Tube sound

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. At first, the concept of tube sound did not exist, because practically all electronic amplification of audio signals was done with vacuum tubes and other comparable methods were not known or used. After introduction of solid state amplifiers, tube sound appeared as the logical complement of transistor sound, which had some negative connotations due to crossover distortion in early transistor amplifiers. The audible significance of tube amplification on audio signals is a subject of continuing debate among audio enthusiasts. 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. At first, the concept of tube sound did not exist, because practically all electronic amplification of audio signals was done with vacuum tubes and other comparable methods were not known or used. After introduction of solid state amplifiers, tube sound appeared as the logical complement of transistor sound, which had some negative connotations due to crossover distortion in early transistor amplifiers. The audible significance of tube amplification on audio signals is a subject of continuing debate among audio enthusiasts. Many electric guitar, electric bass, and keyboard players in several genres also prefer the sound of tube instrument amplifiers or preamplifiers. Tube amplifiers are also preferred by some listeners for stereo systems. 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. Some musiciansprefer 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. The sound of a tube amplifier is partly a function of the circuit topologies typically used with tubes versus the topologies typically used with transistors, as much as the gain devices themselves. Beyond circuit design, there are other differences, such as the differing electronic characteristics of triode, tetrode, and pentode vacuum tubes, along with their solid-state counterparts such as bipolar transistor, FET, MOSFET, IGBT, etc. These can be further divided into differences among various models of the said device type (e.g. EL34 vs. 6L6 tetrodes). In many cases circuit topologies need to account for these differences to either homogenize their widely varying characteristics or to establish a certain operating point required by the device. The low frequency roll-off can be explained by many tube amplifiers having high output impedance compared to transistor designs. The roll-off is due to higher device impedance and reduced feedback margins. (More feedback results in lower output impedance.) Some tube amplifier designs use minimal feedback while others use quite a bit more of it. How much feedback is optimal for tube amplifiers remains a matter of debate. 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. 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.