A valve audio amplifier (UK) or vacuum tube audio amplifier (US) is a valve amplifier used for sound reinforcement, sound recording and reproduction.
Until the invention of solid state devices such as the transistor, all electronic amplification was produced by valve (tube) amplifiers. While solid-state devices prevail in most audio amplifiers today, valve audio amplifiers are still used where their audible characteristics are considered pleasing, for example, music performance or music reproduction.
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Valve amplifiers for guitars (and to a lesser degree vocals and other applications) have different purposes from those of hi-fi amplifiers. The purpose is not necessarily to reproduce sound as accurately as possible but rather to fulfill the musician's concept of what the sound should be. For example, distortion is almost universally considered undesirable in hi-fi amplifiers but may be considered a desirable characteristic in performance.
Small signal circuits are often deliberately designed to have very high gain, driving the signal far outside the linear range of the tube circuit, to deliberately generate large amounts of harmonic distortion. The distortion and overdrive characteristics of valves are quite different from transistors (not least the amount of voltage headroom available in a typical circuit) and this results in a distinctive sound. Amplifiers for such performance applications typically retain tone and filter circuits that have largely disappeared from modern hi-fi products. Amplifiers for guitars in particular may also include a number of "effects" functions.
The electric guitar originates from Rickenbacker in the 1930s but its modern form was popularised by Fender and Gibson (notably the Fender Telecaster (1951) & Stratocaster (1954) and Gibson Les Paul (1952) during the 1950s. The earliest guitar amplifiers were probably audio amplifiers made for other purposes and pressed into service, but the electric guitar and its amplification quickly developed a life of its own, supported by specialist manufacturers.
Guitar amplifiers are typically designed with excess gain, allowing the guitar, when played hard, to generate a signal that is sufficient to overdrive one or more tube stages and thus generate gross distortion deliberately. The characteristics of the tube and the circuit design directly influence the tone that results. Even the power supply can influence the tonal shape, with relatively undersized power supply capacitors producing a characteristic "sag" and subsequent recovery that is often considered musically engaging.[1] In addition, guitarists may employ acoustic feedback, further modifying the resulting sound (noting that the feedback signal has a slight time lag relative to the original signal).
Guitar amplifiers are typically designed to withstand a lot of abuse both electrically and physically (since guitarists often travel to gigs, etc.) In large systems the amplifier is separate from the speaker enclosure(s), but in smaller systems it is often integrated, forming a so-called "combo". Since the amplifier is usually at the top of the combo, the tubes often hang upside down facing the body of the enclosure. They may be held in with clips.
The circuit topology of most modern guitar amplifiers is a class AB1 push pull circuit using the mainstream tubes from the golden age, today usually 6L6 or EL34 but occasionally EL84 or KT88 / 6550 in ultralinear connection . This output stage is normally driven by at least two double triodes, invariably from the noval family (ECC83 or 12AX7).
The earliest mass usage of valve audio amplifiers was for telephony. Valve amplifiers were critical in development of long-distance telephone circuits and submarine telephone cables. Radio applications followed soon after, where valves were used for both the audio (AF) and radio (RF) circuitry. (RF is outside the scope of this article, see valve amplifier).
Among the first applications of sound recording and electronic replay around the 1920s was its use in many cinemas equipping for exhibiting the new 'talkies'. Cinema sound systems of this period were predominantly supplied by "Westrex", related to the Western Electric company, a telecoms supplier, who were also the makers of the 300B DHT tube that today is central to current production DH-SET audiophile amplification.
Almost all amplifiers during this period were (by today's standards) of very low power, tyically using the (Class A) "single ended triode" circuit topology and directly heated tubes. Today this type of circuit retains a niche following at the very extreme of audiophile hi-fi, where it is often referred to by the acronym DH-SET.
Prior to WWII, almost all electronic amplifiers were triodes used without feedback. The linearity of tubes makes it possible to get acceptable distortion performance figures without any form of compensation or error correction. Amplitude distortion in a class A triode stage can be small if care is taken to prevent the anode current from too closely approaching zero, and by ensuring that there is no grid current allowed to flow. In this case, distortion will be largely second harmonic in nature with the percentage of the second harmonic being closely proportional to the output amplitude. Adding modest negative feed back to a circuit with reasonable open loop linearity may also yield further improvements.
During the post war period, widespread adoption of negative feedback in the push pull topology yielded greater power and linearity, notably following the publication in 1947 of the Williamson amplifier, which set the standard (and the dominant topology) for what was to follow.
Widespread adoption of push pull allowed smaller (and thus cheaper) transformers, combined with more power (typically ~ 10 to 15 watts) sufficient to drive higher quality domestic loudspeakers. The high fidelity industry was born.
Other developments included (among others):
Valve amplification peaked as the mainstream technology during the 1960s and 70s, with device and circuits being highly developed, there have been only minor refinements since then.
The last generation of power tubes, typified by KT66, EL34 and KT88, in many ways represent the pinnacle of the technology, and also of production quality. Valve amplifiers produced since that time usually use one of these tubes, which have remained in continuous production (apart from KT66) ever since.
Small signal valves overwhelmingly changed from octal base tubes, notably the audio tube of choice, the 6SN7 family, to the smaller and cheaper noval base ECC81, ECC82, ECC83 (UK, in the US known as 12AX7,12AT7, etc.). The noval base EL84 power tube also became the dominant power tube in a class of ~ 10 watt ultralinear power amplifiers
Commercial tube manufacturers also developed designs using their particular product - most notably, the Mullard 5-10 circuit, which as with the earlier Williamson were subsequently widely cloned either exactly or as derivatives (with and without due credit).
Vacuum tube radios and amplifiers were used in automobiles until being displaced by transistorized radios. This shift occurred for the same reasons as in almost all other electronic devices: transistors are smaller, cooler, cheaper, more durable, use less power and are ready to operate much quicker.
Some enthusiasts prefer "tube amps", so a small number of tube car stereos are still made. Manufacturers include Milbert Amplifiers, Blade, Manley, and Sear Sound. Some are "hybrid" designs with transistorized power supplies and a few "token tubes".
Due to the very poor technical performance of early gramophones, the lack of standardised equalisations, poor components and accessories (including loudspeakers), preamplifiers historically contained extensive and very flexible equalization and tone and filter circuits designed to adjust the frequency response of the amplifier and so the sound produced by the system.
Valve preamplifiers are invariably triode circuits, in order to have low noise as well as good linearity. Mains hum from the heater filaments is a major problem in valve amplifiers, but especially preamplifiers. Modern amplifiers invariably run from the mains, so the heater supply is usually rectified and even regulated to avoid hum
A representative valve preamp from the 1950s is the Leak 'varislope' series of preamps, which included a switchable rumble filter, a switchable scratch filter with selectable slopes and corner frequency, continuously variable treble and bass tone controls and a selection of 4 different gramophone equalisations (RIAA, ortho, RCA, 78). The large number of complex filter circuits, combined with (by today's standards) poor quality switching etc., resulted in slight sonic degradation.
Amplifiers from and prior to this period often have a distinctive sound that today is still widely referred to as "valve sound", which might loosely be described as a "warm" tone.
This tone is not strictly due to valves being used rather than transistors, rather it is just a sound that was originally associated with amplifiers built using valves simply because that is what was available at the time. The origins of that particular tonality are in fact in part due to:
In addition to a huge range of indifferent commodity valve amplifiers made over the years, many very good amplifiers were made, some of which are still highly regarded today. The following is only a sampling of some of the most well-known:
Various basic circuits have been used in designs and as well as various approaches to construction.