All American Five

The radio was called the "All American Five" because the design typically used five vacuum tubes, and comprised 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 requiring 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.[4]

The philosophy of 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.[5]

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 a plate voltage of 160 to 170 volts 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.[6]

Many early examples of the 'All-American Five' posed a shock hazard to users. Lacking a mains transformer, the chassis of the AA5 radio was directly connected to one side of the mains electric supply. The hazard was made worse because the on/off switch was often in the wire of the mains supply which was connected to the chassis, meaning that the chassis could be "hot" when the set was either 'on' or 'off', depending on which way the plug was inserted in the power outlet. Many power plugs had two identical pins, and could be plugged in either way round. The metal chassis securing screws were sometimes accessible from the outside of the Bakelite or wood case, and there were many examples of owners receiving a shock by making contact with these screws while handling a set. Ventilation holes could be large enough to allow children to poke their fingers, or metal objects, through. The same type of hazard was present in European AC/DC sets, at twice the voltage.

The hazard was eliminated from later sets by the use of an internal ground bus connected to the chassis by an isolation network. Underwriters Laboratories required the adoption of the floating chassis, as isolation from the mains (the exact circuit and component values were not specified although the leakage current allowed was) to limit the shock to a "safe" current level. The chassis was maintained at RF ground (for shielding) by a bypass capacitor (typically 0.05 μF to 0.2 μF) usually with a resistor connected across it (typically 220 kΩ to 470 kΩ, although values as small as 22 kΩ were sometimes used or the resistor was simply omitted).[7][8] Over the years, these paper capacitors often become leaky, and could allow sufficient current flow to give the user a shock.

Although four-, six-, and even a few rare eight-tube radios were produced, they were not common. The four-tube version with vacuum tube rectifier was of inferior performance, as they typically had no IF amplifier tube, although some four-tube designs with a selenium rectifier in place of the rectifier tube avoided this problem. The six-tube versions added either an RF amplifier tube, a push-pull audio power amplifier tube, or a beat frequency oscillator tube (to listen to Morse code or single-sideband modulation transmissions). However, these radios cost significantly more and sold in smaller quantities. The eight-tube versions cost even more, adding two or more of the features of the six-tube versions and sometimes an extra IF amplifier tube.

The basic design of the 'All-American Five' had its origins in low-cost sets produced in the early days of radio.

According to various editions of the RCA Receiving Tube Manual, the heater string of an AC/DC radio should be arranged in a particular order to minimize hum. Assuming that all functions are performed by separate tubes, the heaters in the string should be arranged as follows:

1. Input stage
2. Ballast tube or resistor
3. Rectifier
4. Audio power output amplifier
5. RF and IF amplifiers
6. Converter
7. First AF amplifier
8. Detector
9. Ground/B-minus line

Not all manufacturers followed this recommendation.

Many black-and-white and color television receivers were built using All American Five principles, including a hot chassis and series-wired heaters. The designs were found primarily in portable or inexpensive sets ranging from the 1950s to even as late as the GE Portacolor series which was finally discontinued in the 1980s. Early sets tended to use selenium rectifiers in place of a tube; later sets used silicon diodes. Some of these sets were hybrid, using transistors for small signal applications and vacuum tubes in place of then-expensive power transistors. Some also included a rectifier diode in series with the tube filaments; when the set was off, the rectifier kept the filaments partially heated, a technique given a variety of names such as "Instant On".

Since the chassis of the set may be connected directly to the live side of the power line, service shops used an isolation transformer to protect technicians from a shock hazard. Some restorers will rewire the hot chassis set to put the chassis at neutral at all times. Some designs only require polarizing the plug, while others require rewiring the power supply to remove the switch from chassis ground. Power outlets must be wired properly for this modification to be protective.

• Utility Radio
• Volksempfänger

• The All American Five
• Arcane Radio Trivia AA5 Article
• A Review of Developments in Broadcast Receivers of 1933, Radio Engineering (magazine), August 1933, pages 6, 7, 20