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GU-50 produced in 1977 at the Ulianovsk factory

The GU-50, (Russian: ГУ-50) is a power pentode vacuum tube intended for 50 Watt operation as a linear RF amplifier on frequencies up to 120 MHz. It is in fact a Soviet-produced copy of the Telefunken LS-50 power pentode, possibly reverse-engineered from the German (Wermacht) military radios captured during World War II. It is one of the more unusual types of tubes because of its non-standard 8-pin base and a metal "cap" (not connected to any of the tube internal elements) with a plastic "handle" on top of the envelope - which is meant to ease extracting the tube from its socket (especially when the tube is hot). A stock Russian-produced socket includes a rugged die-cast metal cage-like enclosure for the tube, which effectively grounds the top metal "cap" during normal operation. If the "cap" is left electrically floating with respect to ground, this may introduce undesired feedback paths during RF operation at some of the higher frequencies.

In the past the tube was very popular with Soviet and Eastern European amateur radio operators because it was commonly available and allowed to produce fairly large amounts of output power using relatively simple designs.

Although the tube was not originally designed for the purpose, recently some valve sound enthusiasts have used GU-50 in hi-fi tube audio, both in Single-Ended Triode and Push-pull amplifier desings. There were also a few Public Address (PA) amplifiers designed in the Soviet Union, using GU-50 as an output tube, where output power and not sound quality was primarily the issue.

During the Soviet years the tube was produced by one of the major Soviet transmitting tubes manufacturers in the city of Ulianovsk. At this time it is unclear if current production of the tube exists, however, large quantities of New Old Stock (NOS) GU-50s appear to exist in the former Soviet Union and Eastern Europe and the type is far more commonly available, than the original LS-50 tube.

[1]- GU-50 pinout and operating characteristics (in German and English).

[2] - GU-50 pinout and operating characteristics (in Russian).

• [3] - GU-50 at radiomuseum.org.
• [4] - LS-50 at radiomuseum.org.
• [5] - Russian/Soviet tube manufacturers and their logos.
• [6] - an audio amplifier using GU-50 as an output tube (in German.)