For a time receiving and amplifying valves were named in this country "cathode" or "vacuum relay". The first Russian mass valve designed in 1918 at the Radio Laboratory of the town of Nizhny Novgorod under supervision of M.A.Bonch-Brouyevich (on the basis of the first model - "Babushka" tube) was named PR-1 ("vacuum relay, model no.1"). The name of the R-5 valve produced in 1922 by the Petrograd Electro-Vacuum Works meant: "relay, model no.5. A new valve with thoriated cathode produced in 1923 consuming a ten times less heating current than the "R-5", was named the "Micro". Equally economical of heating a two-grid valve with "cathode grid" was named "MDS" ("micro, two-grid").The first low-powered vacuum rectifier was denominated K2-T ("rectifier with two anodes and thoriated cathode").
By 1929 the number of models of radio valves increased very much, which caused the necessity of the introduction of a new integral system of their denomination. A 'letter and figure' system of marking was introduced, which remained till early 1940's. The first letter in the valve marking shows it's category: "P" - receiving, "U" - amplifying, "S" - special, "V" - rectifying, "T" - broadcasting, "N" - low-frequency. The second letter described the cathode - "T" - thoriated, "K" - carbonized, "B" - bariated, "O" - oxide. The figure included in the marking, which was usually a factory number of design, served for division of the valves of the same category. According to this system the valves models R-5, "Micro", MDS, PT-19, KT-2 were renominated as P-7, PT-2, ST-6, ST-19 and VT-14 (in Russian transcription).
In early 1930's a series of economical glass valves of 2 and 4V direct heating appeared (UB-107, UB-110, SB-154 etc.). The receivers using such valves were battery-supplied. From 1935 the so-called 4-V "super"-series of glass valves with indirect heating appeared , which was installed in the AC-supply direct-amplification radios (E4S, EKL) and in the first Soviet superheterodynes (TsRL). In 1931 the first home penthode SO-113 of this series was produced.
he main drawback of this marking system was that it defined the valves rather approximately. For instance, one and the same valve could be referred to the category of both receiving and amplifying valves. From the other side, the valves that strongly differ from one another, such as triodes, tetrodes with "cathode grid", tetrodes with a screening grid, AF penthodes and both kinds of RF penthodes ("varimu" and with short characteristics) were put into the same category ("C") of special valves. Produced in 1937 penthode model SO-183 and a double diode-penthode SO-193 were also included in the same category. Besides, the existing system of marking did not make it possible to define if a given valve was used in a battery radio or it had an indirect heating cathode.
In 1937 our electro-vacuum industry started mass production of quite a new type of valves. There were the tubes in metallic cases - 6A8, 6G7, 6J7, 6K7, 6L7, 6F5, 6F6, 6S5, 6X6, 5C4 (in Russian transcription) and also the glass electronic optical tuning indicator 6E5 - similar to the contemporary American tubes (6A8, 6Q7, 6J7, 6K7, 6L7, 6F5, 6F6, 6C5, 6H6, 5Z4, 6E5). The metallic valve 6L6 (Russian AF power tetrode) and the glass versions of the 5Z4 and 6L6 tubes were produced somewhat later and still later the cheaper glass analogues of many other metallic valves appeared. The marking system of all those valves was more precise than that of 1929. The name of the valve was shorter and its purpose and properties were defined more exactly.
We should note, that even the latter system of marking was not quite consistent. For instance, one and the same letter "F" was used to denominate a high-gain triode and an output AF penthode (6F5 and 6F6). Initially the figure taking the third place in the marking showed the number of outer terminals (including the filament and the outlet of the metallic case). That hindered denomination of new valves similar to the earlier produced ones with the same number of electrodes. And in 1940 a new project of marking of radio valves was worked out , which eliminated this drawback. The most parameters of the system were close to the international one. The first figure approximately showed the heating voltage, the letter in the second place described the basic purpose of the valve or its construction. The figure in the third place did not have a special meaning and then served for division of the valves of the same purpose and construction. For description of the valves' outer design (except the usual metallic ones) another letter was added: "C" - a glass valve of usual size, "M" - glass, mini-sized, "J" - glass, of "acorn" type, etc.
In accordance with the new marking system some valves (in particular, the glass mini-sized ones worked out in 1938) got a new marking. For instance, the valve model SB-242 should be marked 2A1M, the SO-241 - as 2K1M etc. But the new marking was not generally accepted because the radio works continued production of valves with the old marking. Only mini-sized valves of a later design (2K2M, 2J2M) were marked in a new way.
Because by the time there were still many radios in operation using valves of the old 4-Volt glass series our industry produced for some time valves substituting the former types. The names of substituting valves are the following: 4N4S (for SO-118), 4F5S (for SO-122), 4J5S (for SO-124). The inner framework of the tubes 6N7S (one triode), 6F6, 6J7 and 6K7 was used in the substitutes. The electric data (except heating voltage) and the base were the same as in old valves. Such valves are very rare now.
In 1946/47 the "metallic" series was extended by quite a number of new valves. In particular, the newly produced so called "single-base valves" appeared, which were similar to the American 6SA7, 6SK7, 6SQ7 and later - the 6SR7 and 6SJ7. They did not have a control grid terminal in the upper cap any more, but due to a special construction of the base the inter-electrode capacity did not increase at that. The letter "S" in the second position of the marking stands for "single-base". During the first years the valves were produced under their "native" marking, but later they were designated as 6A7, 6K3, 6G2, 6J8 (in Russian transcription). In the author's collection there are such valves with a Latin marking produced in late 1940's and a Soviet valve of the same period 6V6-GT - the ancestor of the well-known AF output tetrode 6P6S (in Russian). The "legendary" 6P3S valve had also undergone several modifications. Its history begins with the metallic 6L6 valve (Russian) of 1937 make. The first version of the home "in glass" 6L6 was no good because of overheating of the under-sized case. The valve 6P3 (Russian, made of glass, though not having the "S" at the end of the marking) produced in early 1950's did not show the required quality. Only in mid-1950's its construction was improved. That was the "classical" 6P3S already. It is produced even now with slight improvements and bears the marking of 6P3S-E and is a valve highly thought of among the audiophiles.
The two-anode direct heating 5C3S kenotrone has also a long history. Among its predecessors there are both the old German valve of the "numeral" series RGN1064 and our VO-166. Similar valves existed in the European "A"-series (AZ1), and also in the US (5U4G, RCA80).
When the standard GOST-5461-50 was introduced in this country, the system of marking of radio valves was put in order. The principles of classification mostly remained the same. The marking of new types of valves ("finger-shape" etc.) were already envisaged in the above-mentioned standard.

(c) V.Brousnikin