4
NOTE: we are not condoning this practice, in fact there are studies that show repeated microshocks
may indeed be detrimental to the nervous system - we're just trying to explain by example!
Current kills
But- let's consider a single ended 300B amp. Now we have a power supply that can supply far less
potential than that TV high voltage supply, maybe only 450 volts, but it may be able to push out 160
mA of current (if it's a stereo amp). Before you grab a bare terminal in that supply while in your bare
feet on the basement floor, you might want to make sure your life insurance policy covers acts of in-
credible stupidity.
It is interesting to note that in the days of tube powered test gear Tektronix corporation use to put a
warning inside its oscilloscopes reminding the technician that the lowly 400 volt supply, which sup-
plied as many as thirty or forty current hogging tubes, was far more dangerous than the intimidating
looking tube rectified voltage tripler circuit.
So yes, those 900V+ 211 and 845 and circuits demand a lot of respect from the builder, but watch
those 250V 2A3 amps too!
OK, so how to stay safe?
First some basics. Always wear shoes when working with electronic gear, preferably rubber soled,
and particularly when standing on concrete floors. This is because the high potential source will want
to find it's way to ground potential. If your body supplies a path to ground, that's where the juice will
go, homes. Right through you! The rubber soles will insulate you from ground.
Another classic path to electrocution is from one hand to the other. If you grab the chassis of an amp
or preamp with one hand, and touch a live terminal with the other, guess where the current will flow.
Right through you! The oldtimers figured out a good way to avoid absent mindedly performing this
shocking display - train yourself to always keep one hand in your pocket when reaching into live
gear.
A heartstopper
The reason these paths are two of the most critical is because they cross through one of the more
electrically sensitive organs in your body - the heart. Because the heart is slighly to the left side of
the chest cavity, it is actually slightly safer to use your right hand than your left hand when reaching
into or touching a probe to live circuits, as the path to ground through your feet does not pass quite
so directly through your heart as current passing through the left hand would.
I'm safe, you're safe, now what?
Make sure you stay away from mains wiring! Remember that the power cord is live even if the equip-
ment is switched off. And don't forget those mains usually supply 15-20A before the circuit breaker
will trip. If you don't need a piece of gear plugged into the wall to test it (say you need to check a re-
sistance), don't leave it plugged in! If it must be plugged in, consider use of an isolation transformer
between the wall socket and the equipment to be tested, which will create a current limit on what the
AC mains can supply.
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