Peter,
thanks for you information again!
GU 50 is Russian transmitting tube , same as the Western SRS552 , LS50 or P50/2, in China,
Chinese made was called FU-50,it was used in some Chinese aircraft and tank radio set.
73! Hsu
And to me, a GU50 is an old British made mercury vapour rectifier tube, with performance similar to an 866A!
73
Peter G3RZP