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http://www.ozvalveamps.elands.com/safety.htm | Last update:
10:54 20/04/07
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Like sex and pyrotechnics, information is the best defence.
Switch off, pull out the plug, and call your friendly electrical serviceman.
- 70's SEC slogan.
There are two kinds of technicians; careless, and old.
Despite working on live gear all my life, some of it at awesome power levels, I got to be an old tech. The time you decide it's too much trouble to follow full safety proceedures will be when you have a memorable narrow escape. Or don't.
One easy rule is don't touch high voltage wiring unless you really have to. When you have to, check with your trusty neon screwdriver just before you do. Strict application of these two rules alone have saved my life when others have done something stupid.
In the old days it wasn't uncommon to cop a bite from the HT line of a mantle radio you were fixing. Two or three hundred volts from a fairly puny power supply without much storage capacity.
The power supply of a typical valve guitar amp or PA is a very different animal - three times the voltage, ten times the current capacity, and heaps of power storage. The risks are real.
“It's the volts that jolts, but the mills that kills.”
The amount of current it takes to kill would bearly light a small torch globe (or LED, 10-20mA). The important thing to avoid is setting up a current path through your chest cavity where it can cause your heart to fibrillate, that is flutter rather than beating and pumping. Take the trouble to learn CPR. It's easy and could literally save a life.
“Beware the lightning that lurks in charged capacitors.”
Electrolytic caps on the power supply lines can remain charged to significant voltages for many hours after power is removed, posing a serious shock hazard for the careless.
A power rail charged up with an innocent Megger (high voltage tester) during a cold test may build up and hold a lethal charge.
Zapping even a partly charged cap with your screwdriver isn't a great idea. With nothing to limit the current hundreds of amps can flow for an instant, enough to chew the end off your favorite screwdriver and bits out of the cap lugs. It's not kind to the cap innards either.
Abolutely never try this on a high-power solid-state amp like a rack amp - you'll get a really nasty surprise.
And that spectacular 'splat' often doesn't fully discharge the cap on the first go either. A light globe load is the right way to go (*).
Live tests under the chassis should be done in quiet and calm with a clear head and fully awake.
Always treat the power mains with the greatest respect. Even a household supply can deliver a stunning amount of current until the fuse blows, and can cause a literal explosion of molten metal which is a particular danger to your eyes.
Fuses are for fire protection, not prevention of electric shock. That's the function of an Earth Leakage Breaker - ELB, Core Balance Relay, RCD, or Safety Switch as they are variously known.
If you use an isolation transformer, UPS or inverter take particular note that an ELB on the input side will will not protect you, and even fitted on the output side will not protect you without a properly established earth, which make the isolation from the line side rather pointless.
Before doing any live work at all on an amp, open the mains plug and check for correct wiring and tight connections, and that the cable is okay.
The most deadly miswiring is transposing the safety earth (green or green/yellow) and neutral (black or blue). The amp will work okay in a correctly wired outlet, but the first time you encounter a miswired outlet the active is on the chassis and you're dead.
Visually inspect the underchassis mains wiring, particularly for perished insulation and any signs of fireworks.
Always check the chassis mains earth is solid.
This preliminary visual inspection should always be carried out, but is particularly important if the amp has been out of service for more than a few months.
Most vintage guitar amps had exposed mains wiring at the switch, fuseholder (if any at all), and tagstrip to the transformer(s) and neon pilot light.
In many cases it is better to dress up this internal mains wiring to a modern no-exposure standard with an insulated terminal block and heat-shrink tubing. (detail) And you can fit a new (longer) mains lead while you are at it.
Make unplugging the mains plug and putting it up on the bench where you can see it, a habitual part of your test cycle. Some old TV tech's would put the set plug in their pocket.
An electricians Megger or high-voltage resistance tester is very handy for static testing a valve amp before you apply serious power, particularly with amps that have been out of service for some time.
When probing high voltages keep the other hand behind you, or in your pocket. Don't rest it on the chassis or other grounded point.
Ensure that your meter and other test instruments are able to withstand the high voltages often found. Most common Digital Voltmeters (DVM's) have limited working voltage.
Do not probe the anodes of the output stage under drive - the peak voltages present can be double the main HT supply voltage and will almost certainly exceed the capabilities of your test equipment.
Always use a globe gimmic(*) to discharge the HT line before cold work. If you have to switch off before the valves warm up the HT filter caps may remain highly charged for a long time.
Many amps were made without bleeder resistors on the main HT. These are a wise retro-fit.
Shocks or burns can also make you skin your knuckles, drop a valve, drop a chassis, knock something over, not just kill.
Output valves retain heat for a long time after they have been switched off, particularly after a fault condition. Take your time.
Watch out for spring retaining clips on power valves that grip the base. These have to be pushed towards the chassis to release the valve. Never try to force a valve in or out.
Some of the materials used in the manufacture of valves are toxic, even radioactive. (detail) Treat a broken valve as toxic and dispose of carefully. Treat internal dust as toxic - do not inhale, and wash off skin.
Most of the 'safe' solvents I have used have turned out to be carcinogenic or otherwise toxic, and frequently flammable. Some will break down to highly toxic gasses if exposed to heat, such as being drawn through a cigarette, splashed on a hot soldering iron, valve, or resistor.
Safety warnings on containers are reduced to the absolute minimum, so take them seriously. Safety bulletins are available from manufacturers and Trade Unions. Get them and read them - understand the true nature of what you are using and exposing yourself to.
A glue I use to stick fabric on speaker boxes uses 'hydrocarbons' as its carrier solvent, which evaporates into the air as the glue sets. If you read 'petrol' for 'hydrocarbons' you should understand why working with such glue in a confined or poorly ventillated space with a heater going is not a good idea. And that's generally true for all solvents.
People who think it is funny to leave charged capacitors or other booby-traps around the workshop for 'laughs' are a serious industrial accident going somewhere to happen. Let it be somewhere else.
There is nothing adult about 'hazard macho' and risky pranks - they can too easily turn tragic. I have barred visitors and dismissed employees for skylarking in my electronics workshops and advise others to take an equally tough line.
(*)Globe gimmic.
I have a dual-function cable with a mains plug, socket, terminal strip, and light globe socket.
Normally the globe is used to safely discharge the HT line after a hot test.
By rewiring the terminal block it places the globe socket in series with the feed, allowing a power limited feed depending on the power of the globe fitted.
Useful for transformer tests and re-forming electro's without alarming explosions.
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