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Tech Tips... Vintage Amplifiers & Electrical Safetyby John W. Tipka Does your vintage guitar amplifier have a possible "death switch" on it? How about the vocalist whose lips get burned or shocked by the metal shell of a microphone plugged into one of the inputs on an amplifier? I know many of us steel and rhythm guitar players use Blackface and Silverface Fender amplifiers with our instruments. When I say "death switch", I'm talking about the GROUND SWITCH on these amplifiers that have a two-pronged plug attached to the power cord. I remember well the days before "double-insulated" or polarity plugs came along. Everything that needed an electrical connection to the mains had a plug with two prongs of the same size. This included my first Gibson BR-6 amplifier which I still own. Plug it in one way and it hummed; rotate the plug and the hum stopped. (By the way, it has a three pronged plug on it now.) Today, appliances are manufactured with two pronged polarized plugs and three pronged grounded plugs. The two pronged polarized plugs have one prong wider than the other to insure the "neutral" or ground/earth from the mains is properly connected to the appliance. This arrangement ensures that the frame or chassis of the appliance is not connected to the high ("hot") potential side of the mains thus preventing possible ELECTROCUTION and possible DEATH if one were to innocently touch, let's say a water faucet, and the appliance at the same time. The double insulated scheme is found primarily on portable tools whereby all electrically conductive parts are hidden under plastic or some under non-conductive material. It uses either a plug with or without polarity. The Best SafetyThe very best safety is with a three pronged plug. It has a high (narrow prong) or hot (black) wire, a neutral (wide prong) or cold (white) wire, and a separate (round post) ground (green) wire. With an amplifier, the black and white wires connect to the amplifier's power transformer through a fuse and a power switch. The separate green ground wire is connected directly to the chassis of the amplifier. This insures that current flows to ground through the ground wire of the cord if something goes awry in the amplifier. The chassis is always attached to ground with this method of connection. This is an extremely low resistance connection that ensures current flow to ground rather than through you if you touch it and another appliance. I recommend that the vintage amplifier with the two conductor cord and two pronged plug be converted to the three conductor cord and three pronged plug because of the hazards I have described above. DISCLAIMER: DO NOT MAKE THE FOLLOWING MODIFICATION YOURSELF if you have no familiarity with electricity or with the internals of an amplifier or electrical equipment. Have an experienced professional electrical or electronic technician make the modification for you. (Ed. Note: to be sure you've got the "right" person for the job, ask to see his/her license or union card.) How to Modify an Old AmpThe Fender amplifiers have the fuse on one side of the line and the power switch on the other side. The fuse and power switch need to be on the same side of the incoming line. 1) The old power cord is removed and replaced with a 16 or 18 AWG three wire cord with an attached three prong plug. The black and white wires of the cord connect to the accessory socket. The black wire to the screw for the small slot or top of the socket when looking at it with the chassis upside down, and the white wire to the bottom screw or wide slot in the socket. 2) The black wire from the power cord (through the accessory socket) is connected to the fuseholder and power switch in series with the high/hot end of the power transformer primary winding, with the fuse coming first. (The black wire from the accessory socket connects to the center, or back terminal, on the far end of the fuseholder. The front terminal of the fuseholder connects to a terminal on the power switch. The high end of the transformer primary winding connects to the other terminal on the power switch.) 3) The other end of the power transformer primary winding is connected to the white wire at the accessory socket. 4) The green wire in the cord is the ground wire and is connected directly to the chassis. A terminal lug and self-tapping metal screw work nicely here. 5) The "death switch" and its associated .047 microfarad capacitor should be removed and thrown into a trash can. Two small washers with a machine screw and nut through them can be used to plug the hole for the former ground switch. With this accomplished, now don't resort to using one of those two prong adapters to float a ground when you have a ground loop problem . Find out why there is a ground loop giving you 60 Hertz hum and eliminate the problem rather than defeating the purpose of the modification described above. Ground loops will be a subject for discussion in a future installment of this column. 
HSGA * HAWAIIAN
STEEL GUITAR ASSOCIATION Homepage URL: www.hsga.org (hsga@lava.net) Last updated: 07/23/02 by Gerald Ross (gbross@umich.edu) |