs.

Class A refers to an output design that doesn't use a Pure Class A single-ended output stage. Class A uses a pair of tubes or multiples terminating in a push-pull output stage.

Class B is rarely used in guitar amplifiers due to crossover distortion issues so manufacturers instead use a combination of Class A and Class B known as Class AB.

Class AB operates by using a pair of tubes. Whereas Pure Class A produces the entire waveform, positive and negative, with a single-ended output, Class AB produces the entire waveform using two tubes with one handling the positive voltage and the other the negative voltage. In comparison to Pure Class A, which uses a full continuous current to achieve a three hundred and sixty degree waveform and is always on at full power even when there is no signal present, Class AB uses a smaller amount of constant current to achieve the same always on state for the two tubes. Class AB is thereby more efficient with regards to power consumption and heat dissipation and the operating life of the tubes are greatly extended.

A tube is based on vacuum technology and requires more energy than a solid state amplifier with the same wattage.

Vacuum tubes are expensive and require replacement every one to four years depending on use.

Amplifiers with tubes are generally heavier than solid state amplifiers due to the need for an output transformer.

Solid state amplifiers use transistors instead of tubes. They are popular with beginners due to their affordability and lower weight. Solid state amplifiers have a fast attack time and are immediately available for use when switched on. Solid state circuitry allows more volume to be applied to the output signal before clipping occurs which makes the amplifiers appealing to amateur and professional jazz or acoustic guitarists who may wish to retain a clean sound at high volumes. A solid state amplifier matched with good quality speakers can produce a wide frequency response. Some solid state amplifiers use field effect transistors (FET) on the preamp stage which at high gain produces a distortion similar to a tube amplifier.

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Text adapted from Guitar/Buying an Amplifier by Wikibooks contributors under CC BY-SA 4.0. Paragraphs have been selected and abridged, and formatting adjusted. This adapted text is shared under the same licence. Contributor history.