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57-424: The 12AX7 was originally intended as replacement for the 6SL7 family of dual-triode amplifier tubes for audio applications. As a popular choice for guitar tube amplifiers, its ongoing use in such equipment makes it one of the few small-signal vacuum tubes in continuous production since it was introduced. The 12AX7 is a twin triode basically composed of two of the triodes from a 6AV6, a double diode triode. The 6AV6

114-413: A d {\displaystyle R_{load}} . If the cathode resistor is bypassed, use R K = 0 {\displaystyle R_{K}=0} . The initial “12” in the designator implies a 12-volt heater requirement; however, the tube has a center-tapped heater so it can be used in either 6.3-V or 12.6-V heater circuits. The 12AX7 is the most common member of what eventually became

171-476: A  = 200 V on the anode. Now suppose we impress on the −1 V bias voltage a signal of 1 V peak-peak, so that the grid voltage varies between −0.5 V and −1.5 V. When V g  = −0.5 V, the anode current will increase to 3.1 mA, lowering the anode voltage to V a = V +  − 10 kΩ × 3.1 mA = 191 V (orange curve). When V g  = −1.5 V,

228-558: A 100k plate load, and a center biased and bypassed cathode, and higher with a larger plate load. A v = μ × R t o t / ( r P + R t o t + ( R K × ( μ + 1 ) ) {\displaystyle A_{v}=\mu \times R_{tot}/(r_{P}+R_{tot}+(R_{K}\times (\mu +1))} Where A v {\displaystyle A_{v}} = voltage gain, μ {\displaystyle \mu }

285-448: A circuit designer can choose the operating point of the particular triode. Then the output voltage and amplification of the triode can be evaluated graphically by drawing a load line on the graph. In the example characteristic shown on the image, suppose we wish to operate it at a quiescent anode voltage V a of 200 V and a grid voltage bias of −1 V. This implies a quiescent plate (anode) current of 2.2 mA (using

342-443: A comeback. Triodes continue to be used in certain high-power RF amplifiers and transmitters . While proponents of vacuum tubes claim their superiority in areas such as high-end and professional audio applications, the solid-state MOSFET has similar performance characteristics. In triode datasheets, characteristics linking the anode current (I a ) to anode voltage (V a ) and grid voltage (V g ) are usually given. From here,

399-470: A driver and phase-inverter in vacuum tube push–pull amplifier circuits. This tube is essentially two 6AB4/EC92s in a single envelope. Unlike the situation with the 6C4 and 12AU7 , both the 6AB4 and the 12AT7 are described by manufacturer's data sheets as R.F. (Radio Frequency) devices operating up to VHF frequencies. The tube has a center-tapped filament so it can be used in either 6.3V 300mA or 12.6V 150mA heater circuits. [REDACTED] As of 2012

456-474: A grid electrode to the thermionic diode ( Fleming valve ), the triode was the first practical electronic amplifier and the ancestor of other types of vacuum tubes such as the tetrode and pentode . Its invention helped make amplified radio technology and long-distance telephony possible. Triodes were widely used in consumer electronics devices such as radios and televisions until the 1970s, when transistors replaced them. Today, their main remaining use

513-462: A heated filament (cathode) and a plate (anode). Triodes came about in 1906 when American engineer Lee de Forest and Austrian physicist Robert von Lieben independently patented tubes that added a third electrode, a control grid , between the filament and plate to control current. Von Lieben's partially-evacuated three-element tube, patented in March 1906, contained a trace of mercury vapor and

570-399: A large family of twin-triode vacuum tubes, manufactured all over the world, all sharing the same pinout (EIA 9A). Most use heaters which can be optionally wired in series (12.6V, 150 mA) or parallel (6.3V, 300 mA). Other tubes, which in some cases can be used interchangeably in an emergency or for different performance characteristics, include the 12AT7 , 12AU7 , 12AV7 , 12AY7, and

627-469: A much more powerful anode current, resulting in amplification . When used in its linear region, variation in the grid voltage will cause an approximately proportional variation in the anode current; this ratio is called the transconductance . If a suitable load resistance is inserted in the anode circuit, although the transconductance is somewhat lowered, the varying anode current will cause a varying voltage across that resistance which can be much larger than

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684-456: A thin metal filament . In some tubes the filament itself is the cathode, while in most tubes there is a separate filament which heats the cathode but is electrically isolated from it. The interior of the tube is well evacuated so that electrons can travel between the cathode and the anode without losing energy in collisions with gas molecules. A positive DC voltage, which can be as low as 20V or up to thousands of volts in some transmitting tubes,

741-418: A triode is about 2000 hours for small tubes and 10,000 hours for power tubes. Low power triodes have a concentric construction (see drawing right) , with the grid and anode as circular or oval cylinders surrounding the cathode. The cathode is a narrow metal tube down the center. Inside the cathode is a filament called the "heater" consisting of a narrow strip of high resistance tungsten wire, which heats

798-423: Is 17 in this case. It is also possible to use triodes as cathode followers in which there is no voltage amplification but a huge reduction in dynamic impedance ; in other words, the current is greatly amplified (as it also is in the common-cathode configuration described above). Amplifying either the voltage or current results in power amplification, the general purpose of an amplifying tube (after all, either

855-460: Is a miniature repackaging (with just a single cathode) of the triode and twin diodes from the octal 6SQ7 (a double-diode triode used in AM radios), which itself is very similar to the older type 75 triode-diode dating from 1930. The 12AX7 is a high-gain (typical amplification factor 100), low-plate-current triode best suited for low-level audio voltage amplification. In this role it is widely used for

912-537: Is in high-power RF amplifiers in radio transmitters and industrial RF heating devices. In recent years there has been a resurgence in demand for low power triodes due to renewed interest in tube-type audio systems by audiophiles who prefer the sound of tube-based electronics. The name "triode" was coined by British physicist William Eccles some time around 1920, derived from the Greek τρίοδος, tríodos , from tri- (three) and hodós (road, way), originally meaning

969-417: Is introduced and each half of a 12AX7 provides a typical voltage gain of about 30; the amplification factor is basically twice the maximum stage gain, as the plate impedance must be matched. Thus half the voltage is across the tube at rest, half across the load resistor. The cathode resistor can be bypassed to reduce or eliminate AC negative feedback and thereby increase gain; maximum gain is about 60 times with

1026-467: Is often of greater interest. When these devices are used as cathode followers (or source followers ), they all have a voltage "gain" of just under 1, but with a large current gain . Although S.G. Brown's Type G Telephone Relay (using a magnetic "earphone" mechanism driving a carbon microphone element) was able to give power amplification and had been in use as early as 1914, it was a purely mechanical device with limited frequency range and fidelity. It

1083-419: Is present on the anode. The negative electrons are attracted to the positively charged anode (or "plate"), and flow through the spaces between the grid wires to it, creating a flow of electrons through the tube from cathode to anode. The magnitude of this current can be controlled by a voltage applied on the grid (relative to the cathode). The grid acts like a gate for the electrons. A more negative voltage on

1140-455: Is the amplification factor of the valve, r P {\displaystyle r_{P}} is the internal plate resistance, R K {\displaystyle R_{K}} is the cathode resistor and R t o t {\displaystyle R_{tot}} is the parallel combination of R P {\displaystyle R_{P}} (external plate resistor) and R l o

1197-553: The Marconi Company , who represented John Ambrose Fleming , the inventor of the diode. The discovery of the triode's amplifying ability in 1912 revolutionized electrical technology, creating the new field of electronics , the technology of active ( amplifying ) electrical devices. The triode was immediately applied to many areas of communication. During World War I, AM voice two way radio sets were made possible in 1917 (see TM (triode) ) which were simple enough that

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1254-464: The 12AX7/ECC83 are available from the following manufacturers: Triode A triode is an electronic amplifying vacuum tube (or thermionic valve in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode , a grid , and a plate ( anode ). Developed from Lee De Forest 's 1906 Audion , a partial vacuum tube that added

1311-555: The 12AX7: Although commonly known in Europe by its Mullard–Philips tube designation of ECC83, other European variations also exist including the low- noise versions 12AX7A, 12AD7, 6681, 7025, and 7729; European versions B339, B759, CV492, CV4004, CV8156, CV8222, ECC803, ECC803S, E2164, and M8137; and the lower-gain low-noise versions 5751 and 6851, intended for avionics equipment. In European usage special-quality valves of some sort were often indicated by exchanging letters and digits in

1368-413: The 7768 has a value of 1.7 pF. The close electrode spacing used in microwave tubes increases capacitances, but this increase is offset by their overall reduced dimensions compared to lower-frequency tubes. In the triode, electrons are released into the tube from the metal cathode by heating it, a process called thermionic emission . The cathode is heated red hot by a separate current flowing through

1425-505: The JFET's drain current is virtually unaffected by drain voltage, it appears as a constant-current device, similar in action to a tetrode or pentode tube (high dynamic output impedance). Both the JFET and tetrode/pentode valves are thereby capable of much higher voltage gains than the triode which seldom exceeds 100. However the power gain , or the output power obtained from a certain AC input voltage

1482-417: The anode current will decrease to 1.4 mA, raising the anode voltage to V a = V +  − 10 kΩ × 1.4 mA = 208 V (green curve). Therefore a 1 V peak-peak signal on the input (grid) causes an output voltage change of about 17 V. Thus voltage amplification of the signal is obtained. The ratio of these two changes, the voltage amplification factor (or mu )

1539-431: The anode current. This is called the "cutoff voltage". Since beyond cutoff the anode current ceases to respond to the grid voltage, the voltage on the grid must remain above the cutoff voltage for faithful (linear) amplification as well as not exceeding the cathode voltage. The triode is somewhat similar in operation to the n-channel JFET ; it is normally on, and exhibits progressively lower and lower plate/drain current as

1596-458: The anode, made of heavy copper, projects through the wall of the tube and is attached to a large external finned metal heat sink which is cooled by forced air or water. A type of low power triode for use at ultrahigh frequencies (UHF), the "lighthouse" tube, has a planar construction to reduce interelectrode capacitance and lead inductance , which gives it the appearance of a "lighthouse". The disk-shaped cathode, grid and plate form planes up

1653-406: The anode. This imbalance of charge causes the grid to exhibit a reactance that is much less than its low-frequency "open circuit" characteristic. Transit time effects are reduced by reduced spacings in the tube. Tubes such as the 416B (a Lighthouse design) and the 7768 (an all-ceramic miniaturised design) are specified for operation to 4 GHz. They feature greatly reduced grid-cathode spacings of

1710-418: The cathode red-hot (800 - 1000 °C). This type is called an " indirectly heated cathode ". The cathode is coated with a mixture of alkaline earth oxides such as calcium and thorium oxide which reduces its work function so it produces more electrons. The grid is constructed of a helix or screen of thin wires surrounding the cathode. The anode is a cylinder or rectangular box of sheet metal surrounding

1767-424: The center of the tube - a little like a sandwich with spaces between the layers. The cathode at the bottom is attached to the tube's pins, but the grid and plate are brought out to low inductance terminals on the upper level of the tube: the grid to a metal ring halfway up, and the plate to a metal button at the top. These are one example of "disk seal" design. Smaller examples dispense with the octal pin base shown in

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1824-497: The current or voltage alone could be increased by decreasing the other just using a transformer, a passive device). 12AT7 12AT7 (also known in Europe by the Mullard–Philips tube designation of ECC81) is a miniature nine-pin medium-gain (60) dual- triode vacuum tube popular in guitar amplifiers . It belongs to a large family of dual triode vacuum tubes which share the same pinout (EIA 9A), including in particular

1881-430: The electrons are attracted, with a grid consisting of a screen of wires between them to control the current. These are sealed inside a glass container from which the air has been removed to a high vacuum, about 10 atm. Since the filament eventually burns out, the tube has a limited lifetime and is made as a replaceable unit; the electrodes are attached to terminal pins which plug into a socket. The operating lifetime of

1938-473: The evolution of radio from a commercial message service to the first mass communication medium, with the beginning of radio broadcasting around 1920. Triodes made transcontinental telephone service possible. Vacuum tube triode repeaters , invented at Bell Telephone after its purchase of the Audion rights, allowed telephone calls to travel beyond the unamplified limit of about 800 miles. The opening by Bell of

1995-473: The first transcontinental telephone line was celebrated 3 years later, on January 25, 1915. Other inventions made possible by the triode were television , public address systems , electric phonographs , and talking motion pictures . The triode served as the technological base from which later vacuum tubes developed, such as the tetrode ( Walter Schottky , 1916) and pentode (Gilles Holst and Bernardus Dominicus Hubertus Tellegen, 1926), which remedied some of

2052-399: The grid will repel more of the electrons, so fewer get through to the anode, reducing the anode current. A less negative voltage on the grid will allow more electrons from the cathode to reach the anode, increasing the anode current. Therefore, an input AC signal on the grid of a few volts (or less), even at a very high impedance (since essentially no current flows through the grid) can control

2109-427: The grid. It is blackened to radiate heat and is often equipped with heat-radiating fins. The electrons travel in a radial direction, from cathode through the grid to the anode. The elements are held in position by mica or ceramic insulators and are supported by stiff wires attached to the base, where the electrodes are brought out to connecting pins. A " getter ", a small amount of shiny barium metal evaporated onto

2166-414: The grid/gate is pulled increasingly negative relative to the source/cathode. Cutoff voltage corresponds to the JFET's pinch-off voltage (V p ) or VGS(off); i.e., the voltage point at which output current essentially reaches zero. This similarity is limited, however. The triode's anode current is highly dependent on anode voltage as well as grid voltage, thus limiting the voltage gain . Because, in contrast,

2223-452: The illustration and rely on contact rings for all connections, including heater and D.C. cathode. As well, high-frequency performance is limited by transit time: the time required for electrons to travel from cathode to anode. Transit time effects are complicated, but one simple effect is input conductance, also known as grid loading. At extreme high frequencies, electrons arriving at the grid may become out of phase with those departing towards

2280-413: The input voltage variations, resulting in voltage gain . The triode is a normally "on" device; and current flows to the anode with zero voltage on the grid. The anode current is progressively reduced as the grid is made more negative relative to the cathode. Usually a constant DC voltage ("bias") is applied to the grid along with the varying signal voltage superimposed on it. That bias is required so that

2337-403: The inside of the glass, helps maintain the vacuum by absorbing gas released in the tube over time. High-power triodes generally use a filament which serves as the cathode (a directly heated cathode) because the emission coating on indirectly heated cathodes is destroyed by the higher ion bombardment in power tubes. A thoriated tungsten filament is most often used, in which thorium added to

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2394-532: The low-voltage 12U7, plus many four-digit EIA series dual triodes. They span a wide range of voltage gain and transconductance. Different versions of each were designed for enhanced ruggedness, low microphonics , stability, lifespan, etc. Those other designs offer lower voltage gain (traded off for higher plate current) than the 12AX7 (which has a voltage gain or A v {\displaystyle A_{v}} of 100), and are more suitable for high-frequency applications. Some American designs similar to

2451-657: The name: the E83CC was a special-quality ECC83. In the US a "W" in the designation, as in 12AX7WA, designates the tube as complying with military grade, higher reliability specifications. The 'E' in the European designation classifies this as having a 6.3 volt heater, whereas the American designation of 12AX7 classifies it as having a 12.6 volt heater. It can, of course, be wired for operation off either voltage. As of 2022 versions of

2508-412: The order of 0.1 mm. These greatly reduced grid spacings also give a much higher amplification factor than conventional axial designs. The 7768 has an amplification factor of 225, compared with 100 for the 6AV6 used in domestic radios and about the maximum possible for an axial design. Anode-grid capacitance is not especially low in these designs. The 6AV6 anode-grid capacitance is 2 picofarads (pF),

2565-537: The outbreak of the First World War . De Forest's Audion did not see much use until its ability to amplify was recognized around 1912 by several researchers, who used it to build the first successful amplifying radio receivers and electronic oscillators . The many uses for amplification motivated its rapid development. By 1913 improved versions with higher vacuum were developed by Harold Arnold at American Telephone and Telegraph Company , which had purchased

2622-470: The pilot in a single seat aircraft could use it while flying. Triode " continuous wave " radio transmitters replaced the cumbersome inefficient " damped wave " spark-gap transmitters , allowing the transmission of sound by amplitude modulation (AM). Amplifying triode radio receivers , which had the power to drive loudspeakers , replaced weak crystal radios , which had to be listened to with earphones , allowing families to listen together. This resulted in

2679-410: The place where three roads meet. Before thermionic valves were invented, Philipp Lenard used the principle of grid control while conducting photoelectric experiments in 1902. The first vacuum tube used in radio was the thermionic diode or Fleming valve , invented by John Ambrose Fleming in 1904 as a detector for radio receivers . It was an evacuated glass bulb containing two electrodes,

2736-401: The positive peaks of the signal never drive the grid positive with respect to the cathode which would result in grid current and non-linear behaviour. A sufficiently negative voltage on the grid (usually around 3-5 volts in small tubes such as the 6AV6, but as much as –130 volts in early audio power devices such as the '45), will prevent any electrons from getting through to the anode, turning off

2793-423: The preamplifier (input and mid-level) stages of audio amplifiers. It has relatively high Miller capacitance, making it unsuitable for radio-frequency use. Typically a 12AX7 triode is configured with a high-value plate resistor, 100 kohms in most guitar amps and 220 kΩ or more in high-fidelity equipment. Grid bias is most often provided by a cathode resistor. If the cathode resistor is unbypassed, negative feedback

2850-427: The rights to the Audion from De Forest, and Irving Langmuir at General Electric , who named his tube the "Pliotron", These were the first vacuum tube triodes. The name "triode" appeared later, when it became necessary to distinguish it from other kinds of vacuum tubes with more or fewer elements ( diodes , tetrodes , pentodes , etc.). There were lengthy lawsuits between De Forest and von Lieben, and De Forest and

2907-506: The shortcomings of the triode detailed below. The triode was very widely used in consumer electronics such as radios, televisions, and audio systems until it was replaced in the 1960s by the transistor , invented in 1947, which brought the "vacuum tube era" introduced by the triode to a close. Today triodes are used mostly in high-power applications for which solid state semiconductor devices are unsuitable, such as radio transmitters and industrial heating equipment. However, more recently

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2964-429: The triode and other vacuum tube devices have been experiencing a resurgence and comeback in high fidelity audio and musical equipment. They also remain in use as vacuum fluorescent displays (VFDs), which come in a variety of implementations but all are essentially triode devices. All triodes have a hot cathode electrode heated by a filament , which releases electrons, and a flat metal plate electrode (anode) to which

3021-550: The tungsten diffuses to the surface and forms a monolayer which increases electron emission. As the monolayer is removed by ion bombardment it is continually renewed by more thorium diffusing to the surface. These generally run at higher temperatures than indirectly heated cathodes. The envelope of the tube is often made of more durable ceramic rather than glass, and all the materials have higher melting points to withstand higher heat levels produced. Tubes with anode power dissipation over several hundred watts are usually actively cooled;

3078-456: The very commonly used low- mu 12AU7 and high-mu 12AX7 . The 12AT7 has somewhat lower voltage gain than the 12AX7, but higher transconductance and plate current, which makes it suitable for high-frequency applications. Originally the tube was intended for operation in VHF circuits, such as TV sets and FM tuners, as an oscillator /frequency converter, but it also found wide use in audio as

3135-424: The yellow curve on the graph). In a class-A triode amplifier, one might place an anode resistor (connected between the anode and the positive power supply). If we choose R a  = 10000 Ω, the voltage drop on it would be V + − V a = I a × R a = 22 V for the chosen anode current of I a = 2.2 mA. Thus we require a power supply voltage V + = 222 V in order to obtain V

3192-681: Was intended to amplify weak telephone signals. Starting in October 1906 De Forest patented a number of three-element tube designs by adding an electrode to the diode, which he called Audions , intended to be used as radio detectors. The one which became the design of the triode, in which the grid was located between the filament and plate, was patented January 29, 1907. Like the von Lieben vacuum tube, De Forest's Audions were incompletely evacuated and contained some gas at low pressure. von Lieben's vacuum tube did not see much development due to his death seven years after its invention, shortly before

3249-815: Was suited only to a limited range of audio frequencies - essentially voice frequencies. The triode was the first non-mechanical device to provide power gain at audio and radio frequencies, and made radio practical. Triodes are used for amplifiers and oscillators . Many types are used only at low to moderate frequency and power levels. Large water-cooled triodes may be used as the final amplifier in radio transmitters, with ratings of thousands of watts. Specialized types of triode ("lighthouse" tubes, with low capacitance between elements) provide useful gain at microwave frequencies. Vacuum tubes are obsolete in mass-marketed consumer electronics , having been overtaken by less expensive transistor-based solid-state devices. However, more recently, vacuum tubes have been making somewhat of

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