The 300B has justly won the reputation for being a superb output tube in single-ended amplifiers. Of course, it actually has a much wider application, such as a push-pull amplifier's output tube and possibly as a headphone amplifier tube or super-buffed line stage tube. But it will take some rethinking to imagine this great tube in a non-audio role. It is sort of like trying to imagine Arnold Schwarzenegger playing Goethe: sure he has the accent, but are the biceps really necessary or even desirable?
The pluses the 300B brings to a voltage regulator is its ability to pass a fair amount of current at low cathode-to-plate voltages, the result of having a low rp. And its 40 watt plate dissipation limit does not hurt either. An advantage few would highlight, however, is its 5 volt heater specification. Why would this count as an advantage? Most high voltage power transformers retain a 5 vac winding for heating a tube rectifier. Rectifiers with rare exception require a floating 5 volt power supply. Had rectifiers used the same nominal 6.3 volts that most tubes use, many would be tempted to attach the rectifier's heater to the common 6.3 vac winding that is referenced to ground. This would spell catastrophe, as the isolation material the separates the heater from the cathode cannot withstand more than 100 volts in most tubes; in fact, most indirectly heated rectifiers have the one leg of the heater element connected to the cathode and thus have a 0 volt heater-to-cathode voltage limit. And those rectifiers that use directly heated cathodes would obviously also short the high-voltage to ground! Choosing 5 volts for rectifiers was a wise decision. Coincidentally, the 300B uses a 5 volt heater. This heater voltage allows us to use the often free 5 vac winding, free because solid-state rectifiers were used instead. (Solid-state rectifiers result in a higher power supply voltage and a much low power supply impedance. High speed rectifiers do perform much better than the slower variety and are heartedly recommended.)