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24 mars 2011 4 24 /03 /mars /2011 07:20

A gas-filled tube, also known as a discharge tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Although the envelope is typically glass, power tubes often use ceramics, and military tubes often use glass-lined metal  Lenovo S10-2 Battery 5200mAh .

Gas tubes are tubes whose electrical characteristics are substantially influenced by the pressure and composition of gas contained inside.

Gas-filled tubes operate by ionizing the gas with applied voltage to start electrical conduction  Lenovo Y730 Battery 5200mAh .

Both hot cathode and cold cathode type devices are encountered. Depending on application, either theglow from the gas or the electric arc or electric glow discharge may be the desired function.

The breakdown voltage depends on the gas composition and electrode distance; the dependencies are described by the Paschen's law  Lenovo Y710 Battery 5200mAh .

Gas-filled tubes exploit phenomenons related to electric discharge in gases.

Gases in use

  • Hydrogen is used in tubes used for very fast switching, e.g. some thyratrons, dekatrons, and krytrons, where very steep edges are required  Lenovo Y530 Battery 5200mAh .
  • The build-up and recovery times of hydrogen are much shorter than in other gases. Hydrogen thyratrons are usually hot-cathode. Hydrogen (and deuterium) can be stored in the tube in the form of a metal hydride, heated with an auxiliary filamentLenovo Y510 Battery 5200mAh ;
  • releasing hydrogen by heating such storage element can be used to replenish cleaned-up gas, and even to adjust the pressure as needed for a thyratron operation at a given voltage.
  • Deuterium is used in ultraviolet lamps for ultraviolet spectroscopy, in neutron generator tubes, and in special tubes (e.g. crossatron) Lenovo S10-3 Battery 5200mAh .
  • It has higher breakdown voltage than hydrogen. In fast switching tubes it is used instead of hydrogen where high voltage operation is required. For a comparison, the hydrogen-filled CX1140 thyratron has anode voltage rating of 25 kV, while the deuterium-filled and otherwise identical CX1159 has 33 kV  Lenovo G450 Battery 5200mAh .
  • Also, at the same voltage the pressure of deuterium can be higher than of hydrogen, allowing higher rise rates of rise of current before it causes excessive anode dissipation. Significantly higher peak powers are achievable. Its recovery time is however about 40% slower than for hydrogen  Lenovo E30 Battery 5200mAh .
  • Metal and nonmetal vapors
    • Mercury vapors are used for applications with high current, e.g. lights, mercury arc valves, ignitrons. Mercury is used because of its high vapor pressure and low ionization potential  Lenovo E40 Battery 5200mAh .
    • Mercury mixed with an inert gas is used where the energy losses in the tube have to be low and the tube lifetime should be long. In mercury-inert gas mixtures, the discharge is initially carried primarily by the inert gas; the released heat then serves to evaporate enough mercury to reach the desired vapor pressure  Lenovo E50 Battery 5200mAh .
    • Low-voltage (hundreds volts) rectifiers use saturated mercury vapor in combination with a small amount of inert gas, allowing cold start of the tubes. High-voltage (kilovolts and more) rectifiers use pure mercury vapor at low pressure, requiring maintenance of maximum temperature of the tube  Lenovo G460 Battery 5200mAh .
    • The liquid mercury serves as a reservoir of mercury, replenishing the vapors that are used up during the discharge. Unsaturated mercury vapor can be used, but as it can not be replenished, the lifetime of such tubes is lower  Lenovo Y460 Battery 5200mAh .
    • The strong dependence of vapor pressure on mercury temperature limits the environments the mercury-based tubes can operate in. In low-pressure mercury lamps, there is an optimum mercury pressure for the highest efficiency  Lenovo Y450 Battery 5200mAh .
    • Photons emitted by ionized mercury atoms can be absorbed by nearby nonionized atoms and either reradiated or the atom is deexcited nonradiatively, too high mercury pressure therefore causes losses of light. Too low mercury pressure leads to too few atoms present to get ionized and radiate photons  Lenovo N200 Battery 5200mAh .
    • The optimum temperature for low-pressure mercury lamps is at about 42 °C, when the saturated vapor pressure of mercury (present as a drop of about 1 mg of liquid mercury in the tube, as a reservoir compensating for losses by clean-up) reaches this optimum  Lenovo T410 Battery 5200mAh .
    • In lamps intended for operation at higher ambient temperatures, and at a wider temperature range, mercury is present in the form of an amalgam with e.g. bismuth and indium; the vapor pressure above amalgam is lower than above liquid mercury  Lenovo T510 Battery 5200mAh .
    • Mercury is used in fluorescent tubes as a source of visible and ultraviolet light for exciting the phosphor; in that application it is usually used together with argon, or in some cases with krypton or neon. Mercury ions deionize slowly, limiting the switching speed of mercury-filled thyratrons  Lenovo SL410 Battery 5200mAh .
    • Ion bombardment with mercury ions of even relatively low energies also gradually destroys oxide-coated cathodes.
    • Sodium vapors are used in light sources.
    • Sulfur vapors are used in sulfur lamps  Lenovo SL510 Battery 5200mAh .
    • Vapors of many metals, alone or together with a noble gas, are used in many lasers.
  • Noble gases are frequently used in tubes for many purposes, from lighting to switching. Pure noble gases are employed in switching tubes  Lenovo C100 Battery 5200mAh .
  • Noble gas filled thyratrons have better electrical parameters than mercury-based ones. The electrodes undergo damage by high-velocity ions. The neutral atoms of the gas slow the ions down by collisions, and reduce the energy transferred to the electrodes by the ion impact  Lenovo T400S Battery 5200mAh .
  • Gases with high molecular weight, e.g. xenon, protect the electrodes better than lighter ones, e.g. neon.
    • Helium is used in helium-neon lasers and in some thyratrons rated for high currents and high voltages  Lenovo Y430 Battery 5200mAh .
    • Helium provides about as short deionization time as hydrogen, but can withstand lower voltage, so it is used much less often.
    • Neon has low ignition voltage and is frequently used in low-voltage tubes. Discharge in neon emits relatively bright red light Ibm ThinkPad G40 Battery 5200mAh ;
    • neon-filled switching tubes therefore also act as indicators, shining red when switched on. This is exploited in the decatron tubes, which act as both counters and displays. Its red light is exploited in neon signage  Ibm ThinkPad X200T Battery 5200mAh .
    • Used in fluorescent tubes with high power and short length, e.g. industrial lighting tubes. Has higher voltage drop in comparison with argon and krypton. Its low atomic mass provides only a little protection to the electrodes against accelerated ions  Apple A1280 battery 5200mAh ;
    • additional screening wires or plates can be used for prolonging the anode lifetime. In fluorescent tubes it is used in combination with mercury  Apple A1281 battery 5200mAh .
    • Argon was the first gas used in fluorescent tubes and is still frequently used due to its low cost, high efficiency, and very low striking voltage. In fluorescent tubes it is used in combination with mercury. It was also used in early rectifier tubes; first thyratrons were derived from such argon-filled tubes  Samsung R522 battery 5200mAh .
    • Krypton can be used in fluorescent lamps instead of argon; in that application it reduces the total energy losses on electrodes from about 15% to 7%. The voltage drop per lamp length is however lower than with argon, which can be compensated by smaller tube diameter  Samsung P467 battery 5200mAh .
    • Krypton-filled lamps also require higher starting voltage; this can be alleviated by using e.g. 25%-75% argon-krypton mixture. In fluorescent tubes it is used in combination with mercury  Hp Pavilion dm3 Battery 5400mAh .
    • Xenon in pure state has high breakdown voltage, making it useful in higher-voltage switching tubes. Xenon is also used as a component of gas mixtures when production of ultraviolet radiation is required, e.g. in plasma displays, usually to excite a phosphor   Hp Pavilion dm4 Battery 5200mAh .
    • The wavelength produced is longer than with argon and krypton and penetrates the phosphors better. To lower the ionization voltage, neon-xenon or helium-xenon are used; above 350 torr, helium has lower breakdown voltage than neon and vice versa  Hp Pavilion cq62 Battery 5200mAh .
    • At concentrations of 1% and less of xenon, the Penning effectbecomes significant in such mixtures, as most of xenon ionization occurs by collision with excited atoms of the other noble gas; at more than few percents of xenon, the discharge ionizes xenon directly due to most energy of the electrons being spent on direct ionization of xenon Hp Pavilion cq42 Battery 5200mAh .
    • Penning mixtures are used where lower ionization voltage is required, e.g., in the neon lamps, Geiger-Muller tubes and other gas-filled particle detectors. A classical combination is about 98–99.5% of neon with 0.5–2% of argon, used in, e.g., neon bulbs and in monochrome plasma displays  Hp Pavilion cq72 Battery 5200mAh .
  • Sulfur hexafluoride is used in special high-voltage applications.
  • Air can be used in some low-demanding applications.
  • Nitrogen at relatively high pressure tends to be used in surge arresters, due to its short build-up time, giving the tubes fast response time to voltage surges  Hp cq20 Battery 2600mAh .
  • Halogens and alcohol vapors absorb ultraviolet radiation and have high electron affinity. When added to inert gases, they quench the discharge; this is exploited in e.g. Geiger-Muller tubes.

The gas pressure influences the following factors  Hp 2230 Battery 2600mAh :

  • breakdown voltage (also called ignition voltage)
  • current density
  • operating voltage
  • backfire voltage  Dell Inspiron Mini 12 battery 7800mAh
  • tube lifetime (lower pressure tubes tend to have shorter lifetimes due to using up of the gas)
  • cathode sputtering, reduced at higher pressures

The gas pressure may range between 0.001 and 1000 torr; most commonly, pressures between 1-10 torr are used Dell XPS M1730 battery 7800mAh .

Above a certain value, the higher the gas pressure, the higher the ignition voltage. High-pressure lighting tubes can require a few kilovolts impulse for ignition when cold, when the gas pressure is low  Dell XPS M2010 battery 7800mAh .

After warming up, when the volatile compound used for light emission is vaporized and the pressure increases, reignition of the discharge requires either significantly higher voltage or reducing the internal pressure by cooling down the lamp Dell Studio XPS 1340 battery 5200mAh/7800mAh .

The gas tends to be used up during the tube operation, by several phenomena collectively called clean-up. The gas atoms or molecules are adsorbed on the surfaces of the electrodes. In high voltage tubes, the accelerated ions can penetrate into the electrode materials  Dell Inspiron 9100 battery 7800mAh .

New surfaces, formed by sputtering of the electrodes and deposited on e.g. the inner surfaces of the tube, also readily adsorb gases. Non-inert gases can also chemically react with the tube components. Hydrogen may diffuse through some metals.

For removal of gas in vacuum tubes, getters are used  Dell Inspiron N4010 battery 5200mAh .

For resupplying gas for gas-filled tubes, replenishers are employed. Most commonly, replenishers are used with hydrogen; a filament made from a hydrogen-absorbing metal (e.g. zirconium or titanium) is present in the tube, and by controlling its temperature the ratio of absorbed and desorbed hydrogen is adjusted Dell Inspiron N3610 battery 5200mAh ,

resulting in controlling of the hydrogen pressure in the tube. The metal filament acts as a hydrogen storage. This approach is used in e.g. hydrogen thyratrons or neutron tubes. Usage of saturated mercury vapor allows using a pool of liquid mercury as a large storage of material Dell Inspiron N5010 battery 5200mAh ;

the atoms lost by clean-up are automatically replenished by evaporation of more mercury. The pressure in the tube is however strongly dependent on the mercury temperature, which has to be controlled carefully.

The gas used dramatically influences the parameters of the tube Dell Inspiron N7010 battery 5200mAh .

Pure inert gases are used where the difference between the ignition voltage and the burning voltage has to be high, e.g. in switching tubes. Tubes for indication and stabilization, where the difference has to be lower, tend to be filled with Penning mixtures Dell Inspiron 1464 battery 5200mAh ;

the lower difference between ignition and burning voltages allows using lower power supply voltages and smaller series resistances.

Large rectifiers use saturated mercury vapor with a small amount of an inert gas. The inert gas supports the discharge when the tube is cold  Dell Inspiron 1564 battery 5200mAh .

The mercury arc valve current-voltage characteristics are highly dependent on the temperature of the liquid mercury. The voltage drop in forward bias decreases from about 60 volts at 0 °C to somewhat above 10 volts at 50 °C and then stays constant Dell Inspiron 1764 battery 5200mAh ;

the reverse bias breakdown ("arc-back") voltage drops dramatically with temperature, from 36 kV at 60 °C to 12 kV at 80 °C to even less at higher temperatures. The operating range is therefore usually between 18–65 °C Dell Inspiron 1320 battery 5200mAh .

The gas in the tube has to be kept pure to maintain the desired properties; even small amount of impurities can dramatically change the tube values; presence of non-inert gases generally increases the breakdown and burning voltages. The presence of impurities can be observed by changes in the glow color of the gas Dell Inspiron 1320n battery 5200mAh .

Air leaking into the tube makes the discharge look pale, milky, or reddish. Traces of mercury vapors glow bluish, obscuring the original gas color. Magnesium vapor colors the discharge green. To prevent outgassing of the tube components during operation, a bake-out is required before filling with gas and sealing Dell Vostro 1710 battery 5200mAh .

Thorough degassing is required for high-quality tubes; even as little as 10?8 torr of oxygen is sufficient for covering the electrodes with monomolecular oxide layer in few hours. Non-inert gases can be removed by suitable getters. for mercury-containing tubes Dell Vostro 1720 battery 5200mAh ,

getters that do not form amalgams with mercury (e.g. zirconium, but not barium) have to be used. Cathode sputtering may be used intentionally for gettering non-inert gases; some reference tubes use molybdenum cathodes for this purpose Dell Studio XPS 1640 battery 5200mAh .

Insulating gases

In special cases, e.g., high voltage switches, gases with good dielectric properties and very high breakdown voltages are needed. Highly electronegative elements, e.g., halogens, are favored as they rapidly recombine with the ions present in the discharge channel  Dell Vostro A840 battery 5200mAh .

One of the most popular choices is sulfur hexafluoride. Other common options are dry pressurized nitrogen and halocarbons.

Lighting and display gas-filled tubes

Fluorescent lighting, CFL lamps, mercury and sodium discharge lamps and HID lamps are all gas-filled tubes used for lighting Dell Vostro A860 battery 5200mAh .

Neon lamps and neon signage (most of which is not neon based these days) are also low-pressure gas-filled tubes.

Specialized historic low-pressure gas-filled tube devices include the Nixie tube (used to display numerals) and the Decatron (used to count or divide pulses, with display as a secondary function) Dell Studio 1535 battery 5200mAh/7800mAh .

Xenon flash lamps are gas-filled tubes used in cameras and strobe lights to produce bright flashes of light.

The recently developed sulfur lamps are also gas-filled tubes when hot  Dell Studio 1735 battery 5200mAh/7800mAh.

A nixie tube is an electronic device for displaying numerals or other information. The glass tube contains a wire-mesh anode and multiple cathodes. In most tubes, the cathodes are shaped like numerals. Applying power to one cathode surrounds it with an orange glow discharge  Dell Vostro 1310 battery 5200mAh .

The tube is filled with a gas at low pressure, usually mostly neon and often a little mercury and/or argon, in a Penning mixture.

Although it resembles a vacuum tube in appearance, its operation does not depend on thermionic emission of electrons from a heated cathode Dell Latitude E6400 battery,10400mAh/7800mAh .

It is therefore called acold-cathode tube (a form of gas filled tube), or a variant of neon lamp. Such tubes rarely exceed 40 °C (104 °F) even under the most severe of operating conditions in a room at ambient temperature  Dell Latitude E6400 Battery,10400mAh/7800mAh .

The most common form of nixie tube has ten cathodes in the shapes of the numerals 0 to 9 (and occasionally a decimal point or two), but there are also types that show various letters, signs and symbols. Dell Latitude E6400 Battery,10400mAh/7800mAh

Because the numbers and other characters are arranged one behind another, each character appears at a different depth, giving Nixie based displays a distinct appearance. A related device is the pixie tube, which uses a stencil mask with numeral-shaped holes instead of shaped cathodes Dell Latitude E6400 Battery,10400mAh/7800mAh .

Some Russian nixies, e.g. the IN-14, used an upside-down digit 2 as the digit 5, presumably to save manufacturing costs as there is no obvious technical or aesthetic reason.

Each cathode can be made to glow in the characteristic neon red-orange color by applying about 170 volts DC at a few milliamperes between a cathode and the anode Dell Latitude E6500 battery,10400mAh/7800mAh .

The current limiting is normally implemented as an anode resistor of a few tens of thousands of ohms. Nixies exhibit negative resistance and will maintain their glow at typically 20 V to 30 V below the strike voltage. Some color variation can be observed between types, caused by differences in the materials and gas mixtures used Dell Latitude E6500 battery,10400mAh/7800mAh .

Longer-life tubes that were manufactured later in the nixie timeline have mercury added to reduce sputtering resulting in a blue or purple tinge to the emitted light. In some cases, these colors are filtered out by a red or orange filter coating on the glass Dell Latitude E6500 battery,10400mAh/7800mAh .

Applications and lifetime

Nixies were used as numeric displays in early digital voltmeters, multimeters, frequency counters and many other types of technical equipment. They also appeared in costly digital time displays used in research and military establishments, and in many early electronic desktop calculators, including the first  Dell Latitude E6500 battery,10400mAh/7800mAh :

the Sumlock-Comptometer ANITA Mk VII of 1961 and even the first electronic telephone switchboards. Later alphanumeric versions in fourteen segment displayformat found use in airport arrival/departure signs and stock ticker displays. Some elevators used nixies to display floor numbers  HP Pavilion DV6-1210SA battery - 5200mAh/8800mAh .

Average longevity of nixie tubes varied from about 5,000 hours for the earliest types, to as high as 200,000 hours or more for some of the last types to be introduced. There is no formal definition as to what constitutes "end of life" for nixies, mechanical failure excepted  HP Pavilion DV7 battery - 7200mAh .

Some sources (Weston 1968, p. 340) suggest that 50% reduction in emitted light would not be acceptable; however cathode poisoning resulting in incomplete digit display, whilst generally not preventing the tube from being used, may also be considered unacceptable Ibm THINKPAD X200 Battery 8800mAh, 11.1V Replacement .

Nixie tubes are susceptible to multiple failure modes, including

Driving nixies outside of their specified electrical parameters will accelerate their demise, especially excess current, which increases sputtering of the electrodes. A few extreme examples of sputtering have even resulted in complete disintegration of nixie tube cathodes  SONY VAIO VGN-FW21E Battery .

As testament to their longevity, and that of the equipment which used them, in 2006 several suppliers still provide common nixie tube types as service replacement parts, new in original packaging. Equipment with nixie tube displays in excellent working condition is still plentiful, though much of it has been in frequent use for 30–40 years or more Asus M52N Battery .

Such items can easily be found as surplus and obtained at very little expense. In the former Soviet Union, nixies were still being manufactured in volume in the 1980s, so Russian and Eastern European nixies are still available.

One advantage of the Nixie tube is that its cathodes are typographically designed, shaped for legibility Dell Latitude E6400 ATG Battery .

In most types, they are not placed in numerical sequence from back to front, but arranged so that cathodes in front of the one that is lit obscure it minimally. The digit sequence is rarely given; one arrangement is 6 7 5 8 4 3 9 2 0 1 from front (6) to back (1) SONY VAIO VGN-FW21J Battery .


The early Nixie displays were made by a small vacuum tube manufacturer called Haydu Brothers Laboratories, and introduced in 1955 by Burroughs Corporation, who purchased Haydu and owned the name Nixie as a trademark SONY VAIO VGN-FW21L Battery .

The name Nixie was derived by Burroughs from "NIX I", an abbreviation of "Numeric Indicator eXperimental No. 1." Similar devices that functioned in the same way were patented in the 1930s, and the first mass-produced display tubes were introduced in 1954 by National Union Co. under the brand name Inditron SONY VAIO VGN-FW21M Battery .

However, their construction was cruder, their average lifetime was shorter, and they failed to find many applications due to their complex periphery.

Burroughs even had another Haydu tube that could operate as a digital counter and directly drive a Nixie tube for display SONY VAIO VGN-FW21Z Battery .

This was called a "Trochotron", in later form known as the "Beam-X Switch" counter tube; another name was "magnetron beam-switching tube", referring to their similarity to a cavity magnetron. Trochotrons were used in the UNIVAC 1101 computer, as well as in clocks and frequency counters SONY VAIO VGN-FW25T/B Battery .

The first trochotrons were surrounded by a hollow cylindrical magnet, with poles at the ends. The field inside the magnet had essentially-parallel lines of force, parallel to the axis of the tube. It was a thermionic vacuum tube; inside were a central cathode, ten anodes, and ten "spade" electrodes  SONY VAIO VGN-FW26T/B Battery .

The magnetic field and voltages applied to the electrodes made the electrons form a thick sheet (as in a cavity magnetron) that went to only one anode. Applying a pulse with specified width and voltages to the spades made the sheet advance to the next anode, where it stayed until the next advance pulse  SONY VAIO VGN-FW27/B Battery .

Count direction was not reversible. A later form of trochotron called a Beam-X Switch replaced the large, heavy external cylindrical magnet with ten small internal metal-alloy rod magnets which also served as electrodes  SONY VAIO VGN-FW27/W Battery .

Glow-transfer counting tubes, similar in essential function to the Trochotrons, had a glow discharge on one of a number of main cathodes, visible through the top of the glass envelope. Most used a neon-based gas mixture and counted in base-10, but faster types were based on argon SONY VAIO VGN-FW31ZJ Battery ,

hydrogen, or other gases, and for timekeeping and similar applications a few base-12 types were available. Sets of "guide" cathodes (usually two sets, but some types had one or three) between the indicating cathodes moved the glow in steps to the next main cathode SONY VAIO VGN-FW27T/H Battery .

Types with two or three sets of guide cathodes could count in either direction. A well-known trade name for glow-transfer counter tubes in the United Kingdom was Dekatron. Types with connections to each individual indicating cathode, which enabled presetting the tube's state to any value SONY VAIO VGN-FW29/B Battery

(in contrast to simpler types which could only be directly reset to zero or a small subset of their total number of states), were trade named Selectron tubes.

Some Nixie-like displays made by other firms were called by various trademarked names including Digitron, Inditron and NumicatorSONY VAIO VGN-FW290JRB Battery .

A proper generic term is "cold cathode neon readout tube", though the phrase "nixie tube" quickly entered the vernacular as a generic name. Hundreds of variations of this design were manufactured by many firms, from the 1950s until the 1990s SONY VAIO VGN-FW290JTB Battery .

Alternatives and successors

Other numeric display technologies concurrently in use included backlit columnar transparencies, a.k.a. "thermometer displays", light pipe, rear-projection and edge-litlightguide displays (all using individual incandescent or neon light bulbs for illumination)SONY VAIO VGN-FW290JTH Battery ;

Numitron incandescent filament readouts; and vacuum fluorescent displaytubes. Before nixie tubes became prominent, most numeric displays were electromechanical, using stepping mechanisms to display digits either directly by use of cylinders bearing printed numerals attached to their rotors SONY VAIO VGN-FW33G/E1 Battery ,

or indirectly by wiring the outputs of stepping switches to indicator bulbs. Later, a few vintage clocks even used a form of stepping switch to drive nixie tubes.

Nixie tubes were superseded in the 1970s by light-emitting diodes (LEDs) and vacuum fluorescent displays (VFDs), often in the form of seven-segment displays SONY VAIO VGN-FW33G/W Battery .

The VFD used a hot filament to emit electrons and phosphor-coated anodes, like a cathode ray tube, shaped to represent segments of a digit, pixels of a graphical display, or complete letters, symbols, or words. Whereas nixies typically require 180 volts to illuminate, VFDs only require relatively low voltages to operate, making them easier and cheaper to use SONY VAIO VGN-FW33GW Battery .

VFDs have a simple internal structure, resulting in a bright, sharp, and unobstructed image. Unlike nixies, the glass envelope of a VFD is evacuated rather than being filled with a specific mixture of gases at low pressure (Dell XPS M1210 Batteryhttp://www.hdd-shop.co.uk .

Specialized high voltage driver chips such as the 7441/74141 were available to drive nixies. LEDs were better suited to the low voltages that integrated circuits used, which was an advantage for devices such as pocket calculators, digital watches, and handheld digital measurement instruments  Dell KM958 battery .

Also, LEDs were much smaller and sturdier, without a fragile glass envelope. LEDs had lower power consumption than both VFDs and Nixie tubes.


Citing dissatisfaction with the aesthetics of modern digital displays and a nostalgic fondness for the styling of obsolete technologyDell Studio 1555 battery ,

significant numbers of electronics enthusiasts in recent years have shown interest in reviving nixies. Unsold tubes that have been sitting in warehouses for decades are being brought out and used, the most common application being in homemade digital clocksSony VGP-BPS13 battery .

This is somewhat ironic, since during their heyday, nixies were generally considered too expensive for use in mass-market consumer goods such as clocks. This recent surge in demand has caused prices to rise significantly, particularly for large tubes Sony VGP-BPS13/B battery .

The largest type known to be in the hands of collectors, the Rodan CD-47/GR-414 (220 mm [8.7 in] tall), have been sold for hundreds of dollars apiece, but these are extremely rare and only found in a few areas of the world by persistent and fortunate seekers Sony VGP-BPS13/S battery .

Prices for other large types displaying digits over 25 mm (1 in) tall have risen by double, triple or more between 1998 and 2005.

In addition to the nixie tube, another important consideration is the circuitry to control the tube  Sony VGP-BPS13A/B battery .

One of the more popular ways to do this is to use the Texas Instruments' SN74141 BCD Decoder Driver IC (or its Russian equivalents, the K155ID1 and KM155ID1, with plastic and ceramic packages, respectively). These have long since been out of production, much like the nixie tubes they were designed for Sony VGP-BPS13B/B battery .

A tube caddy was a very large carrying case sometimes with hundreds of compartments for vacuum tubes. They were carried by repair technicians who did home service calls in the days when radiosand television sets were too large and heavy for the average homeowner to bring to the repair shop Sony VGP-BPL9 battery .

The tube caddy had to be large enough to hold several of each type of tube likely to be needed on a typical day of service calls so that the technician did not need to return to the shop during his rounds. Because tubes are quite light the caddy did not have to accommodate a large payload and could also be constructed with very light materials so the overall package weighed far less than it appeared to Sony VGP-BPS13B/B battery .

Most repairs could be effected with the replacement of tubes since there were few other active components in electronic equipment at that time so this was a very efficient way to accomplish fixes.

Tube sound (or valve sound) is the characteristic sound associated with a vacuum tube-based audio amplifier  Sony VGP-BPL15 battery .

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 a range of popular, rock, funk, blues, reggae and jazz genres also prefer the sound of tube instrument amplifiers or preamplifiers Dell Inspiron E1505 battery .


Before the commercial introduction of transistors in the 1950s, electronic amplifiers used vacuum tubes (known in Great Britain 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 HP Pavilion dv6000 Battery .

Tube amplifiers have retained a loyal following amongst some audiophiles andmusicians. 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 HP Pavilion dv8000 battery .

Sound reproduction

Audiophiles disagree on the relative merits of tube and solid state amplification. Some prefer the sound produced with tube amplifiers on the grounds that it is more natural and satisfying than the sound from typical transistor amplifiers HP PAVILION DV6000 Battery,  8800mAh, 10.8V .

Typically, in sound reproduction systems, accurate reproduction of the sound of the original recording is the goal; gross distortion is something designers do not deliberately seek to introduce HP PAVILION DV8000 Battery,  8800mAh, 10.8V .

Musical instrument amplification

Some musicians also prefer the distortion characteristics of tubes over transistors for electric guitar, bass, and other instrument amplifiers. In this case, generating deliberate (and sometimes considerable, in the case of electric guitars) audible distortion or overdrive is usually the goal SONY VAIO VGN-FZ Battery .

The term can also be used to describe the sound created by specially-designed transistor amplifiers ordigital 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 HP Pavilion DV6000 battery - 8800mAh .

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 HP Pavilion DV8000 battery - 7800mAh .

Audible differences

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  HP Pavilion DV6000 battery - 8800mAh .

Beyond circuit design, there are other differences such as the electronic characteristics of a triode and MOSFET, or a tetrode and a bipolar transistor.

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 HP Pavilion DV8000 battery - 6600mAh .

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", while each higher order harmonic is smaller than the nearest lower order harmonic SONY VAIO VGN-FZ18 Battery .

Some audio professionals[who?] regard the effects of the use of tubes as distortion - which can be used creatively in certain scenarios.

In tube amplifiers high frequencies have been somewhat rolled off SONY VAIO VGN-FZ21E Battery .

Historically this was often the case due to limitations in capacitor performance. Modern audiophile-grade tube amplifiers however, using modern and high quality capacitors can have frequency response that are essentially flat to octaves beyond the audio range: ?3 dB above 85 kHz is quite common (though of arguable utility) SONY VAIO VGN-FW31J Battery - 5200mAh/7800mAh/10400mAh, 11.10V .

The low frequency roll-off can be explained by many tube amplifiers having high output impedance 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 output impedance) SONY VAIO VGN-FW31J Battery - 5200mAh/7800mAh/10400mAh, 11.10V .

A hypothetical amplifier design in two otherwise equal variants with just different amounts of feedback, might result in the higher feedback version having a "drier" mid-range (due to reduced second-order harmonics due to greater reduction of distortion) but a "tighter" bass due to lower output impedance might result SONY VAIO VGN-FW21E Battery .

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 SONY VAIO VGN-NR11S/S Battery .

Instruments such as pianos and vocals sound softer and fatter when compared against transistor amplifiers. The reasons for these effects are not simply related to the gain device type; today an amplifier designer using either technology may make synergistic design compromise choices. Sonic differences are less stereotyped than they used to be as a result SONY VAIO VGN-NR11M/S Battery .

Harmonic content and distortion

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 SONY VAIO VGN-NR11Z/S Battery .

Psychoacoustic phenomena include the effect that high-order harmonics are more offensive than low. Thus, in distortion measurements this should be taken into consideration to weight audible high-order harmonics more than low SONY VAIO VGN-NR11Z/T Battery .

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 Sony VAIO VGN-FZ21E Battery .

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  Sony VAIO VGN-FW21E Battery .

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 Sony VAIO VGN-NR11S/S Battery .

Push-pull amplifiers use two nominally identical gain devices "back to back". One consequence of this is that all even-order harmonic products cancel, leaving odd order products to dominate. A push-pull amplifier is said to have a symmetric (odd symmetry) transfer characteristic, and accordingly produces only odd harmonics Sony VAIO VGN-NR11Z/S Battery .

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  Sony VAIO VGN-NR11M/S Battery .

Push-pull tube amplifiers can be run in class A, AB, or B. Also, a class ABamplifier may have crossover distortion that will be typically inharmonic and thus sonically very undesirable indeed Sony VAIO VGN-NR11Z/T Battery .

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 SONY VAIO VGN-FZ180E Battery .

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 SONY VAIO VGN-FZ220E Battery .

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  SONY VAIO VGN-FZ340E Battery

(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 SONY VAIO VGN-FZ430E Battery ,

principally because tube amplifiers normally use output transformers and they tend to use low feedback.

The design of speaker crossover networks and other electro-mechanical properties may result in a speaker with a very uneven impedance curve SONY VAIO VGN-FZ460E Battery ,

for a nominal 8 ? speaker, being as low as 6 ? at some places and as high as 30–50 ? 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 SONY VAIO VGN-FZ480E Battery .

Design comparison

There has been considerable debate over the characteristics of tubes versus bipolar junction transistors. Triodes and MOSFETs have certain similarities in their transfer characteristics, whereas later forms of the tube, the tetrode and pentode, have quite different characteristics that are in some ways similar to the bipolar transistor  SONY VAIO VGN-FZ4000 Battery .

Despite this, e.g. MOSFET amplifier circuits typically do not reproduce tube sound any more than typical bipolar designs, due to the circuit topology differences between a typical tube design and a typical MOSFET design. But there are always exceptions: for example some very interesting designs such as the Zen series by Nelson Pass which can be found on the web SONY VAIO VGN-FZ31E Battery .

Input impedance

A characteristic feature of most tube amplifier designs is the high input impedance (typically 100 k? or more) in modern designs and as much as 1 M? in classic designs. The input impedance of the amplifier is a load for the source device. Even for some modern music reproduction devices the recommended load impedance is over 50 k? SONY VAIO VGN-FZ31B Battery .

This implies that the input of an average tube amplifier is a problem-free load for music signal sources. By contrast, some transistor amplifiers for home use have lower input impedances, as low as 15 k? SONY VAIO VGN-FZ31J Battery .

Soft clipping

Soft clipping is a very important aspect of tube sound especially for guitar amplifiers, although a Hi-fi amplifier should not normally ever be driven into clipping  SONY VAIO VGN-FZ31M Battery .

A tube amplifier will reproduce a waverelatively linearly to a point, and as the signal moves beyond the linear range of the tube (into overload), it distorts the signal with a smooth curve instead of a sudden, sharp-edged cutoff (or even ringing and/or lockup) as occurs with transistors SONY VAIO VGN-FZ31Z Battery .

The harmonics added to the signal are of lower energy with soft clipping than hard clipping. However, soft clipping is not exclusive to tubes, it can be simulated in transistor circuits (below the point that real hard clipping would occur). (See "Intentional distortion" section) SONY VAIO VGN-FZ38M Battery .

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  SONY VGP-BPS8 Battery ,

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 SONY VGP-BPS13/S Battery .

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 SONY VGP-BPS13A/B Battery .

Complex modern transistor designs often have low level but extremely complex harmonic distortion spectra.

In recording industry and especially with microphone amplifiers it has been shown that amplifiers are often overloaded by signal transients SONY VGP-BPS13B/B Battery .

There is a major difference in the harmonic distortion components of the amplified signal, with tubes, transistors, and operational amplifiers separating into distinct groups  SONY VGP-BPS13A/S Battery .


Early tube amplifiers often had limited response bandwidth, in part due to the characteristics of the inexpensive passive components then available. In power amplifiers most limitations come from the output transformer; low frequencies are limited by transformer core saturation and high frequencies by winding inductance and capacitance SONY VGP-BPS13AS Battery .

Another limitation is in the combination of high output impedance, decoupling capacitor and grid resistor, which acts as a high-pass filter. If interconnections are made from long cable (for example guitar to amp), tube input impedance with cable capacitance acts as a low-pass filter   Dell Studio 1450 Battery .

However, modern premium components make it easy to produce amplifiers that are essentially flat over the audio band, with less than 3 dB attenuation at 6 Hz and 70 kHz, well outside the audible range Dell Studio 1457 Battery .

Negative feedback

Tube amplifiers could not use as much negative feedback (NFB) as transistor amplifiers due to the large phase shifts caused by the output transformers and their lower stage gains Dell Latitude D610 Battery .

While the absence of NFB greatly increases harmonic distortion, it avoids instability, as well as slew rate and bandwidth limitations imposed by dominant-pole compensation in transistor amplifiers. Since transient intermodulation distortion was mainly caused by negative feedback, tube sound never suffered much of that kind of distortion   Toshiba NB100 Battery .

Power supplies

Early tube amplifiers usually used unregulated power supplies. This was due to the high cost associated with high-quality high-voltage power supplies. The typical anode supply was simply a rectifier, an inductor and a filter capacitor Toshiba Satellite M65 battery .

When the tube amplifier was operated at high volume, the power supply voltage would dip as the amplifier draws more current, reducing power output and causing signal modulation. This dipping effect is known as "sag", which may be preferable to some electric guitarists when compared with hard clipping hop.com/toshiba-satellite-m60-series-laptop-battery.htm">Toshiba Satellite M60 battery .

Electric guitar amplifiers are almost invariably using a class AB1 amplifier, whereas in a class A stage the average current drawn from the supply is constant with signal level, and consequently does not cause supply line sag, until the clipping point is reached Dell Latitude D830 Battery .

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  Dell Latitude D620 Battery .

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 resistanceDell Inspiron Mini 10 Battery .

Tube versus solid state rectification

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 Sony VGN-FW11S Battery .

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 Sony VGN-FW11M Battery .

The practical advantage to tube rectification is that rectifier tubes require some time to warm up before they begin to conduct. This provides some 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 Sony VGN-FW139E/H battery .

Some high end manufacturers, such as Welborne Labs in their premium kits, feature ultra-fast 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 Dell Latitude E5400 Battery .

Class A

The special feature of all Class A amplifiers is the absolute absence of crossover distortion (which is substituted with non-linear higher harmonic distortion, q.v.). The crossover distortion was found especially annoying after the first silicon-transistor Class B and Class AB transistor amplifiers arrived on the consumer market Dell Latitude E4200 Battery ;

the earlier germanium-based designs with the much lower turn-on voltage of this technology and the non-linear response curves of the devices had not shown large amounts of cross-over distortion Dell Inspiron 300M Battery .

Although crossover distortion is very fatiguing to the ear and perceptible in listening tests, it is also almost invisible (until looked for) in the traditional Total harmonic distortion (THD) measurements of that epoch  Dell Studio 1737 battery .

Push-pull amplifiers

A Class A push-pull amplifier produces low distortion for any given level of applied feedback, and also cancels the flux in the transformer cores, so this topology is seen by some[who?] as the ultimate engineering approach to the tube Hi-fi amplifier for use with normal speakers  Dell Inspiron E1505 battery .

Output power of as high as 15 watts can be achieved even with classic tubes such as the 2A3[25] or 18 watts from the type 45. Classic pentodes such as the EL34 and KT88 can output as much as 60 and 100 watts respectively. Special types such as the V1505 can be used in designs rated at up to 1100 watts  Dell RM791 battery .

See "An Approach to Audio Frequency Amplifier Design", a collection of reference designs originally published by G.E.C.

Single-Ended Triode (SET) amplifiers

SET amplifiers typically show poor measurements for distortion with a resistive load, have low output power Toshiba Satellite P10 Battery ,

are inefficient, have poor damping factors and high measured harmonic distortion. But they perform somewhat better in dynamic and impulse response. Also, SET-amplifiers allegedly (there is little agreement on this) sound good at low power levels without the perceived need for higher sound levels like transistor amplifiers allegedly do Acer Aspire One battery 10400mAh .

The triode, despite being the oldest signal amplification device, also can (depending on the device in question) have a more linear no-feedback transfer characteristic than more advanced devices such as beam tetrodes and pentodes  SONY VGN-FZ210CE Battery .

However no amplification device has a sufficiently linear characteristic for acceptable distortion performance unless it is linearised by the use of negative feedback. The low gain of a single triode makes it impossible to use more than a minimal level of negative feedback and so THD levels of a few percent are the rule in this type of amplifier Dell Precision M70 Battery .

NFB is used in most post 1950s amplifiers and although it usually reduces the measured distortion level, if poorly implemented it results in an unpleasant combination of harmonics to some ears; this is not the case with a competent implementation  Toshiba Satellite L305 Battery .

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 Toshiba Satellite T4900 Battery .

In the 1970s, designers started producing transistor amps with higher open loop gain to support a greater value of negative feedback. In the following years, amplifiers were built with modest gain but good open loop linearity, deployed with only minimal levels of NFB  Toshiba PA3399U-2BRS battery .

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 Toshiba Satellite A200 Battery .

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. It has been also claimed that a single-ended power amplifier's second harmonic distortion could reduce similar harmonic distortion in a single driver loudspeaker Toshiba Satellite 1200 Battery ,

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 Toshiba Satellite M300 Battery .

The resulting sound pressure level 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 SONY VGP-BPS13A/Q Battery .

For example a 10 W stereo SET uses a minimum of 80 W, and typically 100 W.

Single-ended pentode and tetrode amplifiers

The special feature among tetrodes and pentodes is the possibility to obtain ultra-linear or distributed load operation with an appropriate output transformer SONY VGP-BPS13A/Q Battery .

Ultra-linear connection is a negative feedback method, enabling less harmonic distortion.

Class AB

The majority of modern commercial Hi-fi amplifier designs have until recently used Class AB topology (with more or less pure low-level Class A capability depending on the standing bias current used) SONY VGP-BPS13B/Q Battery ,

in order to deliver greater power and efficiency, typically 12–25 watts and higher. Modern designs normally include at least some negative feedback, although in the old times of High fidelity use of feedback was totally out of question SONY VGP-BPS13B/Q Battery .

It should however be noted that Class D topology (which is vastly more efficient than Class B, and has garnered some respect from audiophiles) is more and more frequently applied where traditional design would use Class AB SONY VGP-BPS13/Q Battery .

Class AB push-pull topology is nearly universally used in tube amps for electric guitar applications that produce power of more than about 10 watts. 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 SONY VGP-BPS13/Q Battery .

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 theintermodulation products of the distortion are generally more closely related to the harmonies of the chord SONY VGP-BPS21A/B Battery .

All sides of the question are inclined to agree about valve guitar amplifiers offering a very useful sound, though there are also some well-respected solid-state designs SONY VGP-BPS21/S Battery .

Tube sound from transistor amplifiers

Some individual characteristics of the tube sound, such as the waveshaping on overdrive, are straightforward to produce in a transistor circuit or digital filter SONY VGP-BPS21B Battery .

For more complete simulations, engineers have been successful in developing transistor amplifiers that produce a sound quality very similar to the tube sound. Usually this involves using a circuit topology similar to that used in tube amplifiers SONY VGP-BPS21A Battery .

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 SONY VGP-BPS21 Battery .

Computer algorithms are currently available that transform digital sound from a CD or other digital source into a distorteddigital sound signal.

Using modern passive components, and modern sources, whether digital or analogue, and wide band loudspeakers Sony VGP-BPS21A/B Battery ,

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" Sony VGP-BPS21/S Battery .

Hybrid amplifiers

Tubes are often used to impart characteristics that many people find audibly pleasant to solid state amplifiers, such as Musical Fidelity's use of Nuvistors, tiny triode tubes, to control large bi-polar transistors in their NuVista 300 power amp Sony VGP-BPS21A Battery .

In America, Moscode and Studio Electric use this method, but use MOSFET transistors for power, rather than bi-polar. Pathos, an Italian company, has developed an entire line of hybrid amplifiers Sony VGP-BPS21 Battery .

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 Sony Vaio PCG-5G2L Battery ,

though other aspects of the "tube sound" would not be duplicated in this exercise.

Tube sound enthusiasts

Different uses of tube amplifiers can be found due to the different personal preferences of the enthusiasts Sony Vaio PCG-5G3L Battery .

From those who opt to restrict their use as active devices to those who opt to include them in the audio circuit, accepting the use of semiconductor gain devices in the power supply or as constant current sources. Others, still, will use tubes for the main amplification circuit but add semiconductors (such as solid-state diodes) for clipping purposes Sony Vaio PCG-5J1L Battery ,

particularly in the preamp section, which is often debated in advertised vintage instrument amplifiers such as the Marshall JCM900 or the Vintage Modern as to their integrity due to their utilization of solid-state devices in the tone-generation circuit Sony Vaio PCG-5K2L Battery .

Other schisms concern the use of triodes vs. tetrodes and pentodes, and the use of directly heated tubes vs. indirectly heated tubes.

Many of the explanations relate to the circuit topologies pioneered using tubes, and traditionally associated with them ever sinceSony Vaio PCG-5J2L Battery ,

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 Sony Vaio PCG-5K1L Battery .

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 Sony Vaio PCG-6S2L Battery .

A tube tester is an electronic instrument designed to test certain characteristics of vacuum tubes. Tube testers evolved along with the vacuum tube to satisfy the demands of the time, and this evolution faded with the tube era, yet with the High-End advent, they became a pricey piece of equipment  Sony Vaio PCG-6S3L Battery .

The first tube testers were simple units designed for specific tubes to be used in the battle fields of World War I by radio operators, so they could easily test the tubes of their communication equipment Sony Vaio PCG-6V1L Battery .

Filament continuity tester

The simplest one is the filament continuity tester, usually with a neon lamp connected in series with the filament and a current limiting resistance fed directly by the mains Sony Vaio PCG-6W1L Battery .

Emission tester

Next in complexities is the emission tester, which basically treats any tube as a diode by carefully connecting the cathode to ground, all the grids and plate to B+ voltage, feeding the filament with the correct voltage, and an ammeter in series with either the plate or the cathode Sony Vaio PCG-6W2L Battery .

This effectively measures emission, the current which the cathode is capable of emitting, for the given plate voltage, which can usually be controlled by a variable load resistor.

Older testers may call themselves Plate Conductance if the ammeter is in series with the plate, or Cathode Conductance if the meter is in series with the cathode Sony Vaio PCG-6W3L Battery .

The problems of emission testers are:

  • they do not measure key characteristics of tubes, like transconductance
  • they do not perform the tests at real load, voltages and currents Sony Vaio PCG-7111L Battery
  • they test the tube under static conditions, which are not even near the dynamic conditions the tube would work with in a real electronic device
  • tubes with grids, might not even show the real emission because of hot spots in the cathode, hidden by the grids under normal conditions  Sony Vaio PCG-7112L Battery

Short circuit test

Usually, emission testers and better testers have a short circuit test which is just a variation of the continuity tester with a neon lamp, and which allows to identify if there is any shortcut between the different electrodes Sony Vaio PCG-7113L Battery .

Mutual conductance tester

The mutual conductance tester tests the tube dynamically by applying bias and an AC voltage to the control grid, and measuring the current obtained on the plate, while maintaining the correct DC voltages on the plate and screen grid  Sony Vaio PCG-7133L Battery .

This setup measures the transconductance of the tube, indicated in micromhos.

Dynamic conductance tester

The dynamic conductance tester is just a variation of the emission tester with its implications, where a proportional AC voltage is applied to each electrode  Sony Vaio PCG-7Z2L Battery .

This tester exists thanks to Jackson mainly to avoid infringing the patents of the mutual conductance tester held by Hickok, but obviously do not provide the same measurements.

In Europe, the principal method of numbering vacuum tubes (or "thermionic valves", as they were often referred to in Europe and Australia, etc.) Sony Vaio PCG-8Y1L Battery

was the nomenclature developed and used by Mullard in the UK and applied Europe-wide thanks to their parentage by Philips who had subsidiaries in Germany (Valvo) and France (Dario), plus the support of Telefunken, from 1934 Sony Vaio PCG-8Y2L Battery .

This system, in use until the end of the tube era, sought to allocate meaningful codes to tubes with a clear understanding of their function. This European common code system became the starting point for the Pro Electronnaming scheme for active devices (including tubes and transistors) Sony Vaio PCG-8Z1L Battery .

It also allowed for easy cross-referencing with the American RETMA tube designation and other systems in use such as Marconi-Osram tube designation, and military numbering systems such asCivilian Valve (CV) numbering in the UK and the JAN tube designation in the USA Sony Vaio PCG-8Z2L Battery .

Although the nomenclature was agreed amongst all the tube manufacturers in Europe there were exceptions in the UK with MOV (Marconi-Osram Valve), Brimar (STC) and Mazda/Ediswan adopting their own systems. The vast majority of MOV valves (tubes) were copies of RCA types (as the two companies had cross-licencing agreements) SONY VAIO VGN-FW31J Battery

but with a British designation, thus a MOV X63 valve was the same as an RCA 6A8 tube, whilst Brimar, which stood for "British Manufactured American Radio" (valves), used all American designations. Brimar/STC was a UK subsidiary of the American giantITT (International Telephone and Telegraph) SONY VAIO VGN-FW31E Battery .

Special quality tubes for military use were sometimes identified by placing the numerical part of the designation between the heater rating and the tube type. For example an EF80 manufactured as a special quality tube would be designated 'E80F'. The system was not universal as other special quality designation systems existed SONY VAIO VGN-FW31B Battery .

For example, an EF91, in its special quality version was designated 'M8084' (the 'M' meant Military) as in this case the standard EF91 was derived from the M8084 military design.

Two or more elements in a single envelope were handled by adding letters after the heater identifier SONY VAIO VGN-FW31M Battery ,

in alphabetical order, so an ECH35 is a 6.3 volt heated triode plus a hexode with an octal base; a PABC80 is a noval 300mA series-heater combination of 3 low-power diodes (two sharing a common cathode), plus a triode SONY VAIO VGN-FW31Z Battery .

Mullard initially handled semiconductor naming by using the "O" heater code (shifting "Cold Cathode" to a code of "Z"), but from 1966 the new Pro Electron standard elegantly codified the allocation of type numbers for all active devices, notably by using initial letters "A", "B" and "C" (rarely used heaters) for germanium SONY VAIO VGN-FW38M Battery ,

silicon and other semiconductor diodes and transistors, as well as codes for integrated circuits, etc. Heater letters A, B, C, F, K, V and Y ceased to be allocated. This meant that most existing European valve type number allocations were compatible with the new system, but sometimes ambiguities could only be resolved by checking the digits in the name SONY VAIO VGN-AW41JF Battery .

For example, it might not immediately be obvious whether a (hypothetical) AD108 is a 4 volt power triode or a germanium power transistor; an AZ41 (still on sale in the 1970s ) might be thought to be a germanium zener diode (although, with only 2 digits for the serial number, it was not really a valid Pro Electron designation) SONY VAIO VGN-AW41JF/H Battery .

By the introduction of the Pro Electron series the vast majority of valve (tube) names started with either D, E, G, P or U, so confusion between the two systems was unlikely.

A valve amplifier or tube amplifier is a type of electronic amplifier that makes use of vacuum tubes to increase the power and/or amplitude of a signal SONY VAIO VGN-AW41MF Battery .

Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers during the 1960s and 1970s. Valve amplifiers are used for applications such as guitar amplifiers, satellite transponders such as DirecTV and GPS, audiophile stereo amplifiers SONY VAIO VGN-AW41MF/H Battery ,

military applications (such asradar) and very high power radio and UHF television transmitters.


Until the invention of the transistor in 1947, all practical amplifiers were made using thermionic valves SONY VAIO VGN-FW30B Battery .

The simplest valve was invented by John Ambrose Fleming while working for the Marconi Company in London in 1904 and named the diode, as it had two electrodes. The diode conducted electricity in one direction only and was used as a radio detector and a rectifier SONY VAIO VGN-FW50B Battery .

In 1906 Lee De Forest added a third electrode and invented the first electronic amplifying device, the triode, which he named the Audion. This additional control grid modulates the current that flows between cathode and anode SONY VAIO VGN-FW51B/W Battery .

The relationship between current flow and plate and grid voltage is often represented as a series of "characteristic curves" on a diagram. Depending on the other components in the circuit this modulated current flow can be used to provide current or voltage gain SONY VAIO VGN-FW51MF Battery .

The first application of valve amplification was in the regeneration of long distance telephony signals. Later, valve amplification was applied to the 'wireless' market that began in the early thirties. In due course amplifiers for music and later television were also built using valves SONY VAIO VGN-FW51MF/H Battery .

The overwhelmingly dominant circuit topology during this period was the single-ended triode gain stage, operating in class A, which gave very good sound (and reasonable measured distortion performance) despite extremely simple circuitry with very few components SONY VAIO VGN-FW51ZF Battery :

important at a time when components were hand made and extremely expensive. Before World War II, almost all valve amplifiers were of low gain and with linearity dependent entirely on the inherent linearity of the valve itself, typically 5% distortion at full power SONY VAIO VGN-FW51ZF/H Battery .

Negative feedback (NFB) was invented by Harold Stephen Black in 1927, but initially little used since at that time gain was at a premium. This technique allows amplifiers to trade gain for reduced distortion levels (and also gave other benefits such as reduced output impedance) SONY VAIO VGN-FW52JB Battery .

The introduction of theWilliamson amplifier in 1947, which was extremely advanced in many respects including very successful use of NFB, was a turning point in audio power amplifier design, operating a push-pull output circuit in class AB1 to give performance surpassing its contemporaries SONY VAIO VGN-FW54FB Battery .

Post-war developments

World War II stimulated dramatic technical progress and industrial scale production economies. Increasing affluence after the war brought about for the first time a substantial and expanding consumer market SONY VAIO VGN-FW70DB Battery .

This enabled more advanced valve designs to be marketed at affordable prices, with the result that the 1960s saw the increasing spread of electronic gramophone players, and ultimately the beginnings of "high fidelity". Hifi was able to drive full frequency range loudspeakers (for the first time often with multiple drivers for different frequency bands) to significant volume levels SONY VAIO VGN-FW71DB/W Battery .

This combined with the spread of TV, produced a 'golden age' in valve development and also in the development of the design of valve amplifier circuits SONY VAIO VGN-FW72JGB Battery .

A range of topologies with only minor variations (notably different phase splitter arrangements and the "Ultra-Linear" transformer connection for tetrodes) rapidly became widespread . This family of designs remains the dominant high power amplifier topology to this day for music application SONY VAIO VGN-FW73JGB Battery .

This period also saw continued growth in civilian radio, with valves being used for both transmitters and receivers.


From the 1970s the silicon transistor became increasingly pervasive  SONY VAIO VGN-FW74FB Battery .

Valve production was sharply decreased, with the notable exception of cathode ray tubes (CRTs), and a reduced range of valves for amplifier applications. Popular low power tubes were dual triodes (ECCnn, 12Ax7 series) plus the EF86 pentode, and power valves were mostly being beam tetrode and pentodes (EL84, EL34, KT88 / 6550, 6L6) SONY VAIO VGN-FW81HS Battery ,

in both cases with indirect heating. This reduced set of types remains the core of valve production today.

The Soviets retained valves to a much greater extent than the West during the Cold War, for the majority of their communications and military amplification requirements SONY VAIO VGN-FW81NS Battery ,

in part due to valves' ability to withstand instantaneous overloads (notably due to a nuclear detonation) that would destroy a transistor .

The dramatic reduction in size, power consumption, reduced distortion levels and above all cost of electronics products based on transistors has made valves obsolete for mainstream products since the 1970s SONY VAIO VGN-FW81S Battery .

Valves remained in certain applications such as high power RF transmitters and the microwave oven.


In audio applications, valves continue to be highly desired by some users, both in the higher-end home audio market and in the guitar amplifier market SONY VAIO VGN-FW82DS Battery.

Amongst stereo enthusiasts, there is a subgroup of audio buffs who advocate the use of tube amplifiers for home listening; they argue that tube amplifiers produce a "warmer" or more "natural" valve sound. Companies in Russia, China and Eastern Europe continue to produce valves to cater to this market SONY VAIO VGN-FW82JS Battery .

In the guitar amplifier market, most performers continue to use tube amps today, including in folk, blues, roots rock, and in harder genres such as metal, where tube amps are used to create heavy distortion. Audio engineers suggest that the subjectively pleasing aspects of tube amplification may be due to the non-linear overdrive that is produced with tubes SONY VAIO VGN-FW82XS Battery.

Tube amplifiers respond differently from transistor amplifiers when signal levels approach and reach the point of clipping. In a tube amplifier, the transition from linear amplification to limiting is less abrupt than in a solid state unit, resulting in a less grating form of distortion at the onset of clipping SONY VAIO VGN-FW83DS Battery .

For this reason, some guitarists prefer the sound of an all-tube amplifier; the aesthetic properties of tube versus solid state amps, though, are a topic of debate in the guitarist community SONY VAIO VGN-FW83JS Battery .


Valves are high voltage and low current devices in contrast to transistors, which typically operate at lower voltages and higher currents for the same power level SONY VAIO VGN-FW83XS Battery .

The high working voltage makes valves well suited for radio transmitters, for example, and valves remain in use today for very high power radio transmitters, where there is still no other technology available. However, for most applications requiring an appreciable output current, a matching transformer is required SONY VAIO VGN-FW90NS Battery .

The transformer is a critical component and heavily influences the performance (and cost) of the amplifier.

Many power valves have good linearity but modest gain or transconductance SONY VAIO VGN-FW90S Battery .

Signal amplifiers using tubes are capable of very high frequency response ranges – up to radio frequency. Indeed, many of the directly heated single-ended triode (DH-SET) audio amplifiers are in fact radio transmitting tubes designed to operate in the megahertz range SONY VAIO VGN-FW91NS Battery .

In practice, however, tube amplifier designs typically "couple" stages either capacitively, limiting bandwidth at the low end, or inductively with transformers, limiting the bandwidth at high end  SONY VAIO VGN-FW91S Battery .


  • Very linear (especially triodes) making it viable to use them in low distortion linear circuits with little or no negative feedback.
  • Inherently suitable for high voltage circuits SONY VAIO VGN-FW92DS Battery .
  • Can be constructed on a scale that can dissipate large amounts of heat (some extreme devices even being water cooled). For this reason valves remained the only viable technology for very high power applications such as radio and TV transmitters long into the age when transistors had displaced valves in most other applications SONY VAIO VGN-FW92JS Battery .
  • Very low "drift" (of specifications) over a wide range of operating conditions, specifically high heat and high power. Semiconductors are very heat sensitive by comparison, forcing compromises in solid state amplifier designs SONY VAIO VGN-FW93DS Battery .
  • Electrically very robust, they can tolerate overloads for minutes which would destroy bipolar transistor systems in milliseconds.
  • Easily removable for testing or replacement – it is much harder to replace a faulty transistor SONY VAIO VGN-FW93JS Battery.
  • Softer clipping when overloading the circuit, which many audiophiles and musicians think gives a more pleasant sound.


  • Heater supplies are usually required for the cathodes, and high voltages are usually required for the anodes SONY VAIO VGN-FW139E/H Battery .
  • They are significantly larger than equivalent solid-state devices
  • High impedance and low current output is unsuitable for direct drive of many real world loads, notably various forms of electric motors SONY VAIO VGN-FW145E/W Battery .
  • Valves may have a shorter working life than solid state parts due to various failure mechanisms (such as heat, cathode poisoning, breakage, or internal short-circuits)  SONY VAIO VGN-FW190EBH Battery .
  • Available in a single polarity only whereas transistors are available in complementary polarities (e.g., NPN/PNP), making possible many circuit configurations that cannot be realized directly with valves.
  • Valve circuits must avoid introduction of noise from ac heater supplies SONY VAIO VGN-FW190EDH Battery .
  • Microphonics – valves may sometimes be sensitive to sound or vibration, inadvertently acting like a microphone.
  • Power consumption due to the heater requirements SONY VAIO VGN-FW21E Battery .


All amplifier circuits are classified by "class of operation" as A, B, AB and C etc. See amplifier classes. Some significantly different circuit topologies exist compared to transistor designs SONY VAIO VGN-FW21J Battery .

  • The grid (where the input signal is presented) needs to be biased substantially negative with respect to the cathode This makes it extremely difficult to direct couple the output of one valve to the input of a following valve as is normally done in transistor designs SONY VAIO VGN-FW21L Battery .
  • Valve stages are coupled with components rated to withstand several hundred volts, typically a capacitor, occasionally a coupling transformer. The phase shifts introduced by coupling networks can become problematic in circuits that have feedback SONY VAIO VGN-FW21M Battery .
  • There is no valve analog of the complementary devices widely used in "totem pole" output stages of silicon circuits. Push-pull valve topologies therefore typically require a transformer SONY VAIO VGN-FW21Z Battery .
  • The very high output impedance of valves (compared with transistors) usually requires matching transformers to drive low impedance loads such as loudspeakers or cutting lathe heads. The transformer is used as the load, in place of the resistor usually used in small-signal and driver stages SONY VAIO VGN-FW25T/B Battery .
  • The impedance of the transformer primary at the frequencies in use is much higher than the DC resistance of the windings, often kilohms. High performance transformers are however severe engineering compromises, are expensive, and in operation are far from ideal  SONY VAIO VGN-FW26T/B Battery .
  • Transformers dramatically increase the cost of a valve amplifier circuit compared to a direct-coupled transistor alternative.
  • The open loop linearity of valves, especially triodes, makes it possible to use little or no negative feedback in circuits whilst retaining acceptable or even excellent distortion performance (especially for small-signal circuits) SONY VAIO VGN-FW27/B Battery .


  • Linear small signal circuits almost invariably use a triode in the single ended gain stage topology (in class A), including the output stage.
  • Broadband valve amplifiers typically use class A1 or AB1 SONY VAIO VGN-FW27/W Battery .
  • Modern high power output stages are usually push pull, often necessitating some form of phase splitter to derive a differential/balanced drive signal from a single ended input, typically followed by a further gain stage (the "driver") prior to the output tubes. For example SRPP) SONY VAIO VGN-FW27T/H Battery
  • single ended" power stages using very large valves exist and dominate in radio transmitter applications. A sidebar is the observation that the niche "DH-SET" topology favored by some audiophiles are extremely simple and typically constructed using valve types originally designed for use in radio transmitters SONY VAIO VGN-FW29/B Battery
  • more complex topologies (notably the use of active loads) can improve linearity and frequency response (by removing Miller capacitance effects).

Output impedance

The high output impedance of tube plate circuits is not well matched to low-impedance loads such as loudspeakers or antennas SONY VAIO VGN-FW290JRB Battery .

A matching network is required for efficient power transfer; this may be a transformer at audio frequencies, or various tuned networks at radio frequencies.

Audio Frequency (AF) and Broadband amplifiers

Valves remain in widespread use in guitar and high-end audio amplifiers due to the sound quality they produce, which is subjectively preferred by some users SONY VAIO VGN-FW290JTB Battery .

They are largely obsolete for most other applications, mainly due to the cost effectiveness advantages of the transistor, and due to the lower weight and heat production of transistor amps.


Telephony was the original, and for many years was a driving application for audio amplification SONY VAIO VGN-FW290JTH Battery .

A specific issue for the telecommunication industry was the technique of multiplexing many (up to a thousand) voice lines onto a single cable, at different frequencies.

The advantage of this is that a single valve "repeater" amplifier can amplify many calls at once, this being very cost effective SONY VAIO VGN-FW52JB Battery .

The problem is that the amplifiers need to be extremely linear, otherwise "intermodulation distortion" (IMD) will result in "crosstalk" between the multiplexed channels. This stimulated development emphasis towards low distortion far beyond the nominal needs of a single voice channel  SONY VAIO VGN-FW51B/W Battery .


Today the main application for valves is audio amplifiers for high-end hi-fi and musical performance use with electric guitars, electric basses, and Hammond organs, although these applications have different requirements regarding distortion which result in different design compromises  SONY VAIO VGN-FW50B Battery ,

although the same basic design techniques are generic and widely applicable to all broadband amplification applications, not only audio.

Post World War II, the majority of valve power amplifiers are of the Class AB-1 "push pull" ultralinear topology, but niche products using the DH-SET and even OTL topologies still exist in small numbers SONY VAIO VGN-FW47GYH Battery .

Instrumentation amplifiers

The basic moving coil voltmeter and ammeter itself takes a small current and thus loads the circuit to which it is attached. This can significantly alter the operating conditions in the circuit being measured  SONY VAIO VGN-FW465GJB Battery .

The vacuum tube voltmeter (VTVM) uses the high input impedance of a valve to buffer the circuit being measured from the load of the ammeter.

Valve oscilloscopes share this very high input impedance and thus can be used to measure voltages even in very high impedance circuits  SONY VAIO VGN-FW465GJ/BE1 Battery .

There may typically be 3 or 4 stages of amplification per display channel. In later oscilloscopes, a type of amplifier using a series of tubes connected at equal distances along transmission lines, known as a distributed amplifier was employed to amplify very high frequency vertical signals before application to the display tube SONY VAIO VGN-FW465J/B Battery .

Valve oscilloscopes are now obsolete.

In the closing years of the valve era, valves were even used to make "operational amplifiers" – the building blocks of much modern linear electronics  SONY VAIO VGN-FW45TJB Battery .

An op-amp typically has a differential input stage and a totem pole output, the circuit usually having a minimum of five active devices. A number of "packages" were produced that integrated such circuits (typically using two or more glass envelopes) into a single module that could be plugged into a larger circuit (such as an analog computer) SONY VAIO VGN-FW45GJB Battery .

Such valve op-amps were very far from ideal and quickly became obsolete, being replaced with solid-state types.

Narrow band and radio frequency tuned amplifiers

Historically (pre-WWII) "transmitting tubes" were among the most powerful tubes available  SONY VAIO VGN-FWZJ/H Battery .

These usually had directly heated thoriated filaments cathodes that glowed like light bulbs. Some tubes were capable of being driven so hard that the anode would itself glow cherry red; the anodes were machined from solid material (rather than fabricated from thin sheet) to withstand heat without distorting  SONY VAIO VGN-FW41M/H Battery .

Notable tubes of this type are the 845 and 211. Later tetrodes and pentodes such as 817 and (direct heated) 813 were also used in large numbers in (especially military) radio transmitters

RF circuits are significantly different from broadband amplifier circuits  SONY VAIO VGN-FW41J/H Battery .

The antenna or following circuit stage typically contains one or more adjustable capacitive or inductive component allowing the resonance of the stage to be accurately matched with carrier frequency in use, to optimize power transfer from and loading on the valve, a so called "tuned circuit"  SONY VAIO VGN-FW41E/H Battery .

Broadband circuits require flat response over a wide range of frequencies. RF circuits by contrast are typically required to operate at high frequencies but often over a very narrow frequency range. For example, an RF device might be required to operate over the range 144 to 146 MHz (just 1.4%) SONY VAIO VGN-FW37J Battery .

Today, radio transmitters are overwhelmingly silicon based, even at microwave frequencies. However an ever decreasing minority of high power radio frequency amplifiers continue to have valve construction SONY VAIO VGN-FW378J/B Battery .

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