• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Safety Practices, General and Ultra-High Voltage

I want to live. :hbeat:

In the interest of safety, I would like to collect whatever safety tips and practices this forum kindly and experienced members will share, so that all may benefit. :grouphug:

First, some warnings and disclaimers:
  • Any voltage can be deadly. :hot: :RIP:
  • Amplifiers are dangerous--each person (me included) is responsible for deciding if they are willing to risk their life, and for ensuring that whatever information they receive is correct
  • This forum and its members accept no responsibility for any death, injury or property damage that result from any of these suggestions--your safety is your own responsibility.

And please note that I have *no* amplifier hardware on hand, and will not for a while. This is a discussion only, and I will only be talking about doing stupid things, not actually doing them.

Okay, with that out of the way, I would like any tube equipment electrical safety advice you care to share, both in general, and for what I will classify (for the purposes of this discussion) as UltraHighVoltage--1000V to 1500V.

I'll start with what I think I know, and you can add more or correct me if I am wrong.

I can think of two basic rules of thumb--
  • Make sure any charge-storing devices remain discharged while working on the equipment
  • Avoid creating a circuit which would pass current through the heart.

From those rules of thumb, I can think of these specific practices:

  1. Put a drain load across all significant charge-storage devices (e.g. B+), and leave them there for the duration of your work--some devices can redevelop voltage if left unloaded.
  2. Verify that all charge-storing devices are discharged before starting work.
  3. Always work with only one hand--keep the other hand in your pocket.
  4. Use hook probe for secure connections and to avoid shorts.
  5. Wear rubber gloves and rubber soled shoes--no bare feet or socks.
  6. Make sure your design has the chassis safety grounded--connected to the ground wire of the power plug.

    Okay, what do you think about these practices, and what can you add?

    One member spoke of a "bench isolating transformer". What is that, what does it do, and how is it installed/used?

    Also, what extra design and usage practices apply to UltraHighVoltage (1000V to 1500V) electronics? I'm thinking about things like component choices, test procedures (standard DMMs are only rated to 600V), design practices (danger of arcing), etc.

    Thanks for your attention and advice. Although I hope for courtesy, I'd rather be flamed figuratively than literally. :hot: ;)


    George Ferguson
Thanks for starting this thread. If it develops well, we'll make it permanent.

Just some general comments that I've made before:

Reflexes, habits, and instincts appropriate to low level low voltage stuff can be deadly. At a previous company, I hired an exceptionally bright young engineer with lots of practical knowledge to do design and prototyping work on some computer peripherals. He took it in his head one day to fix one of the ovens that we used for curing resistors; while probing around with a voltmeter, all interlocks cleverly defeated, he managed to accidently brush against one of the 460V rails. He had probably done that sort of thing a thousand times before with the microcontroller and logic circuits he breadboarded, and though he certainly understood the hazards on an intellectual level, it wasn't reflexive. In the bloodhound heat of concentrating on the quarry of the oven failure, he got careless.

He is no longer a bright young engineer.

When you decide to work on high voltage circuits, spend time around experienced guys. Never work alone. For ultrahigh voltage stuff (high power supplies at 1kV and beyond), all the rules are different and you would be INSANE to the level of suicidal to tackle such a thing without the direct supervision of an experienced guy who wishes you no harm.

Too paranoid is much preferable to not paranoid enough.
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A couple of things I have learned, in addition to those already mentioned:

:att'n: Make sure the unit is turned off at the wall, and preferably unplugged before working on it.
:att'n: If you have to make live measurements, use crocodile clip-leads on your DMM so you can turn on, measure the voltage, turn off.
:att'n: Use the proper insulated top cap when required.
:att'n: Don't use a screwdriver to short a charged capacitor!
Use High Voltage parts, like resistors have a max voltage to, check the specs.

Yeah "Too" all components including wire insulation have a maximum allowable voltage and indeed resitors.

If you don't know how to figure out a resistor's voltage rating, ask for more help! you are no where near ready...

If you were using 250volt 10amp rated wiring in an amplifier that was going to have a 1000v B+ you could not use the wire and not expect electricity to shoot out of the side through the insulation and onto something nearby, like a finger or more commonly a chassis.

Also, don't ever service a TV set, don't even open one up as the chassis in two-conductor televisions are live and will kill you as they are just as if you stuck a fork into the power point, only there is much more contact area. Of course, it may be something simple and you want to see your fav programme, pay a qualified TECH to do it!! Don't take it to your friend's grandfather or your father!

Do you think a retired welfare guy has enough cash to afford all the equipment needed to fully and properly service your TV/VCR?

Believe me, you'll be doing them a favour! especially with all the new TV's out, plastic here plastic there shotty cheap components everywhere... digital this digital that. mongrels...

And never service old radio/amplifier equipment using the same wire which was in there for 50 years, the insulation /will/ fall apart
someday or even worse catch fire from excessive heat.

Always replace electrolyctic capacitors in equipment 10-15+ years old, just because an old set is 'working' doesn't mean it's going to be working for another month or 6. THESE EXPLODE PEOPLE!

Same goes with the old resistors, they are a very high fire risk especially if old-man sprays WD40 over the pots and fires it up straigt afterwoods :p Generally this is okay if you leave it for a few seconds, as it cleans the dirt and corrosion off the pot contact.

However some variants may have a low-evaporative rate and a high flammability, WD40 dries out eventually..

Generally on the output transformers the secondary voltage is too low to become dangerous, however with high impedance taps like 50-150-300 ohm taps for PA work the voltage is stepped up to god-only-knows hundreds of volts, this will give you a nasty tingle or start a fire with poor quality speaker cable!

Aswell, If a tube has a WHITE 'powder' inside it means that the tube has lost it's vacuum!! completely!
if the powder is silver or black it is FINE black actually means it's
got a better vacuum than a silver one so I recommend you don't
return it just because the powder looks 'stressed' or 'burnt'.

You're more than welcome to return it due to any concern, I'm not stopping you. Like a lop-sided tube, wriggly pin, looks weird inside, when you shake it you can hear a ting or ping, etc.

Just my $5, if anyone would like to comment, just notify a moderator or send me an email and I will modify this posting for the next 30mins, I don't get angry if I've been fed bad information at the person who notified me of that fact.

Anyone at any time can place a notification to the moderator about a specific post just by clicking "Report" on the bottom righthand side of the post-box..

DrDeville said:
Sy Said:

Can you tell us some ways, particularly online, to locate such folks?

I've found the National Association for Amateur Radio at http://www.arrl.org/FandES/field/club/ and am contacting them.

Where else should I look?


George Ferguson

Hams are a great start. Look for geezers like me.

Also older TV repairmen; they'll be tough to locate on line, but a wander through your neighborhood or your phone book might be productive. An offer of some free labor will go a long way toward securing lessons- that's how I got started.
djmiddelkoop said:
Use isolation where possible to prevent unwanted flash overs.

Hey DJ,
Nice lederhosen! ;)
Could you explain isolation, in theory, and maybe give an example? If not, I'll try to track it down. :magnify:

Thanks all, for the comments--please keep them coming. At some point, I will summarize them, for further comment.


George Ferguson
We power engineers call anything up to 1000V as low voltage, but then we're a little fuddled by standing too close to HV transmission lines.

The best advice I received when starting out was to keep my hands in my pockets. Anyone who plans to spend any amount of time tooling with electricty would do well to read a book such as the Electrical Safety Handbook. Only sections of this would apply to the hobbyist however, perhaps some more learned members can suggest a more suited publication.
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Well working with high voltage is always dangerous.
What you should use is a isolation transformer on your workbench with ground fault automatic switch. This will save you from almost any electrocution at 200-500 Volts.

When it comes to extremely high voltages like 1000-2000 volts for instance when building SE Amplifiers with RF Amplifier tubes 833, 811 etc you should really know what you are doing. Ask the people who work with RF transmiting Tubes. The ARRL books, websites and info is a good start.

I have heard someone was killed from a radio transmiter when he had his screwdriver too close to the tube anode. Not touching but too close. The arc went through the air through the screw driver. VERY DANGEROUS.
barts said:
Anyone who plans to spend any amount of time tooling with electricty would do well to read a book such as the Electrical Safety Handbook. Only sections of this would apply to the hobbyist however, perhaps some more learned members can suggest a more suited publication.

Thanks Bart! :bulb:

I checked the book out, and it looks very useful and seems to adress the sorts of questions I was wondering about, like arcing. :hot:

I plan to buy this book unless someone recommends something better. It would be great to find one specifically aimed at DIY eletronics, and would not only contain safe usage/test practices, but safe design considerations (e.g. how to avoid arcing). :yikes:

This looks like a great start. :) Any other suggestions?


George Ferguson
Please add: Wear eye protection.

Even at low or modest voltages I have thrice had small components explode in my face (or close enough to make me jump). An LED, a TO-92 transistor and a cap. In each case I was wearing saftey glasses. None of these were life threatening but without the eye protection I could have lost an eye. All were due to mistakes on my part but the dumbest thing I think you can do is believe you will never make a mistake!
Before you do anything,

1) ASSUME that every piece of exposed metal is hot with a high voltage.

2) Act as if every bit of insulation is not truly insulated.

3) Unplug it.

4) Discharge all caps.

5) Work slowly.

6) Be aware of the spark and jerk reaction or the zing and jerk reaction. When you see a spark or get a tingle of voltage, you will jerk you hand to just about any direction. The pulling back can be dangerous and also damage your project.
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Voltage at high frequency and potential

We have not been talking about high freq, directly but it seems to be a subject we are nibbling on.

RF circuits can fool you, voltages are often induced into places you would never expect them to be. Under the right circumstances any piece of conducting material in the area can be hot. When you ground part of a high frequency circuit, it may or may not in fact actually be at ground potential. Combine HF or UHF with high voltage and a whole new set of rules apply.

Know those rules, know the circuit, double check your safety chain.... stay alive!

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Promitheus wrote:

The ARRL books, websites and info is a good start.

Thanks for the tip! I nosed around the site, and could not find anything online that has not been mentioned here. Please let us know if you know of anything online that we have not included already.

Thanks! :up:

Oh, and stay out of the sun! (mythological joke there :clown: )

George Ferguson
HF, VHF, UHF, and HV

In that regard, a neat trick is to use a neon light bulb (like an NE-2) at the end of a long plastic rod (like a meter- use UHMW, not nylon) to probe for high voltage RF. This can save you from getting zapped from unexpected places even when using a supposedly-safe HV probe.

One side benefit of the NE-2 trick is that you can even estimate the frequency from the color of the bulb.
If you are working on a HP'r switchmode power supply then eye protection is a must.....an exploding semi is basically a down sized handgrenade. A boost converter with all that low Z output capacitance plus a defective feeback loop is more lethal than any conventional tranny mains cap input powered DC system.

Ear protection.....whaz's that ? Been/am married....know all about that.....

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Difference Between GRI Outlet and Isolation Transformer

Eye protection--great tip. Thanks! :D

I also appreciate the tip on using an isolation transformer on the workbench.

Could someone explain how an isolation transformer is different from the common Ground Fault Interupt outlets now mandated for bathrooms, pools, and other wet/BareFeet areas?

If it did the same thing, a GFI outlet would be easy to find and install. :hot: :scratch2:

Thanks, and keep the tips coming! :wave:


George "Clueless" Ferguson