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.

The 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.

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