All American Five Radio Schematic
Chanelle Glugla
The term All American Five (abbreviated AA5) is a colloquial name for mass-produced, superheterodyne radio receivers that used five vacuum tubes in their design. These radio sets were designed to receive amplitude modulation (AM) broadcasts in the medium wave band, and were manufactured in the United States from the mid-1930s until the early 1960s.[1][2] By eliminating a power transformer, cost of the units was kept low; the same principle was later applied to television receivers. Variations in the design for lower cost, shortwave bands, better performance or special power supplies existed, although many sets used an identical set of vacuum tubes.
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 very first set of metal tubes produced included 6-volt heater tubes that could be used to make a transformer-powered 6-tube radio. RCA released their first set of these metal octal tubes for this design in 1939, using 12.6-volt 150 mA heaters instead. The original design used the following tubes:
These sets were first marketed in late 1939. Canadian sets would sometimes use a 35L6 in place of the 50L6, as parts of Canada used 110 volts as a design standard. Because areas near Niagara Falls had 25 Hz power, some Canadian sets had slightly larger filter capacitors.
The 50C5, introduced in 1948, is electrically identical to the 50B5, but has a revised pinout to address concerns that high peak voltage between 4 (heater) and 5 (anode) would promote socket breakdown.[1]
A number of other versions of the set appeared, including some that did have a transformer, a version that operated in a motor vehicle off a 6-volt supply, using a vibrator to convert the 6V DC supply to AC which could feed a transformer with higher voltage output, and a version that operated from either dry batteries or the mains supply. The battery version commonly used tubes where the filament was heated by a single 1.5-volt dry cell and plate voltage was supplied by a (nominally) 90-volt battery.
When operating on batteries, this version had almost instant warmup because of the tubes used their filaments as cathodes.This setup was common on Motorola portable radios commonly resembling metal "lunch boxes".
There were even a few "AA4" designs, usually midget sets, only usable in strong-signal metropolitan areas, because most had no IF amplifier (although some replaced the rectifier tube with a selenium rectifier).
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:
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.
In bygone days, before all of the computerized wonders of modern technology, one of the favorite gifts that one could receive during the holiday season was a shiny new radio. The recipient's age was of no matter, it could be a bright red Catalin table radio for grandma, a Snow White radio for little Susie, or a Lone Ranger set for little Johnny.
By the late 1930s, radios had become relatively small, lightweight and reliable. Although the grand console floor model, with its magnificent sound and often the ability to receive international shortwave broadcasts would remain popular until World War II, the little table sets were rapidly gaining popularity. They were often purchased as second sets, for the convenience of having a radio on the breakfast table, while the big console remained in the living room. Battery portables had their own following with those who wanted to remain in the know while on the go.
But what technology made all this possible? Several factors contributed, including the trend towards miniaturization of tubes and other components, but primarily a circuit, which would become known as the "All American Five," because it gave good performance with just five tubes. Prior to its development, radios often contained seven to ten vacuum tubes, more in some cases, and all of the associated components needed to make them work. Edwin Armstrong's heterodyne circuit gave the radio the sensitivity and selectivity to function with a small built-in antenna, but it was the development of a specialized tube known as a pentagrid converter that allowed engineers and manufacturers to greatly reduce the size of the sets.
The tube takes the place of three conventional tubes, and I'll try to describe how it works in layman's terms shortly, but first a bit of its colorful history. Although a number of highly specialized vacuum tubes were developed over time, the pentagrid converter probably had the most noticeable impact from a consumer's standpoint, and by far was the most heavily produced specialty tube ever made. Millions of radios would be built using it. One notable thing about the pentagrid converter is that it evolved, rather than was invented. Donald G. Haines, an engineer with RCA applied for a patent for the tube in 1933, but variations of the tube were already on the market.
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