A tube amplifier is an electronic amplifier in which electron tubes are used to amplify low-frequency electrical signals. With the advent of semiconductor electronics, tube amplifiers in the field of application as audio amplifiers were largely replaced by amplifier circuits with semiconductor components (transistors). However, electron tubes are still used today in guitar amplifiers as well as some high-fidelity amplifiers, especially in the so-called high-end sector.
In the field of classical stage amplifiers for the instruments typical of rock music, such as electric guitar and electric bass, tube technology has asserted itself to this day for various reasons: the special behavior (soft-clipping) of the tube amplifier, its gradually increasing sliding into the signal distortion when an overload is used in a targeted manner, is to be regarded as an inseparable component of the instrument, and gives its individual sound character and does not serve to amplify the sounds produced by the instrument as precisely and unadulterated as possible.
Vacuum Tube Amplifier Vs Transistor Amplifier
Even after the invention of the transistor, there was no alternative to the electron tube for years as an active control element in all areas of electronics applications. The very low transit frequency, noise, and temperature problems of the early germanium transistor types limited the transistor’s uses. With the use of the semiconductor material silicon, a consistent further development of the silicon transistor and its numerous advantages, the semiconductor components bipolar transistor, field-effect transistor and MOSFET increasingly replaced the tube in almost all electronic applications.
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Compared to the electron tube, the transistor has decisive advantages to offer, especially in the following areas: small dimensions, low weight and low price level, mechanical insensitivity, simplest and undemanding power supply paired with high efficiency, extremely long service life, hardly any characteristic curve changes over the entire life cycle, extremely good electrical values of current transistor types due to permanent and continuous research.
One of the most serious disadvantages of using electron tubes is the need for a complex high-voltage power supply, since there are hardly any tubes that can deliver a significant anode current (and thus a significant output power) at a low supply voltage of the order of 50 V and also function with low distortion in this range. The considerable power requirement for heating the cathode and the comparatively large power dissipation also influence the circuit environment. Especially in power tubes, various chemical-physical processes, especially of the cathode, trigger an accelerated aging process, which is why the tube must be replaced after a certain period of operation (see also electron tube).
In the case of a tube amplifier, it is not the tubes, but the output transformer that determines the upper and lower cut-off frequency of the amplifier. Even with special winding techniques, it is hardly possible to exceed the range of 30 Hz to 15 kHz (−3 dB). Transistor amplifiers can easily operate far beyond the LF range (e.g., 5 Hz to 100 kHz) with much smaller tolerances (e.g., −0.5 dB) because they do not require a transformer.
The output resistance should be as low as possible in order to dampen unwanted resonances of the loudspeaker. Because of the much stronger negative feedback and the different circuit topology, it is in principle much smaller in transistor amplifiers than in amplifiers with output transformers. Because of its phase shift, at least at the limits of the bandwidth, the negative feedback in tube amplifiers must be significantly smaller. In addition, the circuit topology plays a major role: tube power amplifiers are usually implemented in cathode-based circuits, which naturally have a high output resistance. On the other hand, transistor power amplifiers are usually designed as collector circuits, since they already have a low output resistance without external negative feedback.
There is also an important difference if the connection between the amplifier and the loudspeaker is disturbed. If this connection is accidentally interrupted in a tube amplifier at high AF power, the resulting high induction voltage will destroy the output transformer and/or damage the power tubes.
Since the 1980s at the latest, semiconductor amplifiers have become common that have become insensitive to critical impedances (short circuit, inductive loads, capacitive loads). For this purpose, the voltage, the current, a power dissipation approximation and the junction temperature of the current drivers are monitored, which in all cases means that the power amplifier cannot be damaged. This is called SOA (Safe Operation Area) operation.
Effective protection of tube power amplifiers against inductive overvoltages, on the other hand, is very difficult and not feasible at all in pure tube technology, which is why hardly any use is made of it in practice (a snubber network is far from sufficient protection). Overcurrent protection of transistor power amplifiers, on the other hand, is very simple and cost-effective, which is why it is present in almost every transistor amplifier.
Compared to semiconductor devices, whose active areas are located in a confined space within a solid, electron tubes are more resistant to short-term electrical overload, radioactivity and electromagnetic pulse (EMP) due to their mechanical design. Neither has anything to do with a hi-fi amplifier.
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Does Vacuum Tube Amplifier Sounds Better?
With sophisticated tube amplifiers, the hi-fi era began in the 1950s, which aimed at the most accurate electroacoustic reproduction of sound events. A few years later, the unstoppable triumph of semiconductor technology in the electronics industry became increasingly apparent, which pushed tube electronics out of the market except for a few niche applications – but it took some time before the first transistor-based hi-fi amplifiers were able to build on the high acoustic quality level achieved by the tube amplifier.
It was not until the mid-nineties that high-quality audio tube amplifiers became socially acceptable again in high-end circles. Hopelessly inferior to their semiconductor-based competitors in many areas in terms of technical data, tube amplifiers often achieve better results when subjectively evaluating their sonic properties. However, some critics describe this as a euphemism of the listener or as a “pleasant distortion” of the sound in today’s high state of semiconductor technology, and the manufacturers of music recordings, not the manufacturers of the players, are responsible for the pleasant sound. The latter should be as neutral as possible.
On the other hand, often only sound engineers with their qualified and appropriately trained hearing skills are able to perceive the subtle differences between very good and excellent amplifiers in a differentiated and reproducible way – they often attest to the outstanding acoustic quality of high-quality tube amplifiers.
Due to their typical characteristics, tubes produce different distortion spectra (spectrum of harmonics) than semiconductors: triodes, pentodes and field-effect transistors have a quadratic characteristic curve in the first approximation, while bipolar transistors have an exponential characteristic curve in the first approximation.
