[Arran]

A point of order, there were 32 volt DC powered sets, but I'm not aware of any 30 volt ones. You may ask why 32 volts was chosen, well apparently 32 volts DC was the standard for electric lights, and fans, on trains at one time, it was also easy to hook up four 8 volt lead acid storage batteries in series to be charged by a farm generating plant. I have a Canadian made Phonola, built by Dominion Electrohome, and apparently based on a Wells Gardner design as it does not use a #48 pentode power output tube, it uses a 6A6 driven by a #43. Other then the #43  the the other tubes are all 6.3 volt AC types in a series-parallel arrangement to add up to 32 volts. Unfortunately someone plugged the set into a 120 volt outlet, fortunately they are all common and relatively inexpensive tubes, and two actually survived this mishap, nothing else was damaged. Suffice it to say it will never have anything resembling an AC line cord with a NEMA 15 plug again. 
Regards
Arran