Valve audio amplifier


Valve audio amplifier

A valve (UK) audio amplifier or vacuum tube (US) audio amplifier is a valve amplifier used for sound recording, reinforcement, or 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.

Instrument and vocal amplification

Valve amplifiers for guitars (and to a lesser degree vocals and other applications) are usually designed to very different specifications from those of hi-fi amplifiers. Distortion may be considered a desirable characteristic, and the valves are electrically driven much harder to maximize output power.

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 origins of electric guitar amplification

The electric guitar originates from Rickenbacker in the 1930s but its modern form was popularised by Fender and Gibson (notably the Fender Telecaster & Stratocaster and Gibson Les Paul) 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. [ [http://www.pentodepress.com/TU-Berlin/class-AB-ripple.html] ] 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 or may not 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 (ECCnn or 12AX7).

Amplifiers for sound reproduction

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 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 NFB 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 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):

* the introduction of the "Point One" series of amplifiers (in 1945) by LEAK in the UK, which first set a performance standard of 0.1% THD
* the Ultra-Linear output stage (a variation of push pull with tetrodes) was originated by Alan Blumlein in 1937 in the UK, but popularised following publication of a paper by David Hafler 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

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).

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 equalization, tone and filter circuits (different kinds of circuits designed to give a controlled non linear frequency response).

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. During the early years batteries were often used so this was not a problem, but modern amplifiers invariably run from the mains. Today 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:

*switchable rumble filter
*switchable scratch filter with selectable slops and corner frequency, at 4.5, 6, 9 kHzin addition to:
*continuously variable treble and bass tone controls
*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, perhaps with a relaxed top end and soft bass, but an excellent midrange.

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
**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

Very often the audiophiles believe that valve sound has an intrinsic quality due to the vacuum tube technology itself; this is not true. In 1972 Matti Otala from Finland demonstrated the origin of the problem in the Transitory Intermodulation Distortion (TIM) [ “Circuit Design Modifications for Minimizing Transient Intermodulation Distortion in Audio Amplifiers”, Matti Otala, Journal of Audio Engineering Society, Vol 20 # 5, June 1972] The TIM distortion do not appear at steady state distortion measurements, then it was not detected by most of the design engineers. TIM is due to the reduced open loop frequency response of the solid state amplifiers. Further works of Otala and other authors, found the solution for TIM distortion [ Distribution of the Phonograph Signal Rate of Change, Lammasniemi, Jorma; Nieminen, Kari, Journal of Audio Engineering Society, Vol 28 # 5, May 1980 ] In high quality modern amplifiers the open loop response is at least 20 kHz, canceling TIM distortion. But TIM distortion is still present in most low price home quality amplifiers

Further important development was conducted by Oscar Bonello of the University of Buenos Aires, Argentina. [ Advanced Negative Feedback Design for High Performance Amplifiers, Oscar Bonello, 67th AES Convention, New York, October 1980 ] Bonello created the “Double Loop Feedback” method for reducing the distortion in audio amplifiers. This technology led to the development of solid state amplifiers far better than the valve amplifiers, at lower cost and with higher power. Oscar Bonello proposed using poles and zeros at the feedback network to get a 9 dB/octave slope instead of the traditional 6 dB/octave. This permits the design of audio amplifiers with virtually no perceived distortion in the treble spectrum. These two contributions led to the development of high quality solid state amplifiers which are virtually “distortion free”.

Notable historic designs

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 :

* LEAK TL/12
* Williamson amplifier
* Mullard 5-10
* Quad II
* Dynaco Mark III and Stereo 70
* McIntosh MC275
* Marantz 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

*Amplifier
*Audio amplifier
*Julius Futterman
*Valve sound
*Valve amplifier

References

*Valve Amplifiers, Morgan Jones, Third Edition 2003 ISBN 0 7506 5694 8 - about the design and construction of valve audio amplifiers
*Tube Amplifiers, Allegro Verlag, Vienna. ISBN 3-901462-00-7 - Contains a short introduction, the rest of the book is lots of photographs of some tube amplifiers.
*Glass Audio. A long running journal devoted to tube amp construction, published by the Audio Amateur (TAA) Corp
*Radiotron Designers Handbook, Classic Edition, F Langford-Smith et al. First published 1934, revised until 1967ISBN 0 7506 3635 1 - Compendium of articles of historic interest to people in this field
*http://www.stereophile.com/reference/70/ - discussion of the limitations of NFB in audiophile systems
* [http://members.aol.com/aria3/otlpaper/otlhist.htm] - Theory paper on OTL designs.

External links

* [http://www.schematicheaven.com Schematic Heaven] — Free schematic archive of vintage guitar amplifier and effect schematics.
* [http://www.audiocircuit.com 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.


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