12AX7

Original 12AX7

The operational amplifier K2-W on a pair of lamps 12AX7 (1952) ^[ K. ^one]

In March 1948, a new miniature lamp appeared in the catalogs of the American companies RCA and - a double triode for industrial automation devices, which was designated 12AX7 ^[1] . 12AX7 was developed by RCA, and was contracted at Sylvania factories - this kind of separation of functions was the norm at that time ^[5] . Companies did not have any particular hopes for the new lamp: it was an outstanding, pass-through development, not even worthy of a note in the corporate RCA magazine ^[1] . The designers only combined two triodes in one cylinder, identical to the triode section of the previously released 6AV6 diode-triode ^[1] .

The combination of a high gain, a low noise level and a low level of a network background embedded in the 6AV6 design turned out to be in demand by the designers of high-quality sound equipment and analog computers ^[1] . It was on the 12AX7 pair that the classic operational amplifier of K2-W ^{[5] was built} . But the main factor behind the unexpected success of 12AX7 was the release of long-playing records and players with low-sensitivity magnetic pickups into the US market ^[1] . Manufacturers of mass equipment urgently needed an inexpensive lamp with a high gain, suitable for use in phono stage ^[1] . Demand grew so rapidly that in 1952, five corporations were already releasing 12AX7: CBS , GE , National Union ^{[K. 2]} , Sylvania and ^[1] .

By this time, the inherent problems of the new lamp had also become clear: cheap intra-tube fittings caused a high microphone effect , and the heterogeneity of wire winding inevitable in mass production caused high nonlinear distortions ^[1] . 12AX7 of the first years of release was noticeably inferior in sound quality to its octal predecessor 6SL7 ^[1] . Despite this, the 12AX7 replaced the “obsolete” 6SL7 and the latest mid-gain lamps from production lines, and by 1956 it became the de facto industry standard for American sound engineering ^[1] . The non-linearity of 12AX7 did not stop this: the designers of the 1950s were able to correct the flaws of the lamps using negative feedback . Therefore, 12AX7 found a place in the professional (tape recorders Ampex ), and in household equipment, and in guitar amplifiers ^[1] .

In the XXI century, 12AX7 and its European counterpart ECC83 - the most common lamps in the circuitry of guitar amplifiers ^[4] . The choice of manufacturers is determined by the economy (12AX7 is still available and therefore available at affordable prices) and conservatism: most modern amplifiers are built according to proven schemes of the 1950s ^[4] . Leo Fender and other designers of the lamp era chose 12AX7 not by chance: the characteristics of this particular lamp were ideally suited for systematic operation in the input overload mode and amplitude limits ^[4] . Excessive amplification 12AX7, on the one hand, made it possible to understaff the guitar amp with a timbral block ^[6] . On the other hand, it increased the subjectively perceived undistorted volume: the threshold beyond which the sound of the guitar “ fell apart” ( English breakup ) at 12AX7 came at much higher levels than its predecessor lamps ^[7] .

ECC83

In the early 1950s, Philips engineers drew attention to the 12AX7 ^[1] . Functionally, the lamp exactly corresponded to the needs of the company, which at that time aimed at conquering the domestic Hi-Fi market, but the quality of the mass American lamps of Europeans did not satisfy ^[1] . The European version of the 12AX7, designated ECC83, and its manufacturing process chain were redesigned by Philips. The lamp received a rigid load-bearing reinforcement, a low-noise spiral cathode heater, but most importantly, the Europeans were able to ensure “military” accuracy of winding grids in mass automated production ^[1] . European lamps had consistently the best non-linear distortion, and the service life of the best ECF83 variants produced by Telefunken exceeded 100 thousand hours unattainable for Americans ^[1] . By 1956, Telefunken lamps conquered not only the European, but also the American market of home audio: they were equipped with amplifiers Eico, Dynaco, Fisher and McIntosh ^[1] . In the late 1950s, high-quality ECC83 and E83CC ^{[k. 3]} production of ( Netherlands ), Mullard ( Great Britain ), ( France ), ( Italy ) ^[1] ; in the 1960s, Japanese ( Hitachi , Panasonic , Toshiba ) and Eastern European ( , Tungsram and others) enterprises engaged in the production of ECC83 and 12AX7 ^[1] .

The American companies could not offer import ECC83 a worthy replacement ^[1] . The improved 12AX7, released by RCA in 1958 under the designation 7025, differed from the base 12AX7 only in a lower level of network background with the same non-linear distortions ^[1] . The lamp found limited demand only in the market of guitar amplifiers ^[1] .

In the USSR, there was no exact analogue of ECC83 or 12AX7: their functional analog 6H2P , copied from the European lamp 6CC41 ^[8] , had a different pinout and different characteristics of the filament. 6Н2П differs from ECC83 in lower input capacity and the presence of an intersection screen between two triodes ^[9] . Grounding the screen reduces crosstalk from one 6H2P section to another by about 6 dB compared to ECC83 ^[9] .

Military and Industrial Series

Already in 1950, GE announced the first advanced analogue of 12AX7 - a high-reliability lamp 5751, which differed from 12AX7 by a lower gain (μ = 70) ^[1] (subsequently this lamp was produced in small series in the UK and Japan ^[1] ). Due to the high cost, 5751 was used only in military and industrial devices; only in the late 1970s with the filing of Conrad-Johnson 5751 entered the arsenal of designers of sound engineering ^[1] . Similar high-quality lamps of their own design with characteristic military designations (CV4004, M8137 and so on) were produced by British companies united in the cartel ^[5] . The release of these episodes ceased in the 1970s and no longer resumed; issue 5751 ended in the 1980s ^{[5] [1]} . The last and, possibly, the most perfect lamp of this series was the extremely rare American series 7729 (GE and CBS, 1960s), designed to work in differential amplifiers of measuring instruments ^[10] .

In 1955, Sylvania and CBS announced the launch of 12AD7, the new low-noise version of 12AX7 for mission-critical tasks ^[11] . The lamp was not in demand in the US and Western Europe, but was extremely successful in Japan ^[11] . Japanese-made 12AD7s, 12AX7 and ECC83 in the domestic market, were an indispensable component of Akai and Sony's 1960s tube technology. Outside of Japan, these lamps had a bad reputation, largely due to the low quality of Japanese mass equipment of that time ^[1] .

All of the listed 12AX7 and ECC83 variants suffered from a high microphone effect . The designers of Telefunken were able to solve this problem by changing the design of the control grid ^[5] . In ordinary lamps, the grid was wound on two vertical cross-beams of circular cross section; in the ECC803S lamp released in 1958, the grid was wound on a rigid stamped frame (supporting frame) made of molybdenum ^[5] . This technically perfect and expensive lamp, produced only at Telefunken factories, became a rarity already in the 1990s ^[5] .

Closing and resuming production

In the 1960s, the slow fading of the vacuum industry began. The first, back in the 1960s, the American Tung-Sol and CBS ^[1] left the game. Telefunken lamp quality has declined; the company began to sell under its own name products of other factories that differed from the original in high noise and high microphone effect ^[1] . Other European companies have also switched to reselling mediocre Japanese lamps; only Amperex and Mullard, as they could, maintained the quality of ECC83 until the 1980s ^[1] . The latter, in the late 1980s, curtailed production of GE, RCA and owned by Philips Sylvania ^[1] . Sophisticated automated equipment - entire plants designed to produce millions of lamps annually - was lost forever. It is only known for certain that the Mullard production line, on which the CV4004 military was produced, ended up in China ^[12] , and Amperex equipment - in Serbia ^[13] .

In the last quarter of the 20th century, the demand for 12AX7 and ECC83 was supported by millions of guitarists still using tube amplifiers. The exact size of the market is unknown; in 2000, it was estimated at at least one million lamps per year ^[13] . Until the mid-1990s, demand was satisfied from old stocks; the American market was swept by a wave of deliberately substandard lamps, pretending to be high-quality, and outright falsifications ^{[1] [2]} . Unscrupulous dealers marked with fake brands Amperex, Mullard and Telefunken all 12AX7 that they could get; as the American and West European stocks are exhausted, low-quality Japanese, East European, and even Indian lamps went into action ^{[1] [2]} .

By 1995, there were four 12AX7 / ECC83 production facilities left in the world: (Serbia), Sino (China), Tesla (Czech Republic), and the Russian Reflector plant ( Saratov ), which began production of three structurally different versions of 12AX7 on request American wholesalers ^{[1] [13]} . All these lamps were inferior to the West European ECC83: the Chinese ones had a short service life, the Serbian ones had an increased microphone effect, the Russian ones had an increased distortion like the old American 12AX7 ^[1] . By 2000, the Chinese plant stopped production, and the plant in Serbia, despite the international embargo , survived and managed to improve the quality of the lamps ^[13] . The Slovak company , which traded in the United States under the Tesla and Teslovak brands, managed to establish at the plant in Chadz not only the base 12AX7, but also an exact copy of the advanced ECC803S ^[13] . By the year 2000, the Kaluga Voskhod plant produced seven different 12AX7 variants by American orders, and in the same year, deliveries of 12AX7 “guitar” production by Svetlana began ^[3] . In the 2010s, Russian-made lamps were sold in the United States under the brands of local dealers and under the classic brands of Genalex Gold Lion ^[14] , Mullard ^[15] , Tung-Sol ^[16] .

Nominal Modes. Triode Parameters

12AX7 is a low-power triode designed solely to amplify low-frequency voltage. The reference documentation thoroughly considers two use cases: a voltage amplification cascade with automatic bias and a phase inverter on two triodes with cathode coupling ^[18] . In both versions, the 12AX7 anodes are loaded with resistance values ​​from 47 to 220 kOhm and are connected to the load through isolation capacitors. For the cathode follower circuit, 12AX7 is poorly suited due to low anode current values ^[19] .

The electrical characteristics of 12AX7, ECC83, 7025 and their full analogues, given by the manufacturers for the two rated modes , are completely identical ^[4] . Values ​​of maximum permissible voltages, currents and capacities may vary depending on the declaration system chosen by the manufacturer (absolute maximum values ^{[k. 4]} or average calculated limit values ^{[k. 5]} ):

The permissible variation in the parameters of the triode (S, μ, and Ri) was not indicated in the documentation for mass-series lamps ^[21] . In practice, it was assumed that for new lamps the permissible deviation of the gain μ is ± 10% (90 ... 110), and the permissible deviations of the slope S and internal resistance Ri are ± 20% ^[21] . Getting a new lamp in a five percent interval for all three parameters is a rare fortunate coincidence ^[21] . As the lamp ages, its steepness irreversibly decreases, and the internal resistance increases; only the gain μ is relatively stable ^[22] .

Select mode

The safe operation area 12AX7 is limited by the maximum allowable voltage at the anode (no more than 350 V) and the maximum allowable power dissipation at the anode (no more than 1 W) ^[17] . Operation at an anode current of less than 0.5 mA is undesirable due to the narrowing of the passband and the unpredictable growth of nonlinear distortions ^[23] . Work in the field of small negative biases (0 ... -1 V) is undesirable due to the flow of grid currents, which also exacerbates distortions ^[23] . In this region, 12AX7 differs disadvantageously from other double triodes by relatively large grid currents and extremely low (units of ohms) input impedance ^[24] . The operation of 12AX7 at positive biases was not standardized in principle ^[25] .

Due to these limitations, the range of possible 12AX7 operating modes is much narrower than the corresponding regions of triodes with an average voltage gain and a relatively wide opening of the current – ​​voltage characteristic ^[23] . Not all modes of this area are practicable: the most advantageous combination of high current, high voltage at the anode, and high resistance load from the point of view of noise and nonlinear distortion requires prohibitively high supply voltage ^[23] . Realizing the potential inherent in 12AX7 is not easy: a lamp requires careful selection of a mode that minimizes noise, nonlinear and frequency distortions ^[23] . Perhaps the opinions about its dissonance are explained precisely by the wrong choice of regime ^[23] . В действительности западноевропейская ECC83 — одна из лучших с точки нелинейных искажений ^[23] , пусть и уступающая в качестве звучания довоенной 6SN7 ^[26] .

Смещение лампы

Большинство каскадов усиления напряжения низкой частоты на лампах, подобных 12AX7, используют автоматическое (катодное) смещение ^[27] . Указанные в справочниках коэффициенты усиления предполагают шунтирование катодного сопротивления конденсатором . Без конденсатора коэффициент усиления каскада снижается примерно вдвое, при этом вследствие эффекта Миллера во столько же раз уменьшается его входная ёмкость, а локальная обратная связь уменьшает нелинейные искажения ^[28] . В серийных усилителях XXI века вместо катодных резисторов используют одиночные красные, жёлтые или зелёные светодиоды ^{[29] [к. 7]}. The LED practically does not affect the nonlinear distortion of the cascade, and due to the small internal resistance (tens of ohms) it does not need a shunt capacitor ^[30] .

12AX7 is also operational in the bias mode by a grid resistor ( grid ) ^[31] . If the cathode of the lamp is grounded, then part of the electrons emitted by it settles on the grid and flows to the ground through a grid resistor ^[32] . The potential of the grid drops below zero and reaches an equilibrium level, which for various instances of 12AX7 and a grid resistor of 10 MΩ is −0.8 ... −1.2 V ^{[33] [к. 8]} . This solution was widely used in early amplifiers, but was rejected due to the instability of lamp characteristics and increased distortion ^{[31] [23]} . In the 21st century, it is used extremely rarely and only in guitar amplifiers, for example, from ^[31] .

Nonlinear Distortion

The second harmonic dominates in the nonlinear distortions of any triode. With a fixed load resistance, the coefficient of the second harmonic is directly proportional to the amplitude of the signal at the anode; as the load resistance decreases, the coefficient of the second harmonic increases nonlinearly ^[35] . The best load from the point of view of distortion is a high-quality active stable current generator (GTS) on field-effect transistors or on a pentode with an internal resistance of the order of tens or hundreds of megohms ^[36] . According to Merlin Blenkow, with such a load the non-linear distortion coefficient of various 12AX7 at a signal voltage at the anode of 10 V rms. does not exceed 0.1% ^{[36] [K. 9]} . In this case, the cascade gain is maximum and equal to μ ^[23] .

Replacing the active load with a resistor leads to an increase in distortion and a decrease in the gain of the cascade. According to Vacuum Tube Valley magazine, with a load resistance of 240 kOhm, a supply voltage of 250 V and a signal voltage at the anode of 10 V eff. the second harmonic coefficient of various 12AX7 and ECC83 is 0.015 ... 0.2%, the third harmonic coefficient is 0 ... 0.02%, and the cascade gain is reduced to 48 ... 80 ^[38] . A further decrease in the load is accompanied by an increase in distortion, which can only be partially compensated by an increase in the supply voltage of the cascade ^[36] , and a decrease in the cascade coefficient (to 50 ... 63 at a load of 100 kΩ and 34 ... 44 at a load of 47 kΩ).

A recommendation widely used in historical literature to use an anode load of 100 kOhm goes back to the “golden rule” of matching a triode with a load: the output power of an ideal triode reaches its maximum when the load resistance is twice the internal lamp resistance (for 12AX7 - about 60 kOhm), at In this case, the voltage gain of the cascade is exactly 2/3 μ ^[35] . When the voltage is amplified , such a load loses to the active GTS in all respects, except for the input Miller capacity ^[35] .

There is no direct connection between the name of the manufacturer, the year of manufacture, and the level of distortion of a particular lamp: US-made lamps show stably mediocre performance, and inexpensive lamps of modern Russian production can outperform classic Mullard ^{[38] [39]} . The scatter of the characteristics of serial lamps was too large and remains ^[39] .

Bandwidth. Frequency Distortion

The bandwidth of the voltage amplification cascade at 12AX7 is limited from above, on the one hand, with a high input Miller capacitance in combination with the output impedance of the signal source, and on the other, with a high output resistance in combination with the load capacitance:

• The input Miller capacitance of the C _CX cascade is approximately equal to the product of the cascade gain by the passage capacitance of the triode C _SA ^[28] . The passport value of С _SA is equal to 1.5 ... 1.7 pF, but in actual operation, taking into account the parasitic capacitance of the panel lobes, С _SA is at least 3 pF ^[28] . Therefore, the Miller capacitance of a real, correctly mounted cascade with k = 60 is at least 200 pF ^[28] .
• The output impedance of the cascade at low frequencies, taking into account the load resistances used in practice, is from 20 to 100 kOhm. Such a high internal resistance makes the 12AX7 amplifier extremely sensitive to stray load capacitance ^[28] .

^{[to. 10]} the cutoff frequencies of both filters , the input and output, usually lie in the ultrasound region, but if the calculation and installation is unsuccessful, the circuits can shift down to the region of sound frequencies ^[28] .

A side effect of the high output impedance is the mediocre electrical isolation of the triode lamp sections. The attenuation of 1 kHz interference from the anode of one section to the anode of another section is approximately –73 dB; at a frequency of 20 kHz, the attenuation worsens to about –47 dB ^[9] .

Intrapump noises

The noise current of the anode of any triode is composed of two components: white , broadband shot noise — current fluctuations due to the finite electron charge , and low-frequency pink flicker noise due to local fluctuations in the work function at the oxide-vacuum interface ^{[41] [c. 11]} . The spectral density of shot noise is constant over the entire operating frequency range; the spectral density of flicker noise is inversely proportional to the frequency ^[43] . With increasing anode current, the density of flicker noise increases, and the density of shot noise decreases ^[ K. ^12] , while the frequency of the separation between regions in which this or that type of noise predominates shifts upward ^[44] . In typical 12AX7 operating modes, this frequency is of the order of 1 kHz ^[44] .

If we consider only shot noise, which is appropriate when designing RF devices, then 12AX7 with its low anode currents and low slope anode-grid characteristics hopelessly loses to triodes with high slope ^[40] . In the nominal operating mode (slope of the characteristic S = 1.2 ... 1.6 mA / V, cathode temperature 1000 K), the calculated noise resistance is 12AX7 R _Ш = 1.3 ... 1.8 kOhm , and the noise voltage in the strip brought to the input of the cascade frequencies 20 ... 20,000 Hz U _W = 0.66 ... 0.8 μV ^[45] - 2.5 times more than that of the triode ECC88 ( S = 12.5 mA / V, the Soviet counterpart - 6N23P ^[46] ).

In the sound range, the real difference in the noise of 12AX7 / ECC83 and ECC88 is not so great due to the lower flicker noise level of 12AX7 ^[40] . With an anode current of 2 mA optimal from the point of view of noise, the voltage of 12AX7 internal tube noise reduced to the input is minimal and equal to 0.7 μV; at lower and higher anode currents, the noise voltage increases to about 1 μV ^[40] . For the same lamp as part of the RIAA phono stage , amplifying low-frequency and attenuating high-frequency components of the signal, the optimal anode current is not more than 1 mA, with a weighted noise level of about 1.0 μV; exactly the same noise level is provided by the theoretically less noisy ECC88 ^[40] .

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