Even now tube amplifiers still sound great perhaps better than ever before. In part that is because we now have access to modern components such as toroidal output transformers, extremely high-quality resistors and capacitors, and many sorts of wire with good acoustic properties. Modern audio sources, such as CD players, and the latest top-end loudspeakers also enable us to appreciate how well tube amplifiers reproduce music even better than before.
Designing Tube Amplifiers - Elektor
This new book from Menno van der Veen looks at tube amplifiers from more than just a theoretical perspective. It focuses primarily on the design phase, where decisions must be taken with regard to the purpose and requirements of the amplifier, and it addresses the following questions: How do these aspects relate to subjective and objective criteria? Which circuits sound the best, and why? If you want to develop and market an amplifier, what problems should you expect? What are the significance and meaning of measurements? Are they still meaningful, or have they lost their relevance?
Thanks to the enormous processing power of computers, we can now measure more details than ever before. How can these new methods be applied to tube amplifiers? Previously it was sufficient to measure the frequency range, power and distortion of an amplifier in order to characterize the amplifier. Are these measurements still sufficient, or should we start measuring according to how we hear, using real music signals instead of waveforms from signal generators? The author sketches a future where amplifier measurements that conform to our sense of hearing enable us to arrive at new insights.
Thermionic tubes are still used in some applications, such as the magnetron used in microwave ovens, certain high-frequency amplifiers, amplifiers for electric musical instruments such as guitars, as well as high end audio amplifiers, which many audio enthusiasts prefer for their "warmer" tube sound.
The non-linear operating characteristic of the triode caused early tube audio amplifiers to exhibit harmonic distortion at low volumes. Plotting plate current as a function of applied grid voltage, it was seen that there was a range of grid voltages for which the transfer characteristics were approximately linear.
By 1940 multisection tubes had become commonplace. There were constraints, however, due to patents and other licensing considerations (see British Valve Association). Constraints due to the number of external pins (leads) often forced the functions to share some of those external connections such as their cathode connections (in addition to the heater connection). The RCA Type 55 is a double diode triode used as a detector, automatic gain control rectifier and audio preamplifier in early AC powered radios. These sets often include the 53 Dual Triode Audio Output. Another early type of multi-section tube, the 6SN7, is a "dual triode" which performs the functions of two triode tubes while taking up half as much space and costing less.The 12AX7 is a dual "high mu" (high voltage gain[40][41][42]) triode in a miniature enclosure, and became widely used in audio signal amplifiers, instruments, and guitar amplifiers.
A beam power tube forms the electron stream from the cathode into multiple partially collimated beams to produce a low potential space charge region between the anode and screen grid to return anode secondary emission electrons to the anode when the anode potential is less than that of the screen grid.[46][47] Formation of beams also reduces screen grid current. In some cylindrically symmetrical beam power tubes, the cathode is formed of narrow strips of emitting material that are aligned with the apertures of the control grid, reducing control grid current.[48] This design helps to overcome some of the practical barriers to designing high-power, high-efficiency power tubes.
Early tubes used a metal or glass envelope atop an insulating bakelite base. In 1938 a technique was developed to use an all-glass construction[53] with the pins fused in the glass base of the envelope. This was used in the design of a much smaller tube outline, known as the miniature tube, having seven or nine pins. Making tubes smaller reduced the voltage where they could safely operate, and also reduced the power dissipation of the filament. Miniature tubes became predominant in consumer applications such as radio receivers and hi-fi amplifiers. However, the larger older styles continued to be used especially as higher-power rectifiers, in higher-power audio output stages and as transmitting tubes.
Sub-miniature tubes with a size roughly that of half a cigarette were used in consumer applications as hearing-aid amplifiers. These tubes did not have pins plugging into a socket but were soldered in place. The "acorn tube" (named due to its shape) was also very small, as was the metal-cased RCA nuvistor from 1959, about the size of a thimble. The nuvistor was developed to compete with the early transistors and operated at higher frequencies than those early transistors could. The small size supported especially high-frequency operation; nuvistors were used in aircraft radio transceivers, UHF television tuners, and some HiFi FM radio tuners (Sansui 500A) until replaced by high-frequency capable transistors.
The tubes developed for Whirlwind were later used in the giant SAGE air-defense computer system. By the late 1950s, it was routine for special-quality small-signal tubes to last for hundreds of thousands of hours if operated conservatively. This increased reliability also made mid-cable amplifiers in submarine cables possible.
A considerable amount of heat is produced when tubes operate, from both the filament (heater) and the stream of electrons bombarding the plate. In power amplifiers, this source of heat is greater than cathode heating. A few types of tube permit operation with the anodes at a dull red heat; in other types, red heat indicates severe overload.
The requirements for heat removal can significantly change the appearance of high-power vacuum tubes. High power audio amplifiers and rectifiers required larger envelopes to dissipate heat. Transmitting tubes could be much larger still.
Large transmitting tubes have carbonized tungsten filaments containing a small trace (1% to 2%) of thorium. An extremely thin (molecular) layer of thorium atoms forms on the outside of the wire's carbonized layer and, when heated, serve as an efficient source of electrons. The thorium slowly evaporates from the wire surface, while new thorium atoms diffuse to the surface to replace them. Such thoriated tungsten cathodes usually deliver lifetimes in the tens of thousands of hours. The end-of-life scenario for a thoriated-tungsten filament is when the carbonized layer has mostly been converted back into another form of tungsten carbide and emission begins to drop off rapidly; a complete loss of thorium has never been found to be a factor in the end-of-life in a tube with this type of emitter.WAAY-TV in Huntsville, Alabama achieved 163,000 hours (18.6 years) of service from an Eimac external cavity klystron in the visual circuit of its transmitter; this is the highest documented service life for this type of tube.[81] It has been said[who?] that transmitters with vacuum tubes are better able to survive lightning strikes than transistor transmitters do. While it was commonly believed that vacuum tubes were more efficient than solid-state circuits at RF power levels above approximately 20 kilowatts, this is no longer the case, especially in medium wave (AM broadcast) service where solid-state transmitters at nearly all power levels have measurably higher efficiency. FM broadcast transmitters with solid-state power amplifiers up to approximately 15 kW also show better overall power efficiency than tube-based power amplifiers.
Some tubes, such as magnetrons, traveling-wave tubes, Carcinotrons, and klystrons, combine magnetic and electrostatic effects. These are efficient (usually narrow-band) RF generators and still find use in radar, microwave ovens and industrial heating. Traveling-wave tubes (TWTs) are very good amplifiers and are even used in some communications satellites. High-powered klystron amplifier tubes can provide hundreds of kilowatts in the UHF range.
Enough people prefer tube sound to make tube amplifiers commercially viable in three areas: musical instrument (e.g., guitar) amplifiers, devices used in recording studios, and audiophile equipment.[97]
Many guitarists prefer using valve amplifiers to solid-state models, often due to the way they tend to distort when overdriven.[98] Any amplifier can only accurately amplify a signal to a certain volume; past this limit, the amplifier will begin to distort the signal. Different circuits will distort the signal in different ways; some guitarists prefer the distortion characteristics of vacuum tubes. Most popular vintage models use vacuum tubes.[citation needed]
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