Tube sound (or valve sound) is the characteristic sound associated with a vacuum tube-based audio amplifiers. Some audiophiles prefer the sound that is produced by the distortion characteristics of tube-based amplifiers. The audible significance of tube amplification on audio signals is a subject of continuing debate among audiophiles. Electric guitar, electric bass, and keyboard players in a range of popular and jazz genres also use tube instrument amplifiers or preamplifiers.
The tube sound is often subjectively described as having a "warmth" and "richness", but the source of this is by no means agreed on. It may be due to the non-linear clipping that occurs with tube amps, or due to the higher levels of second-order harmonic distortion, common in single-ended designs resulting from the characteristics of the tube interacting with the inductance of the output transformer.
Some tube fans attribute the "naturalness" of the perceived sound to the reduced number of components used by tube amplifiers. This is especially so for single-ended triode amplifiers (SET). These are said to reduce the "smearing" of the sound; reducing the imperfections invariably introduced by each electronic component. Much emphasis is placed on phase linearity. This minimalism is not solely the domain of tube amplifiers; there are some transistor amplifiers producing results that are similar. The JFET, for example, behaves much like a triode in its "ohmic" region.
Another advantage of most tube amplifier designs is the high input impedance (typically 100 kΩ) in modern designs and as much as 1 MΩ in classic designs. By contrast, transistor amplifiers for home use may have much lower input impedances, some as low as 20 kΩ. This implies that it requires more energy to excite the input of a typical transistor amplifier to any given voltage than it does a typical tube amplifier. If sensitivity to small signals is a significant goal, then tube designs will typically outperform transistor designs.
Some sonic qualities are easy to explain objectively based on an analysis of the distortion characteristics of the gain device and/or the circuit topology. For example, the triode SE gain stage produces a stereotypical monotonically decaying harmonic distortion spectrum that is dominated by significant second-order harmonics making the sound seem rich or even "fat".
However, other audible differences in sound have proven difficult to define or measure, and it is difficult to explain these sound differences in words as the vocabulary available to describe sound is rather limited -even though the underlying sonic effects may be real. Audiophiles often use words like warm, liquid, smooth and midrange magic to describe the sound from tube amplifiers.
Some claim that the midrange reproduction is more extended and smoother with tube amplifiers, but that high frequencies are somewhat rolled off. Historically this was often the case due to limitations in capacitor performance. Modern audiophile-grade tube amplifiers however, using modern and/or premium quality (and cost) capacitors can have frequency response that are essentially flat to octaves beyond the audio range: −3 dB above 85 kHz is quite common.
Similarly, some would characterize "tube sound" as bass response with less power and/or less definition (perhaps even "sloppy" bass or a bass boom with some speakers). This again can be explained by many tube amplifiers having relatively high output impedance (Z out) compared to transistor designs, due to the combination of both higher device impedance itself and typically reduced feedback margins (more feedback results in a lower Z out).
So for example a hypothetical design in two otherwise equal variants with just different amounts of feedback, might result in the higher feedback version having a "drier" midrange (due to reduced second-order harmonics due to greater reduction of distortion) but a "tighter" bass due lower output impedance. The speaker impedance divided by the Z out is sometimes referred to as the "damping factor" - the amplifier's ability to control the mechanical movement of the speaker.
In general terms the sound from a tube amplifier will typically have a softer attack and the bass frequencies will be more prominent giving a warmer and less "harsh" sound. Instrumentation such as pianos and vocals sound softer and "fatter" than with transistor amplifiers. But as noted the reasons for these effects are not simply and unavoidably related to the gain device type, today a good designer using either technology has to make synergistic design compromise choices. And the sonic differences are less stereotyped than they used to be as a result.
A single-ended amplifier has an asymmetric transfer characteristic, and produces both even and odd harmonics. As tubes are often run single-ended, and semiconductor amplifiers are often push-pull, the types of distortion are incorrectly attributed to the devices (or even the amplifier class) instead of the topology. Push-pull tube amplifiers can be run in class A, AB, or B. When in the class A region the distortion will be as described above, but while in class B the distortion will be SE like. Also, a class AB amplifier will have crossover distortion that will be typically inharmonic and thus sonically very undesirable indeed.
Another factor is that the distortion content of class A circuits (SE or PP) typically monotonically reduces as the signal level is reduced, asymptotic to zero during quiet passages of music. For this reason class A amplifiers are especially desired for classical and acoustic music etc. cf. class B and AB amplifiers, for which the amplitude of the crossover distortion is more or less constant, and thus the distortion relative to signal in fact increases as the music gets quieter. Class A amplifiers measure best at low power, class AB and B amplifiers measure best just below max rated power.
Loudspeakers present a reactive load to an amplifier (capacitance, inductance and resistance). This impedance may vary in value with signal frequency and amplitude. This variable loading affects the amplifier's performance both because the amplifier has finite output impedance (it cannot keep its output voltage perfectly constant when the speaker load varies) and because the phase of the speaker load can change the stability margin of the amplifier. The influence of the speaker impedance is different between tube amplifiers and transistor amplifiers, principally because tube amplifiers normally use output transformers and they tend to use relatively low feedback.
The design of speaker crossover networks and other electro-mechanical properties may result in a speaker with a very un-even impedance curve, for a nominal 8 Ω speaker, being as low as 6 Ω at some places and as high as 30-50 Ω elsewhere in the curve. An amplifier with little or no feedback will always perform poorly when faced with a speaker where little attention was paid to the impedance curve.
Note also that tube circuits often have huge headroom (overload) margins due to the high voltages they run from, so hard clipping is in reality very rare in a tube stage itself. However core saturation in the output transformer may be "designed in" to some guitar amplifiers when driven hard, and/or the tube biasing may be designed so that the tube passes from class AB1 to class AB2 and starts to draw grid current etc. (these effects are perhaps beyond the scope of this article)
Circuit design may also play an important role in the tube sound; tube circuits are often less complex and laid out differently. It is argued that simplicity is usually best, as the length and complexity can change the inductance and capacitance of a circuit. A more complex circuit will have a more complex sonic distortion characteristic. Minimalist DH-SEs for example typically have a dominant very simple harmonic distortion spectrum. Complex modern transistor designs often have low level but extremely complex harmonic distortion spectra.
In contrast, modern amplifiers often use high-quality, well-regulated power supplies. In theory, the output voltage remains constant, but in reality it never does — not least due to resistive losses in the cabling from the power supply to the gain stage. This problem is proportionately much worse in transistor amplifiers because they operate at low voltage and high current, whereas tube voltage amplification stages operate at low currents and high voltages. Ohmic losses are a function of current through resistance.
Some high end tube amplifier designs also include vacuum tube rectifier circuits instead of modern silicon diode or bridge rectifier circuits. A cheap solid state rectifier does introduce audible noise into the circuit. Audibility of the effects is disputed by many. In unregulated power supplies the switching noise from silicon diodes can affect the amplifier's performance by introducing noise into the high voltage circuit. In guitar amplifiers, tube rectification is used in order to intentionally cause the high voltage supply to sag in order to add distortion and compress the output signal.
The practical advantage to tube rectification is that the relatively inexpensive rectifier tubes require some time to warm up before they begin to conduct. This gives the time for the heaters in the output tubes to warm up as well and therefore extend their lifespan. If the high voltage supply is brought up too quickly, the cathodes might be damaged. Some high end manufacturers, such as Welborne Labs in their premium kits, feature ultrafast soft-recovery silicon diodes bridged by snubber networks on the basis that the cost and power required to operate a vacuum tube rectifier does not yield any measurable improvement in the sound.
Class AB push-pull topology is nearly universally used in tube amps for electric guitar applications. Whereas audiophile amps are primarily concerned with avoiding distortion, a guitar amp embraces it. When driven to their respective limits, tubes and transistors distort quite differently. Tubes clip more softly than transistors, allowing higher levels of distortion (which is sometimes desired by the guitarist) whilst still being able to distinguish the harmonies of a chord. This is because the soft profile of the tube amplifier's distortion means that the intermodulation products of the distortion are generally more closely related to the harmonies of the chord.
The triode, despite being the oldest signal amplification device, also has the most linear transfer characteristic, and thus requires little or no negative feedback for acceptable distortion performance. NFB is used in most post 1950s amplifiers and although it usually reduces the measured distortion level, it results in an unpleasant combination of harmonics to some ears.
Audiophiles who prefer SET-amplifiers state that measured sound performance is a poor indicator of real world sound performance and distortion level is not the only criterion for good sound reproduction. There are measurements not using resistive load but actual loudspeakers to back this up. In the 1970s, designers started producing transistor amps with higher open loop gain to support a greater value of negative feedback. These amps produced near perfect measured results but some listeners felt that these amplifiers sounded "cold" or "dull". In the following years, amplifiers were built with modest gain but good open loop linearity, deployed with only minimal levels of NFB.
All amplifiers do distort, so do SETs. This for the most part harmonic distortion is a distortion with a unique pattern of simple and monotonically decaying series of harmonics, dominated by modest levels of second harmonic. The result is like adding the same tone one octave higher. The added harmonic tone is lower, at about 1-5% or less in a no feedback amp at full power and rapidly decreasing at lower levels. Some argue that this "distortion" can actually enhance the music, just like the body of a musical instrument does, making it sound somewhat richer. It has been also claimed that in some cases especially a single-ended power amplifier's harmonic distortion could reduce similar harmonic distortion in a single driver loudspeaker, if their harmonic distortions were equal and amplifier was connected to the speaker so that the distortions would neutralize each other.
SETs usually only produce about 2 watt (W) for a 2A3 tube amp to 8 W for a 300B up to the practical maximum of 40 W for a 805 tube amp. The most expensive amp in existence, the Wavac SH-833 monoblock SETs (which cost about US$350,000) produces about 150 W using an 833A tube. The resulting SPL depends on the sensitivity of the loudspeaker and the size and acoustics of the room as well as amplifier power output. Their low power also makes them ideal for use as preamps. SET amps have a power consumption of a minimum of 8 times the stated stereo power. For example a 10 W stereo SET uses a minimum of 80 W, and typically 100 W.
Audiophiles often believe that tube sound has an intrinsic quality due to the vacuum tube technology itself; this is only partially true. In 1972, Matti Otala demonstrated the origin of a previously unobserved form of distortion: Transient Intermodulation Distortion (TIM), also called "slew rate distortion". TIM was found to occur during very rapid increases in amplifier output voltage. TIM did not appear at steady state sine tone measurements; helping to hide it from design engineers prior to 1972. Problems with TIM stem from reduced open loop frequency response of solid state amplifiers. Further works of Otala and other authors found the solution for TIM distortion, including increasing slew rate, decreasing preamp frequency bandwidth, and the insertion of a lag compensation circuit in the input stage of the amplifier. In high quality modern amplifiers the open loop response is at least 20 kHz, cancelling such distortion mechanisms. However, TIM is still present in most low price home quality amplifiers.
In 1980, a further improvement by Oscar Bonello at the University of Buenos Aires reduced amplifier distortion by employing "Double Loop Feedback" circuitry. This technology led to solid state amplifier designs which could achieve far better distortion measurements than tube amplifiers, at low cost and with high power. At the same time 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 allowed audio amplifiers to be designed without any perceived distortion in the treble spectrum.
In 1982, Tom Scholz, a graduate of MIT and a member of Boston, introduced the Rockman, which used bipolar transistors, but achieved a distorted sound adopted by many well known musicians. Advanced digital signal processing offers the possibility to simulate tube sound. Computer algorithms are currently available that transform digital sound from a CD or other digital source into a distorted digital sound signal.
Using modern passive components, and modern sources, whether digital or analogue, and wide band loudspeakers, it is possible to have tube amplifiers with the characteristic wide bandwidth and "fast" sound of modern transistor amplifiers, including using push-pull circuits, class AB, and feedback. Some enthusiasts have built amplifiers using transistors and MOSFETs that operate in class A, including single ended, and these often have the "tube sound" .
To demonstrate one aspect of this effect, one may use a light bulb in the feedback loop of an infinite gain multiple feedback (IGMF) circuit. The slow response of the light bulb's resistance (which varies according to temperature) can thus be used to moderate the sound and attain a tube-like "soft limiting" of the output, though other aspects of "the tube sound" would not be duplicated in this exercise.
Many of the explanations relate to the circuit topologies pioneered using tubes, and traditionally associated with them ever since, regardless of whether they are built using tubes today, notably the directly heated single-ended triode amplifier circuit, which operates in class A and often has no external negative feedback; this topology is a classic source of the tube sound.
Feedback paths coupled through the secondary of the output transformer reduce distortion because they compensate for the transformer's distortion to some extent. However only limited NFB can be used around the transformer, as there is phase lag caused by the transformer, and this causes instability if NFB is incorrectly (without any phase / frequency correction) used.