2A3

Around 1930, in the American radio-electronic industry, a demand arose for powerful vacuum triodes capable of delivering at least 10 W of output power in mode A to the load in push-pull switching ^[1] . This level was considered optimal for high-end household radios ^[1] . For reasons of cost and safety, the supply voltage of such an amplifier should not exceed 300 V ^[1] . The 45 series triodes most common at that time for this task were too weak: a ten-watt amplifier required four or six of these lamps ^[1] . Powerful triodes of series 50 and 10 required prohibitively high anode voltage and expensive output transformers ^[1] . Recently invented pentodes generated too many distortions , and the use of feedback to reduce them has not yet entered into practice ^[1] .

The solution to the problem was to be a promising development of RCA under the code designation A181C - a direct-glow triode with a low operating voltage of the anode and twice the power compared to lamp 45 ^[1] . The new lamp had a unique cathode design - 24 thin parallel filaments stretched across the suspension cross-members ^[1] . The electrostatic field of such a structure was much closer to the field of an ideal equipotentially plane than the field of four filaments of a traditional W-shaped cathode ^[1] . In practice, the production of a 24-filament cathode was so time-consuming that the developers had to reduce the number of filaments to twenty ^[1] . January 10, 1933, this simplified version went into series under the designation 2A3 ^[1] . By this time, the company, which had never before been engaged in powerful low-frequency triodes, had lost interest in them: the RCA advertising department focused on promoting the latest powerful pentodes, and independent distributors and competing manufacturers engaged in the sale of 2A3 ^[1] .

During 1933-1936, the RCA released at least three different versions of the first generation 2A3, which differed in the design of bearing traverses ^[1] . In 1936, the first generation 2A3 was replaced by another lamp under the same designation ^[1] . 2A3 of the second generation was actually a double triode - two series 45 triodes connected in parallel inside the cylinder ^[1] . Instead of expensive twenty-wire cathodes, conventional W-shaped cathodes were used ^[1] . This two-anode version became the main one for RCA ^[1] . Competitors from Raytheon launched a three-anode 2A3 series, but soon returned to the proven two-anode version ^[2] . Competitors from Western Electric back in 1932 released their own development - a 275A lamp ^[3] . This lamp with a voltage of 5 V, structurally close to the single-anode 2A3, was produced for only a few years and was used only in cinema sound systems Western Electric ^[3] . Another close analogue, developed independently of RCA, is the European single-anode triode AD1 with a voltage of 4 V ( Philips , 1935) ^[4] .

In the mid-1930s, industry switched to a unified glow voltage of 6.3 V ^[2] . Following the new standard, RCA released 6A3 - a modification of 2A3 for a 6.3 V filament voltage, and in the late 1930s its octal version 6B4G appeared, compatible at the level of the pinout with 6L6 beam tetrodes in triode switching ^[2] . A copy of the 6B4G, produced in the USSR , was first labeled 6B4, and since 1950, 6C4C ^[5] . They were produced in the USSR, but in much smaller series, and received the designations 2C6 and 6C6, exact copies of copies 2A3 and 6A3 ^[6] ). All lamps with six-volt incandescence differed from 2A3 in the higher level of network background, especially in single-cycle schemes ^[2] . RCA tried to solve this problem with the release of 6A5G - a modification of 6B4G with a hybrid glow cathode ^[2] . The filaments of this lamp, a traditional W-shaped configuration, were “dressed” in tubular cathodes; four cathodes were electrically connected to the midpoint of the filament and a separate pin of the cap ^[2] . 6A5G was too expensive and was not successful in the market ^[7] . The last constructive option 2A3 of the American development was the lamp of increased reliability 2A3W, or 5930, released in 1948 by Sylvania on the order of the Pentagon ^[2] .

At the end of the XX and at the beginning of the XXI century, small-scale production of 2A3 and its variants of increased capacity (2A3S, 2A3-40 and the like) were resumed by AVVT , Emission Labs (EML) and KR Enterprise in the Czech Republic , JJ Electronic in Slovakia , Sovtek companies and Electro-Harmonix in Russia and Shuguang in China ^{[8] [9] [10]} .

In the mid-1930s, the 2A3 was used in the output stages of most high-end American radios ^[2] . McMurdo Silver preferred to include 2A3 in mode B , and Ernst Humphrey Scott - exclusively in class A ^[2] . In the push-pull output stage of the 48-lamp Quaranta receiver, Scott used twelve 2A3 ^[2] . Not so extravagant, but practical cascades on 2A3 twos worked in a lot of jukeboxes Seeborg and Wurlitzer , and in the organs of Hammond 1935-1940 fours 2A3 were used ^[2] . However, in the late 1930s, the 6L6 beam tetrodes captured the output lamp market, and the use of 2A3 in household appliances ceased ^[2] .

Another area of ​​application of 2A3 is tube voltage stabilizers ^[7] . During the Second World War , despite the existence of specialized powerful 6AS7 triodes, it was 2A3 that was the most common pass-through (control) lamp of American stabilizers ^[7] . The military order supported the production of archaic straight bulbs and after the war, the supply of 2A3 to military depots continued until the mid-1980s ^[7] .

In civilian electronics of the first post-war decades, 2A3 was used only sporadically - in the ULF produced by Brook and Capehart ^[7] . In the markets of the USA and Europe, initially, pre-war beam tetrodes dominated, since the mid-1950s, the latest pentodes EL84 and EL34 developed in Europe, and in the 1960s transistors replaced lamps. The tradition of amplification on direct-burning triodes was not interrupted only in Japan , where a special school of “Japanese Hi-End ” was formed back in the 1960s ^[7] . A typical configuration for this school is a one-stroke VLF on 2A3, preferably a single-channel (first generation), loaded on a highly sensitive horn or on a Lowther single-band speaker ^[7] . Another local school connected with the Japanese through the mediation of the Franco-Japanese ULF designer Jean Hiraghi developed in the 1960s in France ^[12] .

In the USA, the revival of interest in tube sound in the 1980s bypassed direct-current triodes: Americans preferred powerful push-pull amplifiers based on beam tetrodes 6550 ^[12] . Straightforward triodes (primarily 300B, and secondly 2A3 and its analogs) "returned" to the American market only in the first half of the 1990s ^[12] . In 1992-1993, discussions began in audiophile magazines about the advantages and disadvantages of triode and pentode sound, and then - in 1992 - the supply of cheap ($ 14 apiece) Chinese 2A3 manufactured by Shuguang began ^[8] . A little later on the US market appeared 2A3 Russian production ^[8] . Paradoxically, the influx of cheap analogues only accelerated the prices of lamps of the 1930s, especially the first, single-anode generation ^[8] .

The modes recommended by the developers for operating in single-cycle (anode voltage of 250 V, anode current of 60 mA) and in the push-pull cascade (300 V, 40 mA) differ significantly ^[13] . The nominal modes of the push-pull cascade ensure the absence of the grid current during most (but not all) of the signal oscillation period and the almost complete subtraction of the second harmonics of the signal generated by the two shoulders of the cascade ^[13] . The maximum power reaches at control voltage amplitudes from ± 45 V (single-cycle stage) to ± 78 V (two-cycle stage with automatic bias) ^[13] . The load resistance values ​​given in the table were optimized for the maximum output power ^[13] and are not optimal from the point of view of nonlinear distortion and frequency response unevenness ^[14] . To reduce distortion, the load resistance is increased, for example, in a single-cycle version - from 2.5 to 8 kOhm ^[14] . The drop in maximum output power with such a replacement is relatively small ^[14] .

The connection of 2A3 grids with the pre-terminal cascade, according to the developers, is preferably transformer or throttle ^[13] . The high steepness of the characteristic (5.25 mA / V), according to the developers, presupposes separate adjustment of the bias in each lamp of the multi-tube cascade - either along the grid chains or along the cathode chains (which requires a separate filament winding for each lamp) ^[13] .

The safe working conditions given in the RCA documentation for different years of release differ markedly. In the 1934 reference book, the maximum allowable resistance in the grid circuit at a fixed bias is only 10 kOhm ^[13] , in the 1948 reference book - 50 kOhm ^[15] . The 1934 reference book allowed vertical and horizontal lamp arrangement as equal options ^[16] ; in the 1948 reference book, the vertical position is the main one, and the horizontal one is only permissible ^[15] . In both cases, horizontal installation is permissible only when the cathode plane is oriented vertically (otherwise sagging of the cathode and grid threads, up to a short circuit, is possible).

• Barbour, E. 2A3: The Mother of High Fidelity // Vacuum Tube Valley. - 1999. - No. 12. - P. 4-7.
• Barbour, E. History of the WE300B and its relatives // Vacuum Tube Valley. - 1996. - No. 3. - P. 8–12, 32–34.
• RCA Radiotron Cunningham Receiving Tube Manual. - Harrison, New Jersey: RCA Manufacturing Company, 1934. - Vol. RC-12. - (Radiotron Technical Series).
• RCA Receiving Tube Manual. - Harrison, New Jersey: Radio Corporation of America, 1948. - Vol. RC-15. - (Radiotron Technical Series).