I guess they're US made 833c's but I seem to recall the USSR copied them (GU48?) and also other types were copied or based on US and German tubes so in those instances have the benefit of being potential drop in replacements, albeit they may not be exact copies of equal rating so caution/research must be applied before subbing.
The hybrid mu-follower design helps the drive. I can easily get 200mA grid current to the 833 in A2. It only ventures out of A1 operation above about 40W thanks to the extreme anode voltage.When you look at that picture, its hard to believe a 6E5P can drive a transmitting tube...