All American Five - Philosophy

Philosophy

The radio was called the "All American Five" because the design represented the majority of radios manufactured for home use in the USA and Canada in the tube era. 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 powerline, which was a dangerous electric shock hazard and required a thoroughly insulated cabinet; this was more serious with the higher European voltages, and many radios were fitted with transformers, increasing cost but eliminating these issues. 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.

The philosophy behind the design was simple: it had to be as cheap to make as possible. The design was optimized to provide good performance for the price. At least one radio manufacturer, Arthur Atwater Kent, preferred to go out of business rather than attempt to compete with 'midget' or low-cost AA5 designs.

Many design tricks were used to reduce production costs of the five-tube radio. The heaters of all the vacuum tubes had to be rated to use the same current, so they could be operated in series from line voltage. The rectifier and audio output tube required more heater power, so dropped a larger voltage than the other tubes. In many designs the rectifier tube had a tap on the heater to power a dial light. The plate current was routed through that portion of the rectifier heater, in order to make up for the current diverted to the dial lamp. If the dial lamp failed, that part of the rectifier heater would have a larger current which could burn out the tube in a few months. Early radios had a resistor network to minimize the problem but this was soon eliminated as the cost of replacing the tube was not the manufacturer's problem. As with Christmas tree lights, if one tube heater failed, none of the tube heaters would operate.

The radio used a half wave rectifier to produce about 160-170 volts of plate voltage directly from the AC power line; the rectifier, while not needed with a strictly DC supply, did not cause a problem.

The frequency mixer was of the pentagrid converter design to save the cost of a separate oscillator tube. The detector and first audio stage were provided by a dual diode/triode combination tube. When the detector/first audio tube contained a second diode, it could be used to provide automatic gain control (AGC), or AGC bias could be derived from the audio detector diode.