The most common radio to come out of the vacuum tube era was the All-American Five. Consisting of 5 tubes (thus the name,) the heater voltages of the tubes added up to 121 volts, and the radio could be operated directly from the AC line without a transformer - but could also be operated from a sufficient DC source as well as there was no power transformer. Before that all happened, however, battery powered sets were popular. Several types of tubes were designed for this, all with low filament voltages that could be operated off common dry cell batteries. But that was expensive, and as the country became electrified, more people demanded sets that could run on wall current. To meet this demand but still provide battery sets to those who wanted them, the 117Z3 was introduced. This tube performed as the DC power supply for a battery set when plugged in. Having a heater voltage of 117V, it didn't depend on any other part in the set to operate, but also didn't interfere when the set was used on battery. It was created as a fix to allow manufacturers to keep making what the did without changing anything else, but as electricity came to most places, the need for battery sets faded, along with the parts created to make those sets work without batteries. Now, this tube is an oddball, but still useful, you can use it to provide B+ (high voltage for the plate) on tubes but run the rest of the tubes on a DC power supply to reduce hum and other artifacts of AC voltage.