Valve audio amplifiers
Encyclopedia
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.
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.
(notably the Fender Telecaster
(1951) & Stratocaster (1954) and Gibson Les Paul
(1952) during the 1950s. The earliest guitar amplifier
s 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. 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
).
).
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.
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).
radio
s and amplifier
s were used in automobile
s until being displaced by transistor
ized 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".
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.
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:
Valve amplifier
A valve amplifier or tube amplifier is a type of electronic amplifier that makes use of vacuum tubes to increase the power and/or amplitude of a signal. Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers during the 1960s and...
used for sound reinforcement, sound recording and reproduction
Sound recording and reproduction
Sound recording and reproduction is an electrical or mechanical inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recording technology are analog recording and digital recording...
.
Until the invention of solid state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...
devices such as the transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
, 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.
Instrument and vocal amplification
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
Gain
In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...
, 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 origins of electric guitar amplification
The electric guitar originates from Rickenbacker in the 1930s but its modern form was popularised by Fender and GibsonGibson Guitar Corporation
The Gibson Guitar Corporation, formerly of Kalamazoo, Michigan and currently of Nashville, Tennessee, manufactures guitars and other instruments which sell under a variety of brand names...
(notably the Fender Telecaster
Fender Telecaster
The Fender Telecaster, colloquially known as the Tele , is typically a dual-pickup, solid-body electric guitar made by Fender.Its simple yet effective design and revolutionary sound broke ground and set trends in electric guitar manufacturing and popular music...
(1951) & Stratocaster (1954) and Gibson Les Paul
Gibson Les Paul
The Gibson Les Paul was the result of a design collaboration between Gibson Guitar Corporation and the late jazz guitarist and electronics inventor Les Paul. In 1950, with the introduction of the Fender Telecaster to the musical market, electric guitars became a public craze. In reaction, Gibson...
(1952) during the 1950s. The earliest guitar amplifier
Guitar amplifier
A guitar amplifier is an electronic amplifier designed to make the signal of an electric or acoustic guitar louder so that it will produce sound through a loudspeaker...
s 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 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
6L6
6L6 is the designator for a vacuum tube introduced by Radio Corporation of America in July 1936. At the time Philips had already developed and patented power pentode designs, which were fast replacing power triodes due to their greater efficiency...
or EL34
EL34
The EL34 is a thermionic valve or vacuum tube of the power pentode type. It has an international octal base and is found mainly in the final output stages of audio amplification circuits and was designed to be suitable as a series regulator by virtue of its high permissible voltage between heater...
but occasionally EL84
EL84
The EL84 is a vacuum tube of the power pentode type. It has a 9 pin Noval base and is found mainly in the final output stages of amplification circuits, most commonly now in guitar amplifiers, but originally in radios and many other devices of the pre-transistor era.It was developed to eliminate...
or KT88
KT88
The KT88 is a beam tetrode/kinkless tetrode vacuum tube for audio amplification.- Features :The KT88 fits a standard eight-pin octal socket and has similar pinout and applications as the 6L6 and EL34. Specifically designed for audio amplification, the KT88 has similar ratings to the American 6550...
/ 6550 in ultralinear connection . This output stage is normally driven by at least two double triodes, invariably from the noval family (ECC83 or 12AX7
12AX7
12AX7 is a miniature dual triode vacuum tube with high voltage gain. It was developed around 1946 by RCA engineers in Harrison, New Jersey, under developmental number A-4522. It was released for public sale under the 12AX7 identifier on September 15, 1947. The 12AX7 was originally intended as...
).
Early development
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 amplifierValve amplifier
A valve amplifier or tube amplifier is a type of electronic amplifier that makes use of vacuum tubes to increase the power and/or amplitude of a signal. Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers during the 1960s and...
).
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.
The 1940s and 1950s
During the post war period, widespread adoption of negative feedbackNegative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
in the push pull topology yielded greater power and linearity, notably following the publication in 1947 of the Williamson amplifier
Williamson amplifier
A Williamson amplifier refers to a type of vacuum tube amplifier whose circuit design is similar to that originally published by D.T.N. Williamson.- Explanation :...
, 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
High fidelity
High fidelity—or hi-fi—reproduction is a term used by home stereo listeners and home audio enthusiasts to refer to high-quality reproduction of sound or images, to distinguish it from the poorer quality sound produced by inexpensive audio equipment...
industry was born.
Other developments included (among others):
- the introduction of the "Point One" series of amplifiers (in 1945) by LEAKLEAKLEAK is the brand name for high-fidelity audio equipment made by H. J. Leak & Co. Ltd, of London, England. The company was founded in 1934 by Harold Joseph Leak and was sold to the Rank Organisation in January 1969. During the 1950s and 60s, the company produced high-quality amplifiers, radio...
in the UK, which first set a performance standard of 0.1% THD - the Ultra-LinearUltra-LinearUltra-linear is a term used to describe a type of electronic circuit that is used to couple a tetrode or pentode vacuum-tube to a load ....
output stage (a variation of push pull with tetrodes) was originated by Alan BlumleinAlan BlumleinAlan Dower Blumlein was a British electronics engineer, notable for his many inventions in telecommunications, sound recording, stereo, television and radar...
in 1937 in the UK, but popularised following publication of a paper by David HaflerDavid HaflerDavid Hafler was an American audio engineer. He was best known for his work on an improved version of the Williamson amplifier using the ultra-linear circuit of Alan Blumlein....
and Keroes in the USA in 1951, and became the dominant topology during the post war recovery of consumer products - Manufacturers bringing high quality domestic hi-fi to a steadily widening audience, eventually leading to Dynaco selling over 300,000 ST-70's ... to date, the world's most popular hi-fi amplifier (of any type).
Tube hi-fi in the 1960s
The last generation of power tubes, typified by KT66
KT66
The KT66 is a beam tetrode/kinkless tetrode vacuum tube for audio amplification.KT66 is the designator for a vacuum tube introduced by Marconi-Osram Valve Co. Ltd. of Britain in 1937....
, EL34
EL34
The EL34 is a thermionic valve or vacuum tube of the power pentode type. It has an international octal base and is found mainly in the final output stages of audio amplification circuits and was designed to be suitable as a series regulator by virtue of its high permissible voltage between heater...
and KT88
KT88
The KT88 is a beam tetrode/kinkless tetrode vacuum tube for audio amplification.- Features :The KT88 fits a standard eight-pin octal socket and has similar pinout and applications as the 6L6 and EL34. Specifically designed for audio amplification, the KT88 has similar ratings to the American 6550...
, 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
6SN7
6SN7 is a dual triode vacuum tube, on an 8 pin octal base. Although the 6S—series tubes are often metal cased, the 6SN7 is generally found only in a glass GT size envelope. The 6SN7 is basically two 6J5 triodes in one glass envelope.-History:...
family, to the smaller and cheaper noval base ECC81, ECC82, ECC83 (UK, in the US known as 12AX7,12AT7, etc.). The noval base EL84
EL84
The EL84 is a vacuum tube of the power pentode type. It has a 9 pin Noval base and is found mainly in the final output stages of amplification circuits, most commonly now in guitar amplifiers, but originally in radios and many other devices of the pre-transistor era.It was developed to eliminate...
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
Mullard 5-10
The Mullard 5-10 was a circuit for a valve amplifier designed by the British vacuum tube company, Mullard in 1954 at the Mullard Applications Research Laboratory in Mitcham Surrey UK, part of the New Road factory complex, to take advantage of their particular products...
circuit, which as with the earlier Williamson were subsequently widely cloned either exactly or as derivatives (with and without due credit).
Automobile amplifiers
Vacuum tubeVacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
s and amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
s were used in automobile
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
s until being displaced by transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
ized 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".
Valve preamplifiers
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 equalizationEqualization
Equalization, is the process of adjusting the balance between frequency components within an electronic signal. The most well known use of equalization is in sound recording and reproduction but there are many other applications in electronics and telecommunications. The circuit or equipment used...
and tone and filter
Tone control circuits
Tone control is a type of equalization used to make specific pitches or "frequencies" in an audio signal softer or louder. A tone control circuit is an electronic circuit that consists of a network of filters which modify the signal before it is fed to speakers, headphones or recording devices by...
circuits designed to adjust the frequency response
Frequency response
Frequency response is the quantitative measure of the output spectrum of a system or device in response to a stimulus, and is used to characterize the dynamics of the system. It is a measure of magnitude and phase of the output as a function of frequency, in comparison to the input...
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.
Valve sound
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:
- the typical circuit designs of the time (class A or class AB1 with a heavy class A overlap), combined with
- simple circuits, often having little or no feedback, that results in a simple (often monotonically decaying) distortion spectrum
- a frequency response well suited to guitar
- capacitor quality (often very poor compared to modern types),
- under-dimensioned and unregulated power supplies,
- poor quality output transformers in budget equipment,
- low damping factor (High Z out) output stages
Notable historic designs
- LEAKLEAKLEAK is the brand name for high-fidelity audio equipment made by H. J. Leak & Co. Ltd, of London, England. The company was founded in 1934 by Harold Joseph Leak and was sold to the Rank Organisation in January 1969. During the 1950s and 60s, the company produced high-quality amplifiers, radio...
TL/12 - Williamson amplifierWilliamson amplifierA Williamson amplifier refers to a type of vacuum tube amplifier whose circuit design is similar to that originally published by D.T.N. Williamson.- Explanation :...
- Mullard 5-10Mullard 5-10The Mullard 5-10 was a circuit for a valve amplifier designed by the British vacuum tube company, Mullard in 1954 at the Mullard Applications Research Laboratory in Mitcham Surrey UK, part of the New Road factory complex, to take advantage of their particular products...
- QuadQuad ElectroacousticsQuad Electroacoustics is a British manufacturer of hi-fi equipment, based in Huntingdon, England.- Corporate history :The company was founded by Peter J. Walker in 1936 in London, and was initially called S.P. Fidelity Sound Systems. In 1936 the name was changed to the Acoustical Manufacturing Co....
II - DynacoDynacoFounded by David Hafler and Ed Laurent in Philadelphia, Pennsylvania in 1955, Dynaco was an American hi-fi audio system manufacturer popular in the 1960s and 1970s for its wide range of affordable, yet high quality audio components..-Early company history:...
Mark III and Stereo 70 - McIntoshMcIntosh LaboratoryMcIntosh Laboratory is an American manufacturer of high-end audio equipment based in Binghamton, New York. The company was founded in 1949 by Frank McIntosh,. The "classic" vacuum tube components of the 1960s include the MC275 power amplifier, the C22 preamplifier, and the MR67 tuner...
MC275 - MarantzMarantzMarantz is a company that develops and sells upper-mid range to high-end audio products.The first Marantz audio product was designed and built by Saul B. Marantz in his home in Kew Gardens, New York...
8B and 9
Valve audio amplifier technical information
Various basic circuits have been used in designs and as well as various approaches to construction.See also
- AmplifierAmplifierGenerally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
- Audio amplifierAudio amplifierAn audio amplifier is an electronic amplifier that amplifies low-power audio signals to a level suitable for driving loudspeakers and is the final stage in a typical audio playback chain.The preceding stages in such a chain are low power audio amplifiers which perform tasks like pre-amplification,...
- Julius Futterman
- Valve sound
- Valve amplifierValve amplifierA valve amplifier or tube amplifier is a type of electronic amplifier that makes use of vacuum tubes to increase the power and/or amplitude of a signal. Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers during the 1960s and...
External links
- Schematic Heaven — Free schematic archive of vintage guitar amplifier and effect schematics.
- The Audio Circuit - An almost complete list of manufacturers, DIY kits, materials and parts and 'how they work' sections on valve amplifiers.
- http://www.stereophile.com/reference/70/ A detailed discussion of the limitations of NFB as a panacea.