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Since 1929, the SK-3, SK-4, SK-5 and SK-6 radio sets have been produced by the Leningrad artel Standard-Radio since 1929. Superheterodyne 3-tube radio-type "SK-3" is designed to receive from an outdoor antenna and grounding of broadcasting stations in the range of 350 ... 2000 meters to headphones. The SK-4 radio repeats the SK-3 receiver, but with the addition of the 4th lamp for low-frequency amplification and reception of broadcasting stations with medium loudness on the speaker. The SK-5 radio receiver is assembled on 6 radio tubes, repeats the SK-3 receiver circuit, but with the addition of 2 stages at a low frequency, one lamp in the preamplifier and 2 in the two-stroke output stage. The radio provides a loud reception of broadcasting stations on the speaker. The SK-6 radio receiver repeats the SK-5 radio receiver scheme but with the construction of an amplifier output stage according to the "push-bullets" scheme, which allows the receiver to be loaded for 3-4 Record type loudspeakers and providing a loud-speaking reception. A little more detail about the described receivers of the "SK" series in the brochure of the TFR SK for 1931. Unfortunately, there are no pictures or photos of the appearance of the radio!