Of all the connectors used in the Electro Musicians Workshop, perhaps the most important are those carrying the mains supply: if this supply disappears everything stops functioning. Mains voltages also have the unquestioned ability to generate RFI, transient kilovoltages, involuntary fires and pyrotechnic exhibitions and are also particularly adept at electrocution. So with this in mind the following article describes some of the more reliable, high quality mains connectors.

13A Plugs

The 13A plug, a symbol of Britain's oft-maligned genius, is potentially a very reliable and safe connector. But some, having sunk to supermarket level (like records), barely meet acceptable standards. Though being cheap, they tend to find a home amongst the gear of impoverished musicians.

Hard plastic plugs are prone to shatter at inconvenient moments, but are usually acceptable for the sedentary pace of the studio and workshop. Those made by MK and Eveready are accredited with the highest standards, and are recommended. In particular, the old style MK plugs manufactured in the 60's, frequently found in virtual mint condition on second-hand equipment, are particularly tough and well designed. But that's a special exception; most second-hand plugs will be in a poor condition, and if the body is cracked or the terminals are suspect in any way, they should not be used.

For stage work, shatterproof rubber plugs are called for. Obviously, this designation must exclude rubber topped plugs which retain the hard plastic insert. And it's important to be aware that Duraplug, who lead the field, produce two styles of rubber plug. The cheapest and most commonly available is much the same size as a hard-plastic version, but it suffers from an unfortunate malady which made these connectors disreputable. Namely, if the plug pins lie prostrate, and the plug is then trodden/jumped on, nothing breaks — but the pins 'do the splits'. So when the unfortunate plug next meets a socket, great patience and electrocution defying finger-work is needed to persuade the avant-garde V-shaped pin alignment to conform to the socket. So if such treatment is inevitable, the larger and altogether more rugged rubber plug must be sought — it's known as the Husky. Apart from the steadfastness of the pins, this plug also features meatier cable clamping and bushing, suited to the 15 and 20A rubber extension cables it's designed to be used with. And to all intents and purposes, Duraplug's Husky is the nearest one can get to a roadie-proof 13A plug.

When wiring up 13A plugs, spurious disconnection at a future date can be avoided by taking simple precautions in three areas. First examine the fuseholder and fuse. The springy fuse grips will move apart in time, so it's a good habit to squeeze them together whenever changing a fuse. The grips and fuse may also be tarnished. Apart from causing intermittent contact, this can also give rise to overheating. A mild abrasive, e.g. cutting polish, can be used to clean off the corroded layer, but out 'on-the-road', an equally effective cure is to rotate the fuse a few times in the now tightly fitting grips.

Next, the termination must be firm. The 13A plug's course-threaded brass screws are soft, and by judicious tightening, the threads will lock. But the amount of torque this requires is something of a knife-edge between the screws slackening off at a later date (particularly if your music invokes large doses of bass), and the screw-head shearing. As the latter event makes the plug useless, it's wise to play safe and undertighten if you're not adept at sensing the elastic limits in metals. Plugs should be inspected periodically in any case, so marginal undertightening simply makes this precautionary task psychologically more rewarding.

Nonchalant overtightening is also prone to cut through the conductors, particularly the relatively thin 6A (0.75mm²) cross-sections that are standard for sound equipment. To an extent, this can be avoided by tinning the wires, though the conductor may then be prone to break through fatigue as the tinned section is manipulated into the terminal. A better technique is to tin doubled-over and twisted conductor strands. Once a reasonable cross-sectional area of conductor has been built up, it will be possible to tighten the screws such that their crushing, cutting action doesn't significantly weaken the termination, but makes it more reliable through a cold welding effect.

Thirdly, tug the cable sharply to check that it's firmly secured. With a rubber plug, this test must be performed after the plug's lid is in place. Otherwise the tightened grip screws will foul. If the cable is too thin to be thoroughly squashed by the grip, a couple of binding sleeves, a short length of PVC tube or even several turns of good quality insulating tape will enlarge the diameter. And as cable grips aren't infallible, particularly if you insist on using cheap plugs, it's good practice to leave slack in the earth wire, so that this most vital conductor parts company with the plug sometime after the live and neutral wires have been torn away from their terminals.

A neat finishing touch is to mark plugs boldly with unique and indelible figures, e.g. A for amplifiers, X for crossovers, D for mixing consoles, so that the task of identifying them in a large array can take place quickly, without the series of bangs and expletives that signify trial and error unplugging! And of course, in the case of rubber plugs, ink shows up best on the white or orange species...

13A Sockets

Sound equipment invariably requires a large number of mains plugs (n), and you can be sure that the number of outlets available in any home or venue will never exceed (n - 1), and generally tends towards (n/10)! The conventional domestic remedy, in the shape of three-way adaptors won't make much impression on this problem, and for recording or performing, their precarious physical stability leaves much to be desired. They are also prone to shatter. Thus, at an early stage you'll have to build up an array of sockets — a plugboard.

Perhaps the cheapest and simplest way to do this is to mount a number of four-way 'Multibloc' distribution boards on a sheet of plywood. Being fabricated of soft plastic (PVC), the exterior of these boards is certainly rugged, but the socket insert springs and internal connections aren't really up to use 'on-the-road', particularly in respect of the massive peak current requirements of large amplification systems. In addition, these boards don't feature switches, which makes life very tiresome when specific items of gear have to be isolated from time to time.

The universal component for 'on-the-road' use is the MK metalclad socket. This has a hard plastic switch, and is insulated around the terminals, so they're not perfect, and they're also expensive. But the longterm reliability and ruggedness of the body and the terminations is beyond question, and, in practice, they prove very trouble free.

Unlike an integrated 'Multibloc', some effort has to go into mounting and wiring up discrete sockets. Metalclads are either surface mounted on a ply board, being linked with short lengths of threaded conduit and brass male nipples, the conduit protecting the link cables and assuring fail-safe earth continuity. Or the mounting is flush, in a stage-box or flightcase, usually with a recessed surface to protect the sockets should the plugboard be dropped face downwards.

If the plugboard features a permanently attached extension cable, it's essential that this is securely restrained and protected from abrasion at the anchoring point. For stagebox and surface style mounting alike, brass cable glands with butyl rubber inserts are available from electrical wiring contractors. Again, binding sleeves may be used to make a thin cable thicker.

Wiring metalclad sockets calls for the same considerations as plugs; but also leave enough slack cable so that sockets may be removed for inspection, giving the earth conductor the most slack of all.

Finally, with the terminals being designed to fit two or more 2.5mm² conductors, 0.75mm² or 1.5mm² (6 or 13A) extension cables will require substantial building up if the terminal screws are to make reliable contact.

Connectors for the other end

By now, 13A outlets are mercifully almost universal throughout the U.K.. But after 35 years, a few 5 and 15A round pin outlets linger on, and if you habitually play in seedy village halls and decrepit cinemas, it's a good idea to carry round-to-square adaptors, or to make up a pair of tough rubber conversion cables. These consist of a short length of cable, with 13A rubber 'extension' sockets on one end, leading to 5 and 15A plugs, again of the rubber variety. Although you won't engender much sympathy if you seek these in the high street on a Saturday afternoon, round pin connectors are still widely used in stage lighting, and are thus available from lighting equipment distributors.

A less satisfactory situation exists as regards standardisation of the sundry mains connectors that lie at the equipment end of your mains cables. Clearly detachable mains leads are a desirable feature, both in mobile systems and whenever equipment is stacked ceiling high in studios and workshops and equally desirable is a standard connector. In this case, a number of resident spare leads will overcome the nuisance when detachable cables to go astray, and will also enable equipment withdrawn from a rack or other awkward position to be plugged in without the need to unravel the original lead from the usual spaghetti. There's also no need to tie the cable around the gear for transportation, nor the suffering to plugs and feet alike if the cable unravels itself and drags along the floor. On 60's equipment, most connectors are of the round pin Bulgin style, and here again, hard plastic together with tiny screw terminals plus inadequate cable grips and bushing makes an unsatisfactory connector outside the workshop.

The modern equipment — the "IEC" or "Euroconnector" is largely devoid of these shortcomings, provided you choose an upmarket version, typically those produced by British manufacturers such as Bulgin. Versions with integral, moulded cables should definitely not be used. Wiring is straightforward, and being designed for soldered connection long term reliability in the presence of loud Reggae is excellent! The IEC can also be made to latch, making it equal to connectors costing much more. However, manufacturers appear to be reluctant to fit latching clips because these all too readily spring out and disappear in transit. Replacements are cheap, but it's easy to loose hundreds of clips each year, so the IEC remains, for the most part, unlatched. A simple expedient here is to feed 20swg tinned copper wire through the retaining pillar holes, wrapping both ends together around the clip and soldering the joint. The outcome is a very reliable latching mechanism, consummating the IEC's aim — to specify a safe, elegant and trouble free universal equipment connector.

If you have large funds, an even better, albeit less common connector is the XLR-mains. This is very similar in style and idiosyncrasy to standard Cannon XLRs, and apart from the small screws, fragile pins and lower current rating (2A versus 6A for an IEC), it's an altogether much more rugged and reliable connector.

For plugboards and equipment racks, more substantial plugs will be required, to withstand mean currents of 10 to 20A, and in the case of PA systems, peak amplifier currents of 100 to 200A. One standard here is the Cannon EP connector, rated at 20A. Being the XLR's predecessor, it was originally used as an audio connector — and may still be in some installations — and the mains wiring isn't a universal standard, though it sensibly mimics the wiring of familiar 5/15/13A plugs, see Figure 1. So be wary of using other people's EP connectors! As with the IEC and XLR-mains, soldered connections assure long term reliability.

A more bulky, but cheaper and in many respects, a better alternative is the 16A 'Industrial plug'. These are designed for use on building sites, and apart from the screwed terminations, there can be no quibbles about their stamina! Also, they are available in two, non-interchangeable colour-coded styles, one being intended for 110/120V equipment. A particular advantage of the EP series, however, is the wide range of pin configurations, multipin versions being particularly useful for terminating mains multicores in studios and lighting rigs. In addition, the EP is the only-range of mains connectors which caters for both mains inlets and outlets, viz: both male and female varieties are available in cable and chassis mounting format.