I will start with 50 type tubes. These are usually ST shape 1940s. The in demand 1930 Globe types have the numbers, 150/Arcturus. UX250/RCA. UX350/Cunningham. 450/Deforest.
These are in my opinion the best sounding tube ever made of American design. The most naturally real sounding tube i have ever heard. You don't hear a tube, you hear music. So good, as to be on a planet of there own, old school design (like pre 1935) tubes. These tubes where made for the expensive elite amps and radios of the day. Some are just brilliant, others just great so you need to get the right ones. I have about 50 of these tubes.
They have a 7.5 filament voltage and will take 450V. Output is about 4.6 watts
I do not run them hard, 300v or less (around 3 watt). With this voltage they will last longer and sound better (and a big plus) will sound more relaxed. Rule is, run them only as hard as the watts needed.
Back in the 1930s there where many RCAs manufacturing plants and you can tell them by the getter type and its placement.
RCA 250 Globe hat getter. This tube type is sweet and airy, the biggest sound-stage in the RCA range, can be a little brighter than the other RCAs with a little less timbre, good on classical.
RCA 250 Globe side foil getter. This tube type is a good all round performer
RCA 250 Globe small foil getter. This tube type is very good, it has it all in moderation, good timbre, good sound-stage and is not white or bright, great on classical
RCA 250 Globe large foil getter. This tube type is darker sounding, has tremendous timbre, so real, solid flesh on bone, but a smaller sound-stage than the others, Great on male vocals, guitar piano, most important it is not white or bright at all. Maybe the best RCA 250 ever made, rarer than most.
Cunningham UX350 Globe , This tube is exactly the same to look at as the RCAs, but is may as quite as good as the best RCA versions. More romantic less detail. But it it depends on the getter type. I believe they are made in the same factory as the RCA versions by RCA, so they will probably sound the same.
Deforest 450 Globe, This tube lush very romantic, A huge sound stage. Some will say less accurate than the RCAs.
Most later 1940 ST 50 shape tubes are very good, more on these and others later.
Below is a pair of the large foil getter RCA UX 250. And the best sounding 250 ever made. A new made in Holland Phillips PH250 globe 1920-30s