Tube sound (or valve sound) is the characteristic sound associated with a vacuum tube amplifier (valve amplifier in British English), a vacuum tube-based audio amplifier. The concept of tube sound did not exist prior to the invention of transistors, as all electronic amplification of audio signals used vacuum tubes. After the introduction of solid-state amplifiers, tube sound became the logical complement of transistor sound, which had some negative connotations due to crossover distortion in early transistor amplifiers. Modern solid state amplifiers can made to be practically flawless, with a neutral sound compared to tube amplifiers, thus the tube sound means euphonic distortion. The audible significance of tube amplification on audio signals is a subject of continuing debate among audio enthusiasts.

Even though tube amplifiers are generally more expensive than solid state devices, tube instrument amplifiers are still preferred by many musicians for electric guitar, electric bass, and keyboards. Additionally, some audiophiles prefer tube amplification for listening to prerecorded music.

Before the commercial introduction of transistors in the 1950s, electronic amplifiers used vacuum tubes (known in the United Kingdom as valves). By the 1960s, solid state (transistorized) amplification had become more common because of its smaller size, lighter weight, lower heat production, and improved reliability. Tube amplifiers have retained a loyal following amongst some audiophiles and musicians. Some tube designs command very high prices, and tube amplifiers have been going through a revival since Chinese and Russian markets have opened to global trade—tube production never went out of vogue in these countries.^{[further explanation needed]} Many transistor-based audio power amplifiers use MOSFET (metal–oxide–semiconductor field-effect transistor) devices in their power sections, because their distortion curve is more tube-like.

Some musicians prefer the distortion characteristics of tubes over transistors for electric guitar, bass, and other instrument amplifiers. In this case, generating deliberate (and in the case of electric guitars often considerable) audible distortion or overdrive is usually the goal. The term can also be used to describe the sound created by specially designed transistor amplifiers or digital modeling devices that try to closely emulate the characteristics of the tube sound.

The tube sound is often subjectively described as having a warmth and richness, but the source of this is by no means agreed on. Possible explanations mention non-linear clipping, or the higher levels of second-order harmonic distortion in single-ended designs, resulting from the tube interacting with the inductance of the output transformer.

Triodes (and MOSFETs) produce a monotonically decaying harmonic distortion spectrum. Even-order harmonics and odd-order harmonics are both natural number multiples of the input frequency.

A psychoacoustic analysis tells us that high-order harmonics are more offensive than low-order. For this reason, distortion measurements should weight audible high-order harmonics more than low. The importance of high-order harmonics suggests that distortion should be regarded in terms of the complete series or of the composite wave-form that this series represents. It has been shown that weighting the harmonics by the square of the order correlates well with subjective listening tests. Weighting the distortion wave-form proportionally to the square of the frequency gives a measure of the reciprocal of the radius of curvature of the wave-form, and is therefore related to the sharpness of any corners on it. Based on said discovery, highly sophisticated methods of weighting of distortion harmonics have been developed. Since they concentrate in the origins of the distortion, they are mostly useful for the engineers who develop and design audio amplifiers, but on the other hand they may be difficult to use for the reviewers who only measure the output.

A huge issue is that measurements of objective nature (for example, those indicating the magnitude of scientifically quantifiable variables such as current, voltage, power, THD, dB, and so on) fail to address subjective preferences. Especially in the case of designing or reviewing instrument amplifiers, this is a considerable issue because design goals of such differ widely from design goals of likes of HiFi amplifiers. HiFi design largely concentrates on improving performance of objectively measurable variables. Instrument amplifier design largely concentrates on subjective issues, such as pleasantness of a certain type of tone. Fine examples are cases of distortion or frequency response: HiFi design tries to minimize distortion and focuses on eliminating offensive harmonics. It also aims for an ideally flat response. Musical instrument amplifier design deliberately introduces distortion and great non-linearities in frequency response. Former offensiveness of certain types of harmonics becomes a highly subjective topic, along with preferences towards certain types of frequency responses (whether flat or un-flat).^{[citation needed]}