WLW's 500,000-watt "RCA 1" transmitter was in operation between 1934 and 1939, and was the highest power ever used in the United States for public, domestic radio broadcasting. It was designed as an amplifier of the regular 50 kW transmitter. It operated in class C with high-level plate modulation, and required two dedicated 33 kilovolt electrical substation lines and a large cooling pond complete with spray fountains. It operated with a power input of about 750 kW (plus another 400 kW of audio for the modulator) to produce 500 kW. Even after 1939, when regular WLW programming was prohibited from operating with more than 50,000 watts, the station continued post midnight high-powered operation as experimental station W8XO, which helped to greatly improve the RCA 1 transmitter's power and reliability. By the end of World War II, it was capable of producing one million watts, and it "loafed along" at 600 kW.