All American Five
Encyclopedia
The term All American Five is a colloquial name for mass-produced, superheterodyne
radio
receivers that used five vacuum tube
s 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. 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.
because they could operate on DC (which was still in use in several areas of the USA when the design was developed) as well as AC, though this was incidental to the design objective. 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 cost-effective 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 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 (the rectifier and audio output tube) required more-powerful heaters that dropped proportionally more of the line voltage. 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 once manufacturers figured out that the cost of replacing the tube was not their problem. As with Christmas tree lights, if one tube heater failed, none of the tube heaters would operate. 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.
The radio used a 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 provided by a dual diode/triode combination tube.
hazard to their owners. 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. 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.
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 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.
Over the years, these paper capacitors often become leaky, and may allow sufficient current flow to give the user a shock.
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.
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.
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 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 octal tubes.
The voltage distribution has changed around the tube heaters but the total is still a little more than the 120 volt mains supply. This line-up is for an Admiral
radio.
s. With a relatively simple rewiring, the tube heaters could be put in series-parallel to run off 32 volts – the three twelve-volt heaters in series and a 25L6, 35L6 or 43 in parallel. The tubes would still function with the heater voltage somewhat out of specifications. If run from a 32 volt supply the radio had a substantially reduced performance because the B+ voltage would only be 32 volts compared with 160-170 volts when operated from AC. With 32 volts on the plate, the radio tended to be insensitive. Sometimes only the tube heater power was derived from a windmill, and dry batteries were retained for the plate voltage supply. The advantage was that the heaters were a high and continuous load on the battery, whereas the plate voltage battery drain was smaller and intermittent. Often a wet-cell rechargeable battery was used for tube heaters, recharged by a local garage or by exchanging with a vehicle battery.
Many 32 volt farm radios were factory-built for the purpose. They usually had two type 48 power tetrodes that could operate with B+ voltages as low as 28 volts. The type 48 pairs were parallel connected, or connected in push-pull as shown in this schematic of a 1936 farm radio from Sears Roebuck. Some factory 32-volt radios used an electromechanical vibrator
power supply to provide increased voltage. Vibrator power supplies could also be made to work from a 6 volt supply from a dedicated wind-charger or from a car battery
borrowed from a farm 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:
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.
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).
Not all manufacturers followed this recommendation.
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 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.
Superheterodyne receiver
In electronics, a superheterodyne receiver uses frequency mixing or heterodyning to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original radio carrier frequency...
radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
receivers that used five vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
s in their design. These radio sets were designed to receive amplitude modulation
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
(AM) broadcasts in the medium wave band, and were manufactured in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
from the mid 1930s until the early 1960s. By eliminating a power transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
, 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.
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 Japanese-made units. Fewer examples were used in Europe as the 220 volt mains supply meant extra work for designers in providing tube heater voltage. AA5 designs were also called AC/DC receiversAC/DC (electricity)
The description AC/DC refers to equipment designed to be operate on either alternating current or direct current . This term typically describes certain types of vacuum tube radio or television receivers. AC/DC equipment was necessary because in the early days of vacuum tubes, some regions were...
because they could operate on DC (which was still in use in several areas of the USA when the design was developed) as well as AC, though this was incidental to the design objective. When operated on DC, they would only work if the plug was inserted with the correct polarity
Electrical polarity
Electrical polarity is present in every electrical circuit. Electrons flow from the negative pole to the positive pole. In a direct current circuit, one pole is always negative, the other pole is always positive and the electrons flow in one direction only...
. 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 cost-effective to make as possible. The design was optimized to provide good performance for the price. At least one radio manufacturer, Arthur Atwater Kent
A. Atwater Kent
Arthur Atwater Kent, Sr. was an inventor and prominent radio manufacturer based in Philadelphia, usa. In 1921, he patented the modern form of the automobile ignition coil.-Biography:...
, 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 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 (the rectifier and audio output tube) required more-powerful heaters that dropped proportionally more of the line voltage. 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 once manufacturers figured out that the cost of replacing the tube was not their problem. As with Christmas tree lights, if one tube heater failed, none of the tube heaters would operate. When the detector/first audio tube contained a second diode, it could be used to provide automatic gain control
Automatic gain control
Automatic gain control is an adaptive system found in many electronic devices. The average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels...
(AGC), or AGC bias could be derived from the audio detector diode.
The radio used a half wave rectifier to produce about 160-170 volts of plate
Plate electrode
A plate is a type of electrode that formed part of a vacuum tube. The plate is impressed with a positive charge so that it may capture and flow electrons within a circuit....
voltage. This simple rectifier coupled with the wiring of the heaters in series eliminated the mains transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
.
The frequency mixer
Frequency mixer
In electronics a mixer or frequency mixer is a nonlinear electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals at frequencies f1 and f2 are applied to a mixer, and it produces new signals at the sum f1 + f2 and difference f1 -...
was of the pentagrid converter
Pentagrid converter
The pentagrid converter is a radio receiving valve with five grids used as the frequency mixer stage of a superheterodyne radio receiver....
design to save the cost of a separate oscillator
Electronic oscillator
An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. They are widely used in innumerable electronic devices...
tube. The detector
Detector (radio)
A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...
and first audio stage were provided by a dual diode/triode combination tube.
Potential hazards of the design
Many early examples of the 'All-American Five' posed a shockElectric shock
Electric Shock of a body with any source of electricity that causes a sufficient current through the skin, muscles or hair. Typically, the expression is used to denote an unwanted exposure to electricity, hence the effects are considered undesirable....
hazard to their owners. 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. 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.
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
Underwriters Laboratories
Underwriters Laboratories Inc. is an independent product safety certification organization. Established in 1894, the company has its headquarters in Northbrook, Illinois. UL develops standards and test procedures for products, materials, components, assemblies, tools and equipment, chiefly dealing...
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.
Over the years, these paper capacitors often become leaky, and may allow sufficient current flow to give the user a shock.
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 IFIntermediate frequency
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...
amplifier tube, although some designs used a selenium rectifier
Metal rectifier
A metal rectifier is an early type of semiconductor rectifier in which the semiconductor is copper oxide or selenium. They were used in power applications to convert alternating current to direct current in devices such as radios and battery chargers...
in place of the rectifier tube. The six-tube versions added either a RF
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
amplifier tube, a push-pull audio power amplifier tube, or a beat frequency oscillator tube (to allow one to listen to Morse code
Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...
or single-sideband modulation
Single-sideband modulation
Single-sideband modulation or Single-sideband suppressed-carrier is a refinement of amplitude modulation that more efficiently uses electrical power and bandwidth....
). 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 |
The original
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 used the following tubes:- ConverterPentagrid converterThe pentagrid converter is a radio receiving valve with five grids used as the frequency mixer stage of a superheterodyne radio receiver....
: 12A8 - IFIntermediate frequencyIn communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...
amplifier: 12K7 - DetectorDetector (radio)A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...
and first audio amplifier: 12Q7 - AudioSoundSound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
power output: 50L6 - RectifierRectifierA rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
: 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.
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 areas near Niagara Falls
Niagara Falls
The Niagara Falls, located on the Niagara River draining Lake Erie into Lake Ontario, is the collective name for the Horseshoe Falls and the adjacent American Falls along with the comparatively small Bridal Veil Falls, which combined form the highest flow rate of any waterfalls in the world and has...
had 25 Hz power
Utility frequency
The utility frequency, line frequency or mains frequency is the frequency at which alternating current is transmitted from a power plant to the end-user. In most parts of the world this is 50 Hz, although in the Americas it is typically 60 Hz...
, some Canadian sets had slightly larger filter capacitors.
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
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 octal tubes.
The "Power-Saver" version
Another low-power variation changed the tube heaters to run on 100 milliamperes rather than 150 milliamperes. 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: 32ET5 or 34GD5
- Rectifier: 36AM3
The voltage distribution has changed around the tube heaters but the total is still a little more than the 120 volt mains supply. This line-up is for an Admiral
Admiral (electrical appliances)
Admiral is an American appliance brand currently manufactured by Whirlpool Corporation. The brand is sold at The Home Depot.Ross Siragusa founded Continental Radio and Television Corp. as a maker of consumer electronics in Chicago during 1934. This later became Admiral Corp. Its annual sales were...
radio.
Farm radio
A "farm radio" modification (usually done at the point of sale) allowed an AA5 to run off 32 volt DC, commonly generated by farm windmillWindmill
A windmill is a machine which converts the energy of wind into rotational energy by means of vanes called sails or blades. Originally windmills were developed for milling grain for food production. In the course of history the windmill was adapted to many other industrial uses. An important...
s. With a relatively simple rewiring, the tube heaters could be put in series-parallel to run off 32 volts – the three twelve-volt heaters in series and a 25L6, 35L6 or 43 in parallel. The tubes would still function with the heater voltage somewhat out of specifications. If run from a 32 volt supply the radio had a substantially reduced performance because the B+ voltage would only be 32 volts compared with 160-170 volts when operated from AC. With 32 volts on the plate, the radio tended to be insensitive. Sometimes only the tube heater power was derived from a windmill, and dry batteries were retained for the plate voltage supply. The advantage was that the heaters were a high and continuous load on the battery, whereas the plate voltage battery drain was smaller and intermittent. Often a wet-cell rechargeable battery was used for tube heaters, recharged by a local garage or by exchanging with a vehicle battery.
Many 32 volt farm radios were factory-built for the purpose. They usually had two type 48 power tetrodes that could operate with B+ voltages as low as 28 volts. The type 48 pairs were parallel connected, or connected in push-pull as shown in this schematic of a 1936 farm radio from Sears Roebuck. Some factory 32-volt radios used an electromechanical vibrator
Vibrator (electronic)
In early electronics vibrators were used in inverter circuits to provide an alternating current electric power supply from a direct current source....
power supply to provide increased voltage. Vibrator power supplies could also be made to work from a 6 volt supply from a dedicated wind-charger or from a car battery
Car battery
An automotive battery is a type of rechargeable battery that supplies electric energy to an automobile. Usually this refers to an SLI battery to power the starter motor, the lights, and the ignition system of a vehicle’s engine...
borrowed from a farm vehicle.
Battery operated variants
A number of other versions of the set appeared, including some that did have a transformer, a version that operated in a motor vehicleMotor vehicle
A motor vehicle or road vehicle is a self-propelled wheeled vehicle that does not operate on rails, such as trains or trolleys. The vehicle propulsion is provided by an engine or motor, usually by an internal combustion engine, or an electric motor, or some combination of the two, such as hybrid...
off a 6-volt supply and even a version that operated from either dry batteries
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
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 seleniumMetal rectifierA metal rectifier is an early type of semiconductor rectifier in which the semiconductor is copper oxide or selenium. They were used in power applications to convert alternating current to direct current in devices such as radios and battery chargers...
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.
This setup was common on Motorola portable radios commonly resembling metal "lunch boxes".
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 12SK7Vacuum tubeIn electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
as an RF or IF amplifier. This would require using a 35L625L6The 25L6 is an octal-based vacuum tube of the beam-power tetrode type. It found common application in AC/DC radio receivers - such as those of the All American Five type - and was also found in large numbers in early computers, such as the UNIVAC I....
to maintain the heater voltage. - Or, another audio amplifier tube could be added for increased audio output. To keep the total heater voltage at around 120 V, the two output tubes would have to be 25 to 35-volt types, such as the 35L6 or 25L6.
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).
Series string order
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:- Input stage
- Ballast tube or resistor
- Rectifier
- Audio power output amplifier
- RF and IF amplifiers
- Converter
- First AF amplifier
- Detector
- Ground/B-minus line
Not all manufacturers followed this recommendation.
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 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.Servicing precautions
Since the chassis of the set may be connected directly to the live side of the power line, service shops used an isolation transformerIsolation transformer
An isolation transformer is a transformer used to transfer electrical power from a source of alternating current power to some equipment or device while isolating the powered device from the power source, usually for safety...
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 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.