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.
AA5 Radio Signal Flow
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.
The patent office was apparently in no hurry to grant a patent for just adding yet another grid (the element in a tube that controls the flow of electrons) to a tube. They finally granted the patent in 1939, but by then just about every radio was using the tube and every tube manufacturer was cranking out thousands of them. A pentagrid patent was also granted in the U.K. in 1935, but by then U.K. tube maker Farranti had already been producing them for at least two years. I could not find any data suggesting that either party profited from the patents or that any litigation resulted from them. Prior, engineers took what manufacturers had available, and designed radios around these tubes and components. The pentagrid converter marked a turning point in that it was designed to maximize the potential of an existing circuit (Armstrong's heterodyne topology), rather than the circuit being designed to work with existing tubes.
Tube count was further deduced by combining the detector, which extracts the audio from the radio signal, and an audio preamplifier. An additional diode was added to this combination to produce a voltage proportional to the strength of the received signal, which would be used by the automatic volume control circuit (invented by Alan Hazeltine) to keep the volume constant between local and distant stations or when the signal faded due to atmospheric conditions.
For techies, the pre-war mix was typically a 12SA7 pentagrid, 12SK7 Intermediate amplifier, 12SQ7 detector/ preamp, 50L6 audio amplifier and 35Z5 rectifier. This standardization revolutionized the radio industry. Component manufacturers could now make the tubes and support components in vast quantities, resulting in falling prices. This resulted in affordable radios, some selling for under $20, which helped pull surviving radio companies out of the Depression.
The circuit became known as the All American Five, although it was used, to a lesser degree, in Europe. European consumers in general demanded shortwave reception, and some parts of Europe also had additional stations on a very low frequency band below the U.S. standard AM broadcast band. These features required additional tubes, although these radios did make use of the pentagrid converter.
Post-war, the smaller components and the simplicity of the All American Five allowed manufacturers to more room for other components, such as a clock movement. This gave birth to the clock radio, which gave consumers the choice of waking up to the morning news or music. It was a nice alternative to the clanging bell or harsh buzzer of the bedside alarm clock. By the mid 1950s, almost every bedroom had one.
One possibility was that Soviet agents inside the Soviet Embassy were monitoring Watcher two-way radio communication back to MI5 headquarters. Wright came up with a clever plan to prove this was the source of the leak.
Besides the complications of maintaining frequency alignment while simultaneously tuning multiple stages, TRF radios generally lacked sensitivity and were prone to instability (oscillation and annoying whistles). They also required a bulky and expensive power transformer to supply filament/heater voltage to the tubes. I once repaired a TRF radio with a separate transformer and power supply that weighed over 25 pounds!
With the introduction of the superheterodyne receiver, most of these limitations were overcome. Because of the increased sensitivity, the outside antenna was replaced with a loop antenna tuned by a variable capacitor to the AM broadcast signal. The multiple RF stages were replaced with a mixer/local oscillator stage and a single Intermediate Frequency (IF) stage.
The transformer was eliminated by the introduction of tubes with heaters that could be connected in series directly across the emerging, standardized 120 VAC line voltage. The result was a more compact and space-saving design that along with standardization became the All American Five (AA5) radio.
In time, large floor model radios began to disappear from sales floors, replaced by compact tabletop designs using the AA5 circuit. By the late 1940s, a great variety of AA5 models were being mass produced by dozens of radio manufacturers. Internally, they were much the same, so style and outward appearance became the principal selling features.
By 1950, miniature seven-pin versions of the original octal tubes came into use. These were the 12BE6 local oscillator/mixer, the 12BA6 IF amplifier, the 12AV6 detector/first audio amplifier, the 50C5 audio power amplifier, and the 35W4 rectifier. Smaller and sleeker styles were possible, including designs with built-in clocks that could wake you in the morning to your favorite radio station and brew your first cup of coffee via an auxiliary AC outlet.
Over the course of 30 years, millions of AA5 radios were manufactured and sold. Because of the limited lifetimes of tubes and other components, a huge service repair industry flourished. The entry of television in the early 1950s was predicted to spell the doom of radio but that never happened.
Eventually, transistor radios using less power and sporting greater reliability took the market and the AA5 disappeared. The superheterodyne design remained, however, with transistors replacing tubes.
The mixer/local oscillator (12SA7/12BE6) converts the AM broadcast signal to 455 kHz by mixing the AM broadcast and local oscillator signals. Mixing generates two signals identical to the original AM broadcast signal. One is equal to the sum of the mixed signals and the other is equal to the difference. The tuned circuits of the first IF transformer pass only the 455 kHz difference signal (refer to the Sidebar 1).
Second, for the station selection design, I would sense the local oscillator frequency and use it to select the desired FM station. I would need a circuit to capture and condition the local oscillator signal for processing by a microprocessor. The microprocessor could then control station frequency via the TEA5767 FM module.
Shown in Figure 4 is the sine-to-square wave converter circuit. For an antenna to pick up the local oscillator signal (waveform A), I wound a few turns of insulated No. 22 solid wire around the local oscillator/mixer tube. So as not to load down the weak local oscillator signal, I wanted a high impedance input for the first amplifier stage. My choice was a MPF102 JFET transistor shown as Q1.
For the second stage Q2, I chose the NPN transistor 2N3904. Q2 provides an additional gain of 20 and is biased to clip the top of the input half-sine wave (waveform C). I finished with a 75HCT02 two-input NOR gate configured as an inverter. Its purpose is to square up the signal (waveform D) and interface to the Arduino with correct logic voltage levels (0 and 5 VDC).
3. In the TUNE WHILE loop, I apply the same 20 kHz criteria and switch back to SCAN WHILE loop if the leakage signal frequency differs from the current AMFreq table frequency. So long as the leakage signal frequency stays within 20 kHz of an AM table frequency, the TEA5767 stays tuned to the selected FM station. The 20 kHz allowance makes tuning easier and helps eliminate loss of tuning with drift in the local oscillator frequency.
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.
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