designed to run off either alternating current
(ac) or direct current
(dc) power is described as ac/dc
equipment. This usage has become obsolete with the vacuum tube
. Some equipment is inherently able to use either ac or dc, e.g., heating devices, and they are not usually identified as being ac/dc; typically radio
receivers would be described as ac/dc.
Vacuum tube equipment used a number of vacuum tubes, each with a heater requiring a certain amount of electrical power. Tubes require relatively high voltages on some of their electrodes; these voltages can conveniently be derived directly from mains electricity. There are essentially three ways of powering equipment:
- Ac equipment: a transformer converts mains electricity into both a low-voltage (typically 6.3V) supply connected to the parallel-connected heaters of all the tubes in the equipment, all of which require 6.3V but different currents; and a high-voltage supply which is rectified and filtered to give a high dc voltage required by the equipment. Transformers operate on ac only, so that this type of equipment is ac-only.
- 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 a ballast tube or resistor is added so that when the mains voltage is applied across the chain, the required current flows. With mains voltages of around 220V the voltage drop across the additional resistor could be quite high; it was customary to use a resistive power cable (mains cord) of defined resistance, running warm A rectifier and filter is connected directly to the mains. If the mains power is ac, the rectifier converts it to dc; if dc, the rectifier effectively acts as a conductor. In both cases high-voltage dc is generated to drive the circuitry. The tube heaters do just as their name describes, and heat the cathodes, whether ac or dc power is applied. There is no transformer to isolate ac/dc equipment from the mains. Much equipment was based on a metal chassis which had to be connected to one side of the mains.
- Dc-only, from dc mains. No longer applicable.
The operation of the power supply is described further in another article.
In the early days of vacuum tubes some regions were supplied with ac power, others with dc.
- Ac-dc equipment was suitable for use on either ac or dc, an important consideration when both were available. Manufacturers were able to produce a single range of equipment for all power, and users did not have problems when moving house. It was potentially dangerous as metalwork was connected to one side of the mains; even recessed grub screws used for tightening knobs were a potential hazard. The equipment had to be built to prevent any access to metal parts by the user, and was a danger to service personnel. If a 3-pin power plug was used it could be ensured that the chassis was connected to neutral, at almost earth voltage; but 2-pin plugs could be connected either way, allowing the chassis to be at mains voltage. If a resistive power cable was used, an inexpert repairer might replace it by a standard cable, or use the wrong length, damaging equipment and risking fire. Ac-dc equipment did not require a transformer, and was consequently cheaper, lighter, and smaller than comparable ac equipment.
- Ac-only equipment uses a bulky, heavy, and expensive transformer, but the chassis is never live and can be earthed, making for safer operation. Additionally the use of a transformer allowed higher voltages to be generated (e.g., for high-powered audio amplifiers), and allowed multiple independent power supplies from separate transformer windings for different stages.
- Dc-only equipment was a little cheaper than ac-only, but became obsolete as ac power became dominant.
Repair of ac/dc equipment
For safety an isolation transformer
should be used. Technicians used to be accustomed to ensuring that the plug was connected so that the chassis was connected to neutral, and not touching the chassis, but these habits have died out. Resistive ballast resistors and resistive line cords can be replaced by appropriate power resistors
, although it should be noted that ballast tubes have a nonlinear characteristic curve
, and tend to stabilise heater current against mains variations better than a fixed resistor. In old equipment all capacitors
, but particularly electrolytics
, are likely to be faulty; this applies in particulr to the filter capacitors of an ac/dc power supply.
Since solid-state electronics
displaced vacuum tubes, circuits require high currents at relatively low voltages, and the use of transformers has become almost universal. The decreasing cost of complex electronics, with massive functionality available in a single, cheap, integrated circuit
has made it feasible to power equipment safely from either ac or dc mains without a conventional mains transformer: the supply is rectified and filtered if ac, and used to power a high-frequency oscillator
whose output is connected to the primary winding of a small, cheap, high-frequency transformer (which is often part of the tuned circuit used by the oscillator) which isolates the circuitry of the equipment from the mains electricity. In principle it would be simple and cheap to design ac/dc equipment; in practice mains dc mains electricity is no longer used, and dc operation direct from the mains is irrelevant.
Although unrelated to ac/dc as used in the past, modern equipment is often powered from low-voltage DC, typically by a 12V accumulator in a motor vehicle. An inverter can be used to provide AC output; or a DC to DC converter based on a switch-mode power supply (SMPS) for DC output. Modern SMPSes can accept a DC input without any problem, though the DC voltage does need to be around 25% higher than the rated RMS AC voltage. Some specialist suppliers manufacture DC/DC converters.