AC/DC receiver design: Difference between revisions

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	In an AC/DC design there is no transformer to isolate the equipment from the mains. Much equipment was built on a metal ] which was connected to one side of the mains.<ref></ref> 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 to 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.		In an AC/DC design there is no transformer to isolate the equipment from the mains. Much equipment was built on a metal ] which was connected to one side of the mains.<ref></ref> 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 to 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.

	AC/DC radios could have anything from three to eleven tubes,<ref>The 1934 G.E.C. Universal Mains 3, BC3520, had two valves plus rectifier . The 1936 Ferguson 378 came in an AC-only version with eight valves (378AC) and a more expensive 378 Universal with eleven </ref> but a typical low-cost AC/DC ] radio such as the ] would have five. DC-only equipment was usually a little cheaper than AC/DC, but became obsolete as AC power became dominant.		AC/DC radios could have anything from three to eleven tubes,<ref>The 1934 G.E.C. Universal Mains 3, BC3520, had two valves plus rectifier . The 1936 Ferguson 378 came in an AC-only version with eight valves (378AC) and a more expensive 378 Universal with eleven </ref> but a typical low-cost AC/DC ] radio such as the ] would have five. DC-only equipment was usually a little cheaper than AC/DC, but became obsolete as AC power became dominant, solid state circuitry required less power making transformer size requirements less and safer designs evolved.

	===Application===		===Application===

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So-called "All American Five" vacuum tube radio receivers used a power supply that could work on either AC or DC

The description AC/DC refers to electrical equipment designed to operate on either alternating current (AC) or direct current (DC). This term typically describes certain types of transformerless vacuum tube radio or television receivers. Equipment which, by its nature, can use either AC or DC, such as heating devices and incandescent light bulbs, are not usually described as “AC/DC” and not the subject of this article.

Applicability to early radio and television

In the early days of radio, mains electricity was supplied at different voltages in different places, and both direct and alternating current was supplied. Most locations eventually standardised on 110-120V, 60 Hz or 220-240V, 50 Hz AC distribution systems. In the early days different radio sets were required for AC, DC mains, and battery operation; an example is a 1933 Murphy radio, with essentially the same circuit sold as the A4 (AC), D4 (DC mains), and B5 (battery). 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 made transformerless designs remain 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.

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 unidirectional 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 is no longer used in building wiring, but DC-only equipment is common with battery-operated systems such as those in automobiles.

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 450mA) but at different voltages. If necessary, resistance (which can be a ballast tube (barretter), power resistor or resistive cable are added so that, when the mains voltage is applied across the chain, the required current flows. Some types of ballast resistor were built into an envelope like a tube that was easily replaceable. With mains voltages of around 220V, 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 equipment and risking fire.

Transformer

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 are connected directly to the mains. If the mains power is AC, the rectifier converts it to DC. If it is DC, the rectifier effectively acts 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 is 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 to 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.

AC/DC radios could have anything from three to eleven tubes, but a typical low-cost AC/DC superheterodyne radio such as the All American Five would have five. DC-only equipment was usually a little cheaper than AC/DC, but became obsolete as AC power became dominant, solid state circuitry required less power making transformer size requirements less and safer designs evolved.

Application

AC/DC equipment was usable on either AC or DC supplies, an important consideration when DC distribution was still used. Manufacturers were able to produce a single range of equipment for all power, and users did not have problems when moving house.

Regional variations

In the past, 110-120V 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 110-120V tube audio or television equipment needed higher voltages which had to be stepped up by a transformer 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 240V 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 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 description. AC/DC televisions were produced well into the color and semiconductor era (some sets were tube/semiconductor hybrids).