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From: ironma~uno.com (Mike Curtis)
Date: Sat, 21 Jun 1997 09:16:12 PST
Subject: Re: shocking old amps

While it may _seem_ that Mark and I disagree, we both know otherwise :-)

Nothing will make you indestructible, of course. Even Superman is
subject to deleterious effects from Kryptonite :-)

While it is possible that rectified DC from an amp can electrocute you,
it is highly unlikely unless you're repairing or otherwise working on the
amp and have it opened up. Electricity requires TWO contact points.
With an internal power supply, barring serious malfunction there is
simply no way to provide these externally, unless the amplifier was
designed by a total moron (or modified by one). All musical instrument
amplifiers I have ever seen are transformer isolated.

Properly grounding an amp will minimize the dangers of power supply
faulting.

Should a transformer short, you possibly could be exposed to DC, but it's
very unlikely because the transformer is grounded. This rarely happens,
and when it does, the transformer will usually blow the fuse. Still, as
Mark mentions, it can happen.

But AC is a different story. Electrocution via AC and improperly
grounded amplifiers is VERY common. TOO common. Probably most of us
have been "bitten" my mics and instruments at one time or another - and
EACH of these COULD have KILLED us had we made a better contact (e.g.
grabbed harder, been more sweaty, had current flowing from left/right arm
to right/left leg right through the heart; etc.).

A fuse offers no direct protection against lethal electrocution from
improperly wired electrical outlets or improperly grounded gear. It only
requires a few thousandths of an amp through the right organs (I believe
the "official" figure is somewhere around 30 mA) to kill you. Guitar
amps typically use fuses rated around three amps, enough to kill a
HUNDRED band members at the same time! And of course a foil covered blown
fuse (what I call a 10,000 amp fuse :-) can do a LOT more damage.

What a proper fuse WILL buy you is some very limited protection against a
malfunctioning amp. But mostly what a fuse buys you is protection FOR
THE AMPLIFIER. The fuse is rated at considerably more current than the
amplifier will ever draw during normal operation. If the amplifier
malfunctions, it may help keep destruction of amp componeents to a
minimum, and it offers some protection against the device starting on
fire from massive current.

You should ALWAYS carry spare fuses for your amps. I keep in my guitar
case a box of five exact replacements for each fused device I have, as
well as a couple of spare wall warts. If your amp blows a fuse,
sometimes it's just a bad fuse, so try replacing it with the EXACT fuse
called for (the same amps, and either fast blow or slow blow as the case
may be - don't use one for the other). If it blows again, your amp is
probably bad and usually needs to be serviced (although this can happen
if the speaker wires are shorted, so try disconnecting the speaker wires
before replacing the fuse a second time and if that fixes it, check the
speaker wires and connections for a short). If ity still blows, just
play through the bass players rig or PA.

The UK, I believe, uses 220 volts AC. This is roughly four times as
dangerous as the USA's 117 VAC, according to Ohms Law expressed as a
function of voltage and (your body) resistance:

P=E^2/R

But in either case, you're better off being overly cautious.

-- IronMan Mike Curtis
The One Man "Better'n A" Band
Electric harmonica, guitar, bass pedals, vocals
Cassette available - Email for details