![xUSSR GU-50 (ГУ-50) Tube](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg1pqTZIw5IOByaQv7ZfVpZzvFM7u2pRWk5VaMnatia23A8MBvw15ocrwWzibEAgai1BXVi1m4KNvePJav8-cMFFc6093HCwKXQR4NsyRra34lZpVxlEt2S47kDYEcro_UwFZ2k1roLErg/s1600/gu-50.jpg)
These power pentodes has been used as a transceiver final (one or pair of ones in parallel), in a modulator for the more powerful tube in AM transmitters, in a driver stages and (when SSB times comes) in the most common general class amplifier - three (or four) GU50 in parallel with all grids connected to the ground.
GU-50 is russian version of WWII Luftwaffe LS 50 general purpose HF power pentode by Telefunken, also known as RL 12 P 50 tube in Wermacht's transmitters. Main difference between russian and german versions - socket construction and dimentions, so I'm not quite sure that it is possible to use russian GU-50 as a spare part for a vintage german transmitter.
There are several other members in this family, such as DDR SRS 552 M tube, SL 152 (~12.6V filament) and EL 152 (~6.3V), but I don't know any more about these tubes, only that these tubes exists.