[mikethedruid]

Here are data sheets for both the type 27 http://frank.pocnet.net/sheets/021/2/27.pdf and type 37 http://frank.pocnet.net/sheets/021/3/37.pdf tubes. The main differences are that the type 37 uses a 6.3 V filament and the type 27 uses a 2.5 V heater. The type 37 also has higher output than the 27. Both tubes can be used on either AC or DC since they use a separate heater and cathode. On older sets with tubes which used the filament itself for the cathode, often one finds a resistor with a variable center tap, a potentiometer, used across the filament, and the cathode connection made to the center tap, where such a tube was to be powered by AC. The center tap was then adjusted to minimize "hum". Very soon though, when indirectly heated cathodes began to be used, with 5 pin bases, such as the 27 and 37, this was no longer necessary, and by merely changing to a 5 pin base, and appropriate re-wiring, one could convert a battery powered set to use AC for the filament supply, the biggest hog of electrical power, eliminating the need for bulky car batteries and battery chargers.