B+ Safety Tips
From datc–(at)–ol.com Sat Dec 27 23:00:28 CST 1997
From: datc–(at)–ol.com (Datc1)
Subject: Re: DEADLY VOLTAGE IN TUBE AMPS
Date: 28 Dec 1997 04:55:28 GMT
Xref: geraldo.cc.utexas.edu alt.guitar.amps:77639
Having ingested the best that most great amps have to offer in B+, as well as
an alarming tendancy to put both hands into an amp and take it across the
chest, I have to say it’s best to avoid getting shocked. It does not feel
good, and tends to burn parts of the skin.
Here’s some tips:
1) Don’t “sleep” on the chassis. That is, don’t lay your hands or arms on the
chassis as you look for things. This creates a comfortable situation that can
come back to haunt you later when you do this with a live amp and hit some
voltage. This is one of my biggest bad habits.
2) It is best to keep one hand off the amp at all times to avoid the
possibility of taking a hit across the chest. Your cardiovascular will thank
you for your diligence.
3) Do not stand on cement. This is a very good path to ground, especially in
humid climates. Use a rubber mat or at least some carpet and sit on a stool.
4) Do not work on an amp bare-footed. Rubber-soled shoes are better than
5) In general avoid being a path to ground.
6) Avoid picking up or moving a chassis while it’s powered up. If your’59
Bassman starts to tip over the edge of the bench and you try to catch it, you
may just grab 120vac wall current, or you may stand back and see it shatter on
the floor. Neither is a welcome situation.
7) Discharge the filter caps through a 1K 1w resistor after you power down
before you start to work.
8) WATCH where you are putting your fingers. You may think you are grabbing
that clipped out resistor but your knuckle is headed straight for the AC
switch. This is another bad habit of mine, especially when I am working fast.
9) If you do take a hit, pull back but don’t panic so that you rip your skin
on the chassis or kick over the table. Even the sweet kiss from the rails of
an SVT will cause little harm if the ground is weak and your reaction swift.
10) Don’t let people hang over your shoulder when you work, and don’t leave an
open chassis on the bench plugged in. Assume that non tech-types know NOTHING
about voltage risks.
From mgarvi–(at)–anix.com Sun Dec 28 11:59:02 CST 1997
From: mgarvi–(at)–anix.com (Mark Garvin)
Subject: Re: DEADLY VOLTAGE IN TUBE AMPS
Date: 28 Dec 1997 06:14:25 -0500
Xref: geraldo.cc.utexas.edu alt.guitar.amps:77660
In <19971228095601.EAA0631--(at)--adder01.news.aol.com> jsimons92–(at)–ol.com (JSimons920) writes:
>amps (current) is what kills..1 amp will cook your heart if you complete a
>circuit.. work with one hand in your back pocket on live circuits, less of a
>chance to kill..
>voltage will wake you up, but it won’t kill you..
I know that the statement above is paraphrased often, but…
Voltage thru resistance makes current. When you touch both B+ and
ground, you become the unwitting resistor in a simple Ohm’s law
equation. In other words, considering that your own skin resistance
is relatively constant, voltage will have a very direct bearing on
Skin resistance *does* vary from one individual to another, and can also
be affected by contact with electolytes, subcutaneous contact, etc. (insert
electrodes under the top layer of skin for quantum increase in current).
This relates to all relatively low-impedance voltage supplies, like
tube amps. The place where it does not relate is in static electricity
discharge, where there is an inherent limit in source current. Even then,
a big enough sheet of charged plastic film has been known to knock people
on their butts.
There are standards for what constitutes a potentially lethal voltage.
Considering the above, the standards are relatively conservative.
I believe that in NY, anything above 32 v or so requires inspection/
licensing for use near bathtubs and such.
By the way, the figure is in the low milliamps for current thru heart
muscle, so the 32 volt figure is understandable. Next time you hold a
9v battery on your tongue, consider whether you’d want to run that
kind of current thru your heart.
Even lower voltages can be hazardous. Mechanics who get metal bracelets
or rings jammed between 12v car battery contacts find that the resulting
high current heats the metal to meat-cooking temps quite fast.