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Philip Galkin
Philip Galkin

Valve And Transistor Audio Amplifiers

A valve amplifier or tube amplifier is a type of electronic amplifier that uses vacuum tubes to increase the amplitude or power of a signal. Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers in the 1960s and 1970s.Valve amplifiers can be used for applications such as guitar amplifiers, satellite transponders such as DirecTV and GPS, high quality stereo amplifiers, military applications (such as radar) and very high power radio and UHF television transmitters.

Valve and Transistor Audio Amplifiers

Until the invention of the transistor in 1947, most practical high-frequency electronic amplifiers were made using thermionic valves.[1] The simplest valve (named diode because it had two electrodes) was invented by John Ambrose Fleming while working for the Marconi Company in London in 1904. The diode conducted electricity in one direction only and was used as a radio detector and a rectifier.

The first application of valve amplification was in the regeneration of long distance telephony signals. Later, valve amplification was applied to the 'wireless' market that began in the early thirties. In due course amplifiers for music and later television were also built using valves.

The overwhelmingly dominant circuit topology during this period was the single-ended triode gain stage, operating in class A, which gave very good sound (and reasonable measured distortion performance) despite extremely simple circuitry with very few components: important at a time when components were handmade and extremely expensive. Before World War II, almost all valve amplifiers were of low gain and with linearity dependent entirely on the inherent linearity of the valve itself, typically 5% distortion at full power.

Negative feedback (NFB) was invented by Harold Stephen Black in 1927, but initially little used since at that time gain was at a premium. This technique allows amplifiers to trade gain for reduced distortion levels (and also gave other benefits such as reduced output impedance). The introduction of the Williamson amplifier in 1947, which was extremely advanced in many respects including very successful use of NFB, was a turning point in audio power amplifier design, operating a push-pull output circuit in class AB1 to give performance surpassing its contemporaries.

From the 1970s the silicon transistor became increasingly pervasive. Valve production was sharply decreased, with the notable exception of cathode ray tubes (CRTs), and a reduced range of valves for amplifier applications. Popular low power tubes were dual triodes (ECCnn, 12Ax7 series) plus the EF86 pentode, and power valves were mostly being beam tetrode and pentodes (EL84, EL34, KT88 / 6550, 6L6), in both cases with indirect heating. This reduced set of types remains the core of valve production today.

The Soviets retained valves to a much greater extent than the West during the Cold War, for the majority of their communications and military amplification requirements, in part due to valves' ability to withstand instantaneous overloads (notably due to a nuclear detonation) that would destroy a transistor.[2]

The dramatic reduction in size, power consumption, reduced distortion levels and above all cost of electronics products based on transistors has made valves obsolete for mainstream products since the 1970s. Valves remained in certain applications such as high power RF transmitters and the microwave oven, and audio amplification equipment, particularly for the electric guitar, recording studios, and high-end home stereos.

In audio applications, valves continue to be highly desired by most professional users, particularly in recording studios' equipment and guitar amplifiers. There is a subgroup of audio enthusiasts who advocate the use of tube amplifiers for home listening. They argue that tube amplifiers produce a "warmer" or more "natural" valve sound. Companies in Asia and Eastern Europe continue to produce valves to cater to this market.

Many professional guitar players use 'tube amps' because of their renowned 'tone'. 'Tone' in this usage is referring to timbre, or pitch color, and can be a very subjective quality to quantify. Most audio technicians and scientists theorize that the 'even harmonic distortion' produced by valve tubes sounds more pleasing to the ear than transistors, regardless of style. It is the tonal characteristics of valve tubes that have sustained them as the industry standard for guitars and studio microphone pre-amplification.

Tube amplifiers respond differently from transistor amplifiers when signal levels approach and reach the point of clipping. In a tube amplifier, the transition from linear amplification to limiting is less abrupt than in a solid state unit, resulting in a less grating form of distortion at the onset of clipping. For this reason, some guitarists prefer the sound of an all-tube amplifier; the aesthetic properties of tube versus solid state amps, though, are a topic of debate in the guitarist community.[3]

Power valves typically operate at higher voltages and lower currents than transistors - although solid state operating voltages have steadily increased with modern device technologies. High power radio transmitters in use today operate in the kilovolt range, where there is still no other comparable technology available. ([power = voltage current], so high power requires high voltage, high current, or both)

Valves remain in widespread use in guitar and high-end audio amplifiers due to the perceived sound quality they produce. They are largely obsolete elsewhere because of higher power consumption, distortion, costs, reliability, and weight in comparison to transistors.

The advantage of this is that a single valve "repeater" amplifier can amplify many calls at once, this being very cost effective. The problem is that the amplifiers need to be extremely linear, otherwise "intermodulation distortion" (IMD) will result in "crosstalk" between the multiplexed channels. This stimulated development emphasis towards low distortion far beyond the nominal needs of a single voice channel.

Today, the main application for valves is audio amplifiers for high-end hi-fi and musical performance use with electric guitars, electric basses, and Hammond organs, although these applications have different requirements regarding distortion which result in different design compromises, although the same basic design techniques are generic and widely applicable to all broadband amplification applications, not only audio.

Post World War II, the majority of valve power amplifiers are of the Class AB-1 "push pull" ultralinear topology, or lower cost single ended i.e. 6BQ5/EL84 power tubes, but niche products using the DH-SET and even OTL topologies still exist in small numbers.

Today, radio transmitters are overwhelmingly silicon based, even at microwave frequencies. However, an ever-decreasing minority of high power radio frequency amplifiers continue to have valve construction.

Valves (vacuum tubes) vs. transistors (yes, again ) ... this somewhat simplified article looks at the primary differences between the two. I have included some measured data from a valve amp that may help understanding the differences in real terms. Apart from the obvious differences set out below, there are some important considerations that for reasons I do not understand, seem to have been completely ignored for the most part. These will be covered in detail (together with the measurements) later in this article.

There are differences - not only between valved and transistorised amplifiers, but between different valve (or transistor) amps amongst their peers. However, the fact remains that when similarly specified amplifiers are compared using a double blind test (DBT) methodology, it is rare that listeners are able to pick the difference with statistically significant reliability.

While this could be because of 'test stress' as commonly complained of by many subjectivists (and assuming you actually believe it happens), it is generally more likely that the differences are inaudible. I do not propose to discuss the audibility or otherwise of various distortion mechanisms here. I will just describe, in simple and measurable terms, the primary differences between vacuum tube amps and 'solid state' versions of similar specifications.It's important to realise that this article simply looks at some specific differences between valve and transistor amps, in particular the conduction 'efficiency' of the output devices. Whether you prefer valves or transistors is immaterial, and while I actually like valve amps, I don't have one in my system because the ongoing maintenance is time-consuming and expensive. This is especially true with a system that uses six power amps (plus one for the subwoofer) because I use an active crossover.It's also important to understand that valves are not 'better' than semiconductors, nor are they 'worse'. The final result is down to engineering, how much the user is willing to pay for an amp that has enough power for his/her needs, and the overall aesthetics (to some this is very important, to others it's meaningless). While a valve amp may appear to be simpler than an equivalent transistor power stage, this is an illusion. The complexity is in the details that aren't readily apparent - in particular the output transformer.

The following list (and most of this discussion) is based on using valves in power amplifiers, rather than line level (preamplifier) stages. The analysis that follows compares like with like (insofar as possible), and assumes push-pull, Class-AB amplifiers, driving resistive and loudspeaker loads. While other topologies may give (marginally) different results in some cases, the general (and overwhelming) factors remain much the same.Firstly, we shall look at the physical characteristics of the amplifying devices themselves ...

While the above is not an exhaustive list, it gives a reasonable overview of the primary differences between the devices. By necessity, the list is not as detailed as it might be, and in some cases I have made assumptions that are reasonable for audio amplifiers. For example, many valves have extremely high amplification factors, but generally have poor linearity as a result. While some high gain valves are used in audio, they are not common in modern equipment. Likewise device linearity. Both valves and semiconductors range from being highly non-linear to having very good linearity (before feedback is applied). 041b061a72


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