[Arran]

I don't think that the type #50 tubes were that common to begin with, they had a rather short reign between the decline of the 71As in popularity and the advent of the type #45s. I know of hundreds of sets that used 71As, thousands that used a pair of #45s, but maybe a dozen models that used type 50s, RCA, Atwater Kent, Sparton, and Majestic, all top end models from 1929-31. Then after that the 45s, then the 2A3s, and 47/2A5s came about and then you're into the 6.3 volt types, which lasted until the end of the tube era
Part of the reason may be that to get the performance out of a type #50 tube you need about 400-450 VDC on the plates (especially a single ended one like soem RCAs used), which means you need close to 1 kv ac coming out of the high voltage winding, so that means a large and expensive power supply and transformer. They also needed a 7.5 volt winding, not really a big deal since it was common to use a different filament winding for the power output tube or tubes from the rest of the tubes in that era. Compare that to it's contemporary the type #45 which only needed 250 vdc, they didn't produce the same power as a pair of #50s but they were a more efficient tube.
Apparently they are making type #50s again, and with a balloon envelope, although I think anyone thinking of building an audio amp with even a new production type #50 is crazy. Sure they were the top tube to use in 1929, but there are lots of other better and less expensive tubes to experiment with, even triodes. There were valid reasons why the manufacturers stopped using them in radios, anyone who has owned a 1929 RCA with a smoked power transformer can explain it to you.
Regards
Arran