All American Five
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The All American Five was a mass-produced, superheterodyne radio receiver with five vacuum tubes first designed and produced in the USA in the 1930s.
The radio was called the "All American Five" because the design represented the majority of radios manufactured for home use in the USA in the tube era. The design was also popular in Canada, with the last examples made being inexpensive Japanese units. Fewer examples were used in Europe as the 220 volt mains supply meant extra work for designers in providing filament voltage. They were manufactured in the millions by hundreds of manufacturers from the 1930s all the way through to the end of the vacuum tube era.
The philosophy behind the design was simple: it had to be as cost-effective to make as possible. The design was optimised 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.[1] AA5 designs were also called AC/DC receivers because they could operate on DC mains (which was still in use in several areas of the USA when the design was developed) voltage as well as AC, though this was incidental to the design objective. Of course, when operated on DC, they would only work if the plug was in the correct polarity.
Many design tricks were used to reduce production costs of the five-tube radio. The heaters of the vacuum tubes were all rated to use the same current, so they could be operated in series from line voltage. The more-powerful tubes requiring more-powerful heaters (the rectifier and audio output tube) dropped proportionally more of the line voltage across their heaters. The rectifier tube had a tap on the heater to accommodate the dial light. The plate current was often routed through that portion of the rectifier heater, in order to eliminate the need for a separate fuse. As with Christmas tree lights, if one tube heater failed, the entire heater circuit became inoperable.
The radio used a simple half wave rectifier to produce about 160-170 volts of plate voltage. This simple rectifier coupled with the wiring of the heaters in series eliminated the mains transformer.
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 combined with a dual Diode/Triode combination tube. One diode section sometimes provided a separate automatic volume control function.
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[edit] Potential hazards of the design
Many early examples of the 'All-American Five' posed a shock hazard to their owners. Lacking a mains transformer, the chassis of the AAF 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 connected to the chassis, This meant 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. The metal chassis securing screws were often 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.
After public outcry and several sensational press accounts of the problem, the hazard was eliminated from later sets by the use of an internal ground buss 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 specified) 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). See here for a typical schematic of a 1948 model AC/DC radio with a 220K isolation resistor.[2]
Over the years, these paper capacitors often become leaky, and may allow sufficient current flow to give the user a shock. When repairing these sets, one should be careful to connect them to the line so the chassis is connected to the neutral side, or alternatively to use an isolation transformer.
[edit] Variations on the theme
Although four-, six-, and even a few rare eight-tube radios were also produced, they were not common. The four-tube version was of inferior performance, as they might have had no IF amplifier tube, although some designs used a selenium rectifier in place of the rectifier tube. The six-tube versions added either a RF amplifier tube, a push-pull audio power amplifier tube, or a beat frequency oscillator tube (to allow one to listen to Morse code or single-sideband modulation). However, these radios cost sufficiently more that they did not sell well. 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.
| # Tubes | RF Amp | BFO | Converter | IF Amp | Det/Pre-amp | Audio Amp | Rectifier |
|---|---|---|---|---|---|---|---|
| 4 | X | X | X | X | |||
| 4 | X | X | X | X | (selenium) | ||
| 5 (standard) | X | X | X | X | X | ||
| 6 | X | X | X | X | X | X | |
| 6 | X | X | X | X X (push-pull) | X | ||
| 6 | X | X | X | X | X | X | |
| 8 | X | X | X | X X (push-pull) | X X (push-pull) | X | |
| 8 | X | X | X X | X | X X (push-pull) | X | |
| 8 | X | X | X | X X (push-pull) | X X (push-pull) | X | |
| 8 | X | X | X | X | X | X X (push-pull) | X |
[edit] Specific implementations
[edit] The original
The original design was a gradual evolution from the early days of Radio. The basic design of the 'All-American Five' had its origins in low-cost sets produced in the early days of radio. 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 metal octal tubes for this design in 1939. The original design comprised the following tube set:
- Converter: 12A8
- IF amplifier: 12K7
- Detector and first audio amplifier: 12Q7
- Audio power output: 50L6
- Rectifier: 35Z4.
This series had the grids brought out as top caps on the signal tubes, and the 35Z4 did not have a provision for a dial light.
[edit] The most popular
The tube array in the early days of the Octal tubes was:
- Converter: 12SA7
- IF amplifier: 12SK7
- Detector and first audio amplifier: 12SQ7
- Audio power output: 50L6
- Rectifier: 35Z5
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 part of the area near Niagara Falls was provided with 25 Hz power, some Canadian sets had slightly larger filter capacitors.
[edit] The "Loctal" variant
The tube line up of the Loctal tubes was:
- Converter: 14Q7
- IF amplifier: 14A7
- Detector and first audio amplifier: 14B6
- Audio power output: 50A5
- Rectifier: 35Y4 or 35Z3
[edit] The redesign using miniature tubes
After the Second World War the set was redesigned to use miniature 7-pin tubes and the line up became:
- Converter: 12BE6
- IF amplifier: 12BA6
- Detector and first audio amplifier: 12AV6 or 12AT6
- Audio power output: 50C5 or the less-common 50B5
- Rectifier: 35W4
The 50C5 was introduced in 1948, to address concerns that the set might pose more of a shock hazard to the user if the 35W4 and 50B5 were to be accidentally interchanged. The 50C5 is a 50B5 with a different pin-out.
In the postwar period, some makers built sets with a mixture of miniature and some octal tubes.
[edit] The 'Power-Saver version
Another rare variation, by changing the tube heaters to run on 100 milliamperes rather than 150 milliamperes, a low-power variation was produced. Unfortunately the lower power also resulted in a slightly longer warm-up time:
- Converter: 18FX6
- IF amplifier: 18FW6
- Detector and first audio amplifier: 18FY6
- Audio power output: 34GD5
- Rectifier: 36AM3
Note: that how even though the voltage distribution has changed around the tube heaters, the total is still a little more than the 120 volt mains supply (the leading numbers are the heater voltage).
[edit] Variations
Since the AA5 was a minimalist design, there was plenty of room for enhanced versions, resulting in an "AA6":
- A few sets added an extra 12SK7 as an RF or IF amplifier. This would require using a 35L6 to maintain the heater voltage.
- Or another tube could be added to the other end for increased audio output. In order to keep the heater voltages consistent, the two output tubes would have to be 25 to 35-volt types, such as the 35L6 or 25L5.
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).
To reduce costs further sometimes a pair of 12AU6 pentodes was used, one as an autodyne converter (instead of a pentagrid converter) and the other as an audio preamplifier.
[edit] Other variants
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 and even 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.
One version, called a Three-way portable because it could be operated any of three ways: batteries, the AC line, or the DC line; typically had the following tube array:
- Converter: 1R5
- IF amplifier: 1U4
- Detector and first audio amplifier: 1U5
- Audio power output: 3V4
- Rectifier: 35W4, 117Z3, or a selenium rectifier
This version used a 7.5 V A battery and a 90 V B battery. Note that the A battery did not need to heat the rectifier tube because, when operating from the batteries, the rectifier was not needed.
When operating on batteries, this version had almost instant warmup because of the tubes used their filaments as cathodes.
[edit] Farm radio
There was also the "farm radio" modification (usually done at the point of sale) that allowed an AA5 to run off 28 volt DC, commonly generated by farm windmills. With a relatively simple rewiring the tubes could be put in series-parallel to run off 28 volts – the two twelve volt heaters in series and a 25L6 or 35L6 in parallel. The tube heaters were flexible enough to still run with the voltages somewhat out of specifications. But with only 28 volts on the plates, the tubes would just barely amplify, just enough audio for listening in a relatively quiet room.[citation needed]
[edit] Effect on television design
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 completely 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—these sets were often sold as "Instant On" or "Insta-Play" or any number of similar variations.
During servicing, absolutely all All American Five radios (or AA5-inspired TV sets) must be operated using an isolation transformer. Some restorers will rewire the hot chassis set to put the chassis at neutral at all times. Some TV 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 work.
Even today many solid-state televisions and other electronic equipment using two-prong plugs use the floating chassis design, even though they may use switching power supplies. Many TV sets use bridge rectifiers off the line, thus resulting in a "hot" internal ground no matter which way the power plug is plugged in. These definitely require the use of an isolation transformer. You can observe this if running your finger along one of the television rabbit ears antenna causes an electrical tingle in your finger in one orientation of the plug in the outlet.
[edit] External links
- History of the AA5 (All American 5ive) AM tube radio
- The All American Five
- Arcane Radio Triva AA5 Article
[edit] Notes
- ^ Douglas, Alan, Radio Manufacturers of the 1920s (Vol. 1) Vestal, New York: Vestal Press, Ltd. (1988); Schiffer, Michael, The Portable Radio In American Life, Tucson: Univ. of Ariz. Press (1991)
- ^ In older schematics, "M" was used to indicate "thousand" and not "megohm". Later on, "K" for "kilo" or "thousand", and "Meg" for "mega" or "million" became the standard, with "M" deleted to avoid confusion. Today, the symbols are kΩ and MΩ.
