AC/DC receiver design
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AC/DC receiver design discusses a development in the power supply section of vacuum tube based radio and television receivers eliminating the bulky and expensive mains transformer. An unintentional feature of the design was that the receiver was able to operate from a DC supply as well as an AC supply and consequently they were known as "AC/DC receivers".
Applicability to early radio and television
In the early days of radio, mains electricity was supplied at different voltages in different places, and either direct current (DC) or alternating current (AC) was supplied. There are three ways of powering electronic equipment. AC-only equipment would rely on a transformer to provide the voltages for heater and plate circuits. AC/DC equipment would connect all the tube heaters in series to match the supply voltage; a rectifier would convert AC to the direct current required for operation. When connected to a DC supply, the rectifier stage of the power supply performed no active function. DC-only equipment would only run from a DC supply and included no rectifier stage. DC mains power is no longer supplied to residential users.
Different radio set models were required for AC, DC mains, and battery operation. For example a 1933 Murphy radio with essentially the same circuit had different models for AC supply, DC supply, and battery operation. The introduction of AC/DC circuitry allowed a single model to be used on either AC or DC mains as a selling point, and some such models added "Universal" to their name (such sets usually had user-settable voltage tapping arrangements to cater for the wide range of voltages.)
The first ever AC/DC design of radio was the All American Five. The sole aim of the design was to eliminate the mains transformer. The lower cost of transformerless designs remained popular with manufacturers long after DC power distribution had disappeared. Several models were produced which dispensed with the power transformer, but had circuit features which only allowed operation from AC. Some early models were available in both AC-only and AC/DC versions, with the AC/DC versions sometimes slightly more expensive.
Series tube heaters
Vacuum tube equipment used a number of tubes, each with a heater requiring a certain amount of electrical power. In AC/DC equipment, the heaters of all the tubes are connected in series. All the tubes are rated at the same current (typically 100, 150, 300, or 450 mA) but at different voltages, according to their heating power requirements. If necessary, resistance (which can be a ballast tube (barretter), a power resistor or a resistive mains lead are added so that, when the mains voltage is applied across the chain, the specified heating current flows. Some types of ballast resistors were built into an envelope like a tube that was easily replaceable. With mains voltages of around 220 V, the power dissipated by the additional resistance and the voltage drop across it could be quite high, and it was common to use a resistive power cable (mains cord) of defined resistance, running warm, rather than putting a hot resistor inside the case. If a resistive power cable was used, an inexperienced repairer might replace it with a standard cable, or use the wrong length, damaging the equipment and risking a fire.
AC/DC equipment did not require a transformer, and was consequently cheaper, lighter, and smaller than comparable AC equipment. This type of equipment continued to be produced long after AC became the universal standard due to its cost advantage over AC-only, and was only discontinued when vacuum tubes were replaced by low-voltage solid-state electronics.
A rectifier and a filter capacitor were connected directly to the mains. If the mains power was AC, the rectifier converted it to DC. If it was DC, the rectifier effectively acted as a conductor. When operating on DC, the voltage available was reduced by the voltage drop across the rectifier. Because an AC waveform has a voltage peak that is higher than the average value produced by the rectifier, the same set operating on the same root mean square AC supply voltage would have a higher effective voltage after the rectifier stage. In areas using 110–120 volt AC, a simple half-wave rectifier limited the maximum plate voltage that could be developed; this was adequate for relatively low-power audio equipment, but television receivers or higher-powered amplifiers required either a more complex voltage doubler rectifier or warranted the use of a power transformer with a conveniently high secondary voltage. Areas with 220–240 volt AC supplies could develop higher plate voltage with a simple rectifier. Transformerless power supplies were feasible for television receivers in 220–240 volt areas. Additionally, the use of a transformer allowed multiple independent power supplies from separate transformer windings for different stages.
In an AC/DC design there was no transformer to isolate the equipment from the mains. Much equipment was built on a metal chassis which was connected to one side of the mains. Because no power transformer was used, so-called "hot chassis" construction was required and the equipment power supply was conductively connected to the input power source. Any exposed metal on the device connected to the circuit common was also connected to the power supply. For safety, no exposed metal could be connected to the circuit common. Service personnel working on energized equipment had to use an isolation transformer for safety, or be mindful that the chassis could be live. AC-only vacuum tube equipment used a bulky, heavy, and expensive transformer, but the chassis was not connected to the supply conductors and could be earthed, making for safer operation.
Transformerless "hot chassis" televisions continued to be commonly manufactured long after transistorisation rendered live-chassis design obsolete in radios. By the 1990s, inclusion of audio-video input jacks required elimination of the floating ground as TVs needed to be interconnectable with VCRs, game consoles and video disc players. The widespread replacement of cathode ray tubes with liquid crystal displays after the turn of the millennium resulted in televisions using primarily low voltages, obtained from switching power supplies. The potentially-hazardous "floating chassis" was no more.
In the past, 110–120 V was not high enough for high-power audio and television applications. Therefore, it was used to operate low-power audio equipment such as radio receivers. Higher-powered 110–120 V tube audio or television equipment needed higher voltages which were obtained using step-up transformer based power supply, or sometimes a voltage doubler, therefore operating off AC only.
Some AC/DC equipment was designed to be switchable to be able to operate off either 110 V AC (possibly with a voltage doubler) or 220–240V AC or DC. Television receivers were produced which could run off 240 V AC or DC. The voltage was not high enough to power some circuits, so energy was recovered during the flyback period from the primary of the line output transformer to provide a boosted HT (high tension) supply. In a typical vacuum tube colour TV set, the line output stage had to boost its own HT supply to between 900 to 1200 volts (depending on screen size and design). Transistor line output stages, although not requiring supply voltages above the rectified mains voltage, nevertheless still developed extra voltage over the normal supply rail to avoid complicating the power supply circuitry. A typical transistor stage would produce between 20 and 50 'extra' volts. Some details of the way in which the nominally 190 volts HT supply was boosted to nearly 500 volts in the 1951 Bush TV22 are described in a technical publication. AC/DC televisions were produced well into the color and semiconductor era (some sets were tube/semiconductor hybrids).
With widespread adoption of solid-state design in the 1970s, voltage and power requirements for tabletop portable radio receivers dropped significantly. One common approach was to design a battery-powered radio (typically 6 volts DC from four dry cells) but include a small built-in step down transformer and rectifier to allow mains electricity (120V or 240V AC, depending on region) as an alternative to battery-powered operation.
Notes and references
- "Murphy Radio Model A4 From 1933". Classicwireless.co.uk. Anonymous. Retrieved June 21, 2013.
- "Sunbeam radio" Classicwireless.co.uk. Anonymous. Retrieved June 21, 2013. (Offers AC/DC operation as a selling point).
- "Decca 'Universal 55' radio". Classicwireless.co.uk. Anonymous. Retrieved June 21, 2013.
- "Technical Bulletin: Model 'PS'" (PDF). Astor Radio Corporation Pty, Ltd. February 22, 1952. Via KevinChant.com. Retrieved June 21, 2013. (Manual of 1952 Astor with instructions on use with AC and DC mains of different voltages)
- "The All American Five". Fun with Tubes. Max Robinson. Angelfire.com. Retrieved June 21, 2013. (Third sentence.)
- "History of the AA5 (All American 5ive) AM tube radio". WA2ISE personal webpage. Netcom.com. Retrieved June 21, 2013.
- "An eight-valve 110 V AC or 220 V AC/DC superheterodyne receiver with push-pull output stage" Data and Circuits of Radio Receiver and Amplifier Valves IIIa. Philips Technical Library. p. 264-269. Ed. N.S. Markus & J. Otte. Elsevier Press. 1952 (English edition).(Detailed description and circuit diagram)
- "Pye B18T AC/DC Television Chassis". Wireless World. December, 1948. Via r-type.org. Retrieved June 21, 2013. (True AC/DC 240V (190–220 V operation needed an additional AC-only autotransformer) monochrome TV and other equipment. While DC operation was possible, it was not an advertised feature; the transformerless design was to save size and weight.)
- "1935 catalogue". Murphy Radio Co. Retrieved June 21, 2013. (Showing AC/DC models GB£0.5.0 (about 2%) more expensive than AC only.)
- "All About Ballast and Resistor Tubes". Radio Craft (from National Union Radio Corp), January 1939. Via Antiqueradios.com.
- "VII. A five-valve receiver for AC/DC mains" (PDF). Data and Circuits of Radio Receiver and Amplifier Valves IIIa. Philips Technical Library. p. 254-258. Ed. N.S. Markus & J. Otte. Elsevier Press. 1952 (English edition). (With ballast (barretter), detailed description and circuit diagram. Retrieved June 21, 2013.
- "Resistive Line Cords And Ballast Tubes". CHRS Journal. California Historical Radio Society. Via Antiqueradios.com.
- Seal, D.J. (1971). The MAZDA Book of PAL Receiver Servicing. Foulsham Technical Books / Thorn Radio Valves & Tubes Ltd. 1971. pp. 173–174. Via Archive.org.
- Seal, 1971, p. 173.
- This is the range from a large collection of TV servicing data. 20 volts is the ITT FS12 (12″ B&W), and 50 volts is the BRC2000 chassis used in a fair number of early transistorised 25″ colour TV sets.
- Burrell, Malcolm (December 1979). "Vintage TV: The Bush Model TV22" (PDF). Television. UK. pp. 88–89. Archived from the original on 2012-03-23. Retrieved 2013-06-21 – via domino405.co.uk.