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 German (Wehrmacht) military radios captured during World War II, or based on documentation, machines and materials captured as a trophy. It is one of the more unusual types of tube 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). One stock Russian-produced socket includes a rugged die-cast metal cage-like enclosure for the tube with spring-loaded locking lid. (The cap was made for fixation of the tube in the metal can, since tubes were designed for mobile operations). Another stock Russian-produced socket is stamped of light aluminium sheet metal, without a lid on top.
In the past the tube was very popular with Soviet and Eastern European amateur radio operators because it was commonly available (from military and government warehouses) and could produce fairly large amounts of output power using relatively simple designs. Also, it was popular in Soviet era DIY audio amplifiers for hi-fi and musical instruments. GU-50 tubes were used also in early TV sets as a horizontal sweep output stage, and in medical devices as RF generators. The tube was never used in mass production amplifiers for civil usage because of relatively expensive technology and materials used.
During the Soviet years the tube was produced by one of the major Soviet transmitting-tube manufacturers in the city of Ulianovsk. It was also made in Poland, in the Unitra factory. 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 be available in the former Soviet Union and Eastern Europe and the type is far more readily found than the original LS-50.
GU-50 was not alone. There were other successors of LS-50 designed by Telefunken. Switzerland's BBC produced P50/1 and later modified version P50/2. They had the same base and pinout as the original LS-50. German Democratic Republic made a P50 that had the same bulb as Soviet ГУ-50. Later RFT made a P50-1 version of their tube. Unlike Soviet TV designers that used stock military GU-50 for their first TV sets, Eastern German manufacturers offered P50-2 especially made for TV horizontal sweep duties. There were also versions with simplified bulbs made for civil usage, but internals were still too expensive to compete with civil tubes like rugged clones of RCA 6L6 and especially designed later sweep tubes like EL500. The Western German version was called FL152 (or, with 6.3 volts heater, EL152); Eastern German version was called SRS552N.
The tube, despite its European popularity, was not well known in U.S.A. and Canada until audiophiles rediscovered it after the collapse of Eastern political block and Soviet Union.
The Chinese version of this tube is called FU-50 and production still continues, probably by Shuguang, the major Chinese tube builder.
The tube was originally designed for mobile operations, so it's filament draws only 0.705 A of current, while anode power dissipation allowed is up to 40w, quite big for such low filament power. It determines the need for high anode voltage, 800V typically. The screen grid is quite dense, so it's voltage when working in pentode mode is limited by 300V, otherwise power dissipated by the grid when anode voltage goes below screen grid voltage would damage it. That's why the tube can't be efficiently used in so called "Ultra - Linear" regime when anode and screen grid have the same voltage at idle. However, in triode mode anode voltage never goes below screen grid voltage, so 400-450V of B+ is acceptable.
The tube can work also in so called "Right handed triode" mode, or "High Mu" mode, when all 3 grids are connected together and used with positive bias. In such regime B+ voltage can be up to 1200V, and anode curves resemble pentode ones, with higher than in the "left handed" triode mode impedance.
Anode of the tube is made of a special nickel alloy, specially coated, so it can shortly dissipate more than 100W on anode without destruction. However, such regime causes too high temperature to be used continuously, but instead of destroying the tube like it can be expected in case of so-called "Audio Tubes" like 6L6 and successors, it causes absorption of gases making vacuum cleaner.
-  - GU - 50 with red anode, run away due to gases; after that works perfectly.
- - GU-50 pinout and operating characteristics (in German and English).
-  - GU-50 pinout and operating characteristics (in Russian).
- - GU-50 PDF datasheet (in Russian and English).
-  - GU-50 curves from Radio #9 1961 magazine.
-  - GU-50 at radiomuseum.org.
-  - LS-50 at radiomuseum.org.
-  EL-152 tube (modified LS-50)
-  Original LS-50
-  Picture of recent FU50 (china)
-  - a prototype of High-end stereo amplifier using GU-50 output tubes
-  - an audio amplifier using GU-50 as an output tube (in German.)
-  - Russian/Soviet tube manufacturers and their logos.
-  - HF Amplifier with single GU50 in military rugged socket and shield.
-  - GU50 Old type and new type comparison.