[Doug Houston]

Back in the mid-twenties, the only power amplifier was the 20. Amplifier, yes; power......well hardly any.

The 10 came along during the same period, and was able to kick out around a whole 1 1/2 watts! Then, around 1928, the amplifiers appeared that used 50's in place of the 10. Both 10 and 50, by the way, used 450 jolts to get full power out of them. Power supplies were very critical, as well you might imagine, what with those halacious voltages to filter. The HV winding for the RCA Radiola 64 measures 1500 VCT!

I have a couple of Victor's Electrolas, that used a 10 to drive the speaker. I am totally astonished at the output that that feeble 10 sent to those spearkers. They were the RCA "104" electro-dynamic jobs. They sounded almost hard to believe!

I have several sets that use 50's; around 21 of them. If you poke through some of the radio mags in the 192-33 period, there are PA amplifiers that use push-pull 50's, and claim around 40 watts output. No question, they're pushing those 50 jugs well into class "B"!.

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