To be somewhat brief, common radio tube voltages are 1.5, 2.5, 5.0, 6.3, 12.6, 25, 35, 50, and 117, with 6.3 being probably the most common, though I could be wrong, because AC/DC sets are probably the most common, and they usually use 12.6, 35, and 50 volt tubes, though the earlier ones used 6.3 and 25 volt tubes. There are still other voltages than these, found in earlier radios, or in much later radios with FM, or in televisions where the tubes are strung in series. Also, while most 6.3, 12.6, 25, 35, and 50 volt tubes have standard amperages (and they aren't the same), there will be exceptions to the rule, and so, when dealing with series string sets, you can't plug in every 6.3 or 12.6 volt tube you come across, whether it is otherwise an electrical equivilant or not.

T.