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What valve amplifiers are

What they are

The valve sound or tube sound is the sound often associated with music amplified by a valve amplifier, although this association is not strict. The valve sound is often described as being "warm", "rich", "relaxed" etc, in contrast to a so-called transistor sound that is sometimes said to be "faster" or even "harsh" and "brittle". Both audio enthusiasts and musicians with amplified instruments have a strong interest in these questions.

One important aspect of tube sound is the soft ceiling in the dynamic range of tube amplifiers. A tube radio or tube amplifier will increase in volume to a point, and then as the volume is further increased, it gently reduces in volume. Unlike transistor amplifiers, tube amplifiers don't tend to produce the harsh clipping of saturation and cutoff that transistor amplifiers do. Thus turning up the volume control, the sound will get louder and then quieter again in a very smooth and gentle sort of way. This soft ceiling behaves very much like a dynamic range compressor/limiter. In the same way that photographic film has a filmic look especially owing to a soft compression of highlights (i.e. even when massively overexposed, the film still responds, to some degree, to increases in light), tube amplifiers provide a nice compressive effect. For this reason, tubes are often still used to impart a pleasant response characteristic to solid state amplifiers (e.g. through the use of one 12AX7 tube within an otherwise transistorized amplifier). Alternatively, one may use a light bulb in the feedback loop of an infinite gain multiple feedback (IGMF) circuit. The sluggish response of the light bulb can thus be used to moderate the sound and attain a valvelike "soft ceiling" in dynamic range. Microprocessor-based or microcontroller-based solid state signal processing devices can also be used for this purpose, and there are a wide variety of products that can shape the dynamic range of an audio signal. These typically operate in four regimes: expansion at low levels; no change at mid levels; compression at high levels; limiting at extremely high levels.

Some other aspects of valve sound have more to do with the circuit topology and circuit design of the amplifier, than with the use of valves rather than transistors as the active gain devices. There is also a degree of influence from the passive components used.

Early amplifiers were by definition valve amplifiers since the transistor did not become common in consumer amplifiers until the late 1960s. The very earliest ampifiers usually had single ended topologies with the most basic type of valve, known as a triode. An audio amplifier using this topology will always be in class A. Class A single-ended triode ampliers (Known as SET's) have a characteristic distortion spectrum, a simple and monotonically decaying series of harmonics, dominated by modest levels of second harmonic distortion. Second harmonic distortion is musically equivalent to adding the same tone one octave higher, to form a chord. in this case the added tone is at a lower level (typically 5% or less at full power) but the effect is to "fatten" the sound.

Another aspect of the valve sound is that early valve amplifiers often had only limited bandwidth, in part due to passive component technology available at the time, notably capacitors. It must be remembered that the source material available at this time (usually vinyl records) lacked the definition of today's CDs. There was no electronic music with extreme bass, and high frequencies mostly contained noise and distortion, so it was advantagous to not emphasise these.

Transistor amplifiers are almost always class AB push pull, and this topology tends to cancel even order harmonic distortion products. The resulting distortion is therefore dominated by odd order harmonics, which to human sensibilities sound "harsh" etc. Transistor amplifiers made during the 1980s typically also had extremely high gain, but poor open loop linearity, and relied on large amounts of negative feedback (NFB). Some consider that NFB does not sound "natural" or "musical", due to errors in the way it reacts to transients. These errors also reveal themselves in very complex distortion spectra, that humans find "discordant".

In contrast audio valves typically have only modest gain, and are extremely linear. This makes it possible to design very simple valve circuits that rely on this inherent open loop linearity and have little, or indeed no, NFB, and thus have very simple distortion spectra.

The subject is further complicated by the way triodes and MOSFETS have certain similarities in their transfer characteristics, whereas later forms of the valve, the tetrode and pentode have quite different characteristics, in some ways reminicient of the transistor.

All these factors of contribute to the valve sound for audio applications. Valve amplifiers however are also widely used for electric guitars, in this case for quite different reasons : the way they distort when overdriven is quite different, again leading to a very distinctive sound.

In more modern times, transistor amplifiers have become dominant, mainly because they are cheaper to produce, and operate on lower voltages (including batteries, in applications such as car radios.) However, valve amplifiers have retained a loyal following among audio fanatics, especially for SET's, in Japan, and in recent years there has also been a resurgence in the west.

Using modern passive components, and modern digital sources and wide band speakers, it is possible to build valve amplifiers that have the characteristic wide bandwidth and "fast" sound of modern transistor amplifiers, including using push pull circuits, class AB and feedback. And some enthusiasts have built amplifiers using transistors and MOSFETS that operate in class A, including single ended, and these often have the valve sound.

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