[MrFixr55]

Some of the original Edison DC generators were in use until the 1940s. The NYC subway and electrified Long Island Rail Road branches still run on 600VDC. The NY Central (Metro North) and Amtrak run on 2.5KV AC I believe. The Waldorf Astroria lighting circuits were DC until a major renovation in the 1970s. It is interesting that (I believe) that some of the first "AC-DC" sets were more expensive than transformer powered sets. Obviously, with a heater string that could drop 15-120V without resistors increased reliability, and lowered cost. Sound output power and sensitivity both suffered in the "AA5" AC-DC Set when compared to transformer powered units.

The universality of the induction motor in consumer refrigerators (See GE "Monior Top" refrigerator) starting in the 19 teens and 1920s and the need to efficiently transmit power generated by hydroelectric dams almost required the conversion of power systems from DC to AC. Granted, toasters, light bulbs and most small appliances that used brush type motors could use either DC or AC. Load balancing of DC generation systems was difficult at best. The 2 major electric manufacturers were Westinghouse and Edison General Electric. Google "The Current Wars" for the full story. Nicolai Tesla, the inventor of the induction motor worked for Edison. When Edison reneged on a promised bonus, Tesla went to work for Westinghouse and helped develop the AC power transmission system as well as practical induction motors. Edison claimed that AC was more unsafe than DC and even electrocuted animals, including a rogue circus elephant, with Westinghouse alternators. These demonstrations caused New York to be the first state to use electrocution using AC as a humane (???!!!) way of executing condemned criminals. Westinghouse won the contract for the Niagra Falls Hydroelectric plant and the "Current Wars". Not the dropping of the name Edison from General Electric.

DC outlets and line cords were not "polarized" therefore the plug could be inserted so that the plate of the rectifier was connected to the negative pole, therefore not being able to pass current to the B supply. The tube heaters would light, but there would be no B+, therefore no sound.

Several late 1940s or early 1950s Philcos used a full wave voltage doubler rectifier, especially in their radio - phono combinations. The use of the voltage doubler limited the set to AC operation, even though there was no power transformer. However, this was not really an issue because the phono motor required AC. The level of performance for these sets was very near the performance of a 5 tube transformer powered set, especially when they used an 8" speaker.