Shop Talk: 6-String Sparkler

January 14, 2008

At a rehearsal the other night, I broke the B-string on my Telecaster. The string snapped right at the bridge and was swinging around from my headstock when suddenly it spontaneously combusted—a flame arced across it, and in an instant it burned like a firecracker fuse all the way up to the tuners! My guitar amp and my pedalboard—along with some powered P.A. speakers, a mixer, and a laptop—were all plugged into a power strip that was connected to an ordinary household extension cord via a ground-lifting three-prong adaptor. My amp was connected via an XLR cable to the mixer, and we were rehearsing on a cement floor. My theory is the that at some point the broken string swung down and made contact with the concrete floor, completed a circuit, and ignited when a massive amount of current went through it. I can’t remember whether I had my hands on the other strings when the fireworks occurred, because I was looking for spare strings at the time, but either way I’m thinking I was pretty lucky to have been insulated from the floor by my rubber-soled sneakers. My main question is this: How close did I come to being electrocuted?
—Earl Amador, San Francisco, California

You’re correct that the broken string completed some kind of circuit, causing enough current to flow through it to heat the metal to its flash point. The string could have contacted a mic stand or a microphone that was at a different electrical potential than the string. As your amp was plugged into a ground lift, there could have been a charge on the strings and a different polarity charge on the mixer chassis, which was connected directly to the microphone via the XLR connectors. Since neither the amp nor the mixer was grounded, and each had its own 120-volt power supply, this may have been the cause. The only way to know for sure would be to duplicate the setup and start probing different things with a voltmeter. Check between the strings and the mic, between the string ground and true earth ground, and between the mic stand and the strings. Once you know where the difference of electrical potential exists, you’ll be better able to solve the grounding problem and—hopefully—avoid a potentially lethal situation. I’ve seen sparks jump from my lip to a microphone more than once. Knock on wood, though, I’ve never had my lip—or guitar string—burst into flames.
—Gerald Weber, Kendrick Amplifiers

I play a 1965 blackface Super Reverb amp. If I blow a fuse, especially at a gig, what is the best way to get the amp working again?
— Fred Hilkey, Boulder, Colorado

When a fuse blows on a tube amp, 99 percent of the time the problem is with the output tubes or the rectifier tube. For starters, I recommend that you bring some extra fuses (2 amp Slo-Blo type for your Super Reverb), an extra rectifier tube, and a spare set of output tubes to every gig. If a fuse blows, change the fuse, the rectifier, and both output tubes, and your amp will most likely be up and running in less than five minutes. If you are not on a gig, follow this procedure to pinpoint the faulty component: First, remove the rectifier and output tubes. Replace the fuse and turn on the amp. If the fuse blows, it is likely that the power transformer is blown (a rare condition). If the fuse holds, replace the tubes one at a time, starting with the rectifier. Insert the rectifier tube and turn on the amp and leave it in the “standby” mode. If the fuse blows during warm up, the rectifier tube is bad. If the amp warms up fine but the fuse blows when you switch from “standby” to “play,” then you’ve got a shorted filter cap. If the fuse holds, insert one of the two output tubes and try again. If the fuse blows, this output tube is bad. If
the fuse holds, replace the other output tube and see if it is the one causing the problem.
—Gerald Weber, Kendrick Amplifiers

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