They were manufactured in the millions by hundreds of manufacturers from the 1930s onward, with the last examples being made in Japan. The heaters of the tubes were connected in series, all consequently drawing the same current, but with different voltages across them. The standard line up of tubes were designed so that the total rated voltage of the five tubes was 121 volts, slightly more than the electricity supply voltage of 110-117V. An extra dropper resistor was therefore not required. Transformerless designs had a metal chassis connected to one side of the power line, which was a dangerous electric shock hazard and required a thoroughly insulated cabinet. Transformerless radios could be powered by either AC or DC (consequently called AC/DC receivers)—DC supplies were still not uncommon. When operated on DC, they would only work if the plug was inserted with the correct polarity. Also, if run from a DC supply the radio had a reduced performance because the B+ voltage would only be 120 volts compared with 160-170 volts when operated from AC.