| Last update: 20:38 04/04/09


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.

Danger! sign Shock Risk sign
kidandpower02heli.jpg Edit: 20/4/07, edit, append: 4/4/09

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 (particularly when you “know” it's dead) don't touch high voltage wiring unless you really have to. I once had someone “liven up” a machine I had isolated and was working on, while I was out to lunch - and left it that way for me as a death trap.

So, when you must touch normally live wiring, check with your trusty neon screwdriver just before you do; then always make your first touch gingerly with the back of your hand. You may punch yourself in the face, but you won't grip the conductor. Strict application of these two rules alone has 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. It hurt, but was unlikely to kill.

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 particular issue with valve amps is where the amp is switched on, then switched off again before the valves heat up (or have been removed for some reason). If the amp has a solid-state rectifier and no HT bleeder (most) then the HT rail will remain charged to peak voltage.

Similarly, a power rail charged up with an innocent Megger (or other 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 (*).

“Pay attention!”

Live tests under the chassis should be done in quiet and calm with a clear head and fully awake.

The reality of fixing stuff is that static or cold test can only take you so far and you often have to probe the innards “while the soup is boiling”, the equipment is live.

You don't need any distractions such as TV, radio, music, children, pets etc., during live tests. You need to be able to hear the hum of the power transformer, any sparking or arcing, frying resistors, the valves “crinkling” as they heat up - normal or abnormal.

Meter probes are useful, but prone to slip off test points when you look away to read the meter or CRO. In this situation clip-on hook probes or small insulated croc clips are a better proposition, allowing you to pay full attention to the positioning of the clip on the live circuit, then switch your full attention to your instruments.

When you are using a probe you can steady your hand by resting your wrist on the chassis. This also means that if you do touch high voltage it will tend to only flow through your hand and not through your chest (because your other hand is in your back pocket, right?)

For signal tests I use a millivoltmeter as a front end that also feeds the CRO and frequency counter with a standardised level. This means only only one probe to connect, and generally only one sensitivity control to set.

Power mains

Always treat the power mains with the greatest respect. Even a household supply can deliver a stunning amount of current until the fuse blows or circuit breaker opens, and can cause a literal explosion of molten metal which is a particular danger to your eyes. (I wear glasses, and years ago skeptically changed from glass to triple-metal coated plastic. I have found them more scratch-resistant, much lighter, and impact-safer.)

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.

Whatever the name these all work by detecting an imbalance in the “go” and “return” currents in the active and neutral conductors, and thus must be leaking to earth somewhere. For this to work the neutral must be bound to local safety earth on the supply side, not the load side. A workshop ELB is a really good idea, and normally an ELB will provide considerable protection against accidental contact - but should never be deliberately relied on since an earth fault on the load side neutral will defeat its operation.

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. You really need to do this sometime, and right at the beginning is the best time. Look also of older rubber-insulated power lead which may well be rotting and should be replaced with modern mains cable.

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.

High tension

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. Typically these are about 1k ohm per volt, but remember that smaller modern resistors have a limited voltage rating, so it's a good idea to either check the data or assume it's around 200 volts and use two or more in series. This also distributes the power/heat.

Particularly with direct-on-mains equipment, such as stage lighting controllers, and larger valve amps, I like the idea of neon “alive” indicators within the equipment, but these are not a substitute for a bleeder as they turn off around 50-odd volts.

Shocks or burns can also make you skin your knuckles, drop a valve, drop a chassis, knock something over, not just kill.

Hot valves

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.

Broken valves

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.


Additional safety tips 4/4/09 from Brian Symons ...

I'm a retired electrician & tech but that was a long time ago so I need to keep refreshing my memory & I like to read practical articles.

I would like to make a couple of suggestions.

I liked that you made special reference to the Killer Leads to remind people - we called them “fool killers” especially those found with two crocodile clips on the end - often uninsulated.

While a dangerous item many people say they can't do without them so I like to suggest to people to buy a set of cheap meter leads with safety banana plugs or buy the plugs from JayCar & fit them to the end of a power cord (cut the leads in half & solder the short leads on), then get a couple of the new safety plastic crocodile clips with metal inserts similar to the Fluke type to plug onto the safety banana connector ends. An inline switch on the lead doesn't hurt either. If you use an inline switch then you can just wire the short test leads to the switch.

You can also use a foot switch near the bench on a separate outlet for temporary powering up.

I got caught several times by Varistors or VDRs exploding - especially if the equipment had been through a thunder storm. It isn't nice if it happens near your face. A bit of clear heat shrink on VDRs is a great safety aid. Clear heat shrink allows ease of inspection. Many commercial items have heat shrink fitted - it is often black worse luck. Fitting it before powering an item up for inspection is not a bad idea.

Some techs use Double Pole switches on their benches as an extra safety measure & some equipment has double pole mains switches. I saw a tech thrown across a room because the neutral contact opened with the active welded shut. It was on a power point. At my suggestion, he bought one of the cheap three neon type power point testers & fitted it permanently as an outlet indicator. It is just one thing to keep in mind with double pole switches - I have seen that fault quite often.

The inductive test pencils are handy too. If you rub the nose of one of them on a synthetic cloth it will light proving it is operating.

A safety banana socket on your bench with 240V active to it allows you to prove operation of your test pencil before & after testing - i.e. Safety Testing - Test the tester, test the item, test the tester

An exposed bulb for a load or for a series tester is a concern. We had a client killed by touching a metal item after a bulb broke & one of the filament supports touched the metal.

A cage from a lead light works well or rewiring a lead to use one of the newer plastic lead lights that has a plug moulded into the base. The other easy alternative is to mount the lamp holder onto a PVC pipe end cap (even a batten holder can be used) with a piece of PVC pipe rising above the bulb with holes drilled for visibility.

I mounted miniature circuit breakers on a lead with a blown fuse on the end to allow fitting into a fuse holder so that you don't keep blowing fuses when testing. I have a plug & socket to allow different fuse sizes & insulated croc clips. It is far easier to just reset a circuit breaker & a lot cheaper than blowing HRC fuses on some equipment. It you use MCBs then you can have a few breakers wired in parallel & just turn on the rating you wish.

Liquid Electrical tape is handy. I was concerned about using it until I found it listed as acceptable on a MilSpec site. JayCar have it & even SuperCheap Auto.

The heat shrink with a meltable inner wall is quite affordable from JayCar. It is great to stop moisture ingress, extra protection or as an aid for stress relief.

One practical tip too is regarding Silastic™. I have seen it used as a tack to hold wires in place on PCBs, as an encapsulating material etc. If used anywhere near metal then it must be the neutral cure variety - if it smells like vinegar then it is acid cure & mustn't be used. I saw a PCB from a large international company that had used acid cure to tack wires on a PCB. Every where that it had a PCB track under it, the track was no longer there. It had also corroded switch terminals.

Many people also don't realise that you can get QC (Quick Conect) crimp connectors that are insulated all over. These are great on switch terminals etc., for safety. Fully insulated blade connecors for them are also available for safety.

Brian Symons

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