Safety Practices, General and Ultra-High Voltage

[FRE]

There are probes available that clamp on to the terminals to be tested. I haven't used one myself, but probably they are a good idea. They could be connected before powering up the device. Surely they would enhance safety by making it easier to use only one hand, or to use no hands. Also, they would reduce the liklihood of having a probe slip and cause a short curcuit.

When building, it might be a good idea to take clamp-on probes into consideration by shaping connections to expedite using clamp-on probes.

[k9swc]

Don't rely on isolation transformers or GFI's to protect you from voltages over 1000! At those potentials, a lethal current can be passed through you without tripping the GFI, popping circuit breakers or blowing fuses.

All the rules and tips posted here are good; but the number one rule in my book is to use CARE and pay attention to what you're doing.

The one time I got zapped by HV - about 50 years ago when I was a young ham working on a HV power supply - it was because I got careless (and forgot to discharge the filter caps). Luckily I was sitting in a swivel chair on wheels and the muscle reaction from my forearms which were in contact with the bench shot me backwards to the far wall - where I ended up with a nice knot on my head where it hit the wall to go along with the tingling sensation in my arm from the shock! ...Never again!

Be careful out there...

Bud

[nuvistordave]

If it is less than 1000 volts...then I'm not interested!!! Trained on radar and radiotelephone equipment in the Air Force. No jewlery, one hand rule lock out tag out, etc. Still working on high voltage going on over three decades in the private sector.(digital WAS a fad all those jobs are in China)

[seth_space]

If it is less than 1000 volts...then I'm not interested!!!

that's because you already took precautions before. It's all about the precautions here..

[UncleArt]

I'll add that no matter what the voltage, after replacing components in anything, find a way to make sure all your little clipped leads, wire strands, and solder drippings ALL come out before you power up.

Use a brush, compressed air, something... along with a bright light, and make sure none of your "debris" is going to fall free, now, or in the future, and land across a couple dangerous places.

Regards...
art

[Globulator]

I actually had a transistor amplifier (a 50 wpc Pioneer SX838) that kept a 40V charge in each of the big capacitors (80V total) for days.

Days. Fully assembled and working, obviously when off there was no current path, even via the main output transistors.

I added bleeder resistors after I discovered that!!

[Bigun]

Very helpful thread. I'm about to start on my first tube amp and this are good precautions. I hope I've captured them all. What I have taken away from this thread is:

Safety - preparing the Environment:
a) use a well lit workspace
b) insulated floor surface (rubber mat preferred, dry newspapers for 'plan B') and wear sneakers
c) mains isolation transformer is recommended
d) fit bleeder resistors to all hv capacitors - there's no need to ever remove them

Safety - preparing for work
a) make sure you are not working alone
b) double check everything, e.g. no foreign parts stuck in the chasis, insulation on hv parts, earth straps secure
c) if feeling tired, stop
d) before plugging in the power check isolation between chasis and prongs on power plug using ohm meter (chasis to power should be at least 1M)
e) after plugging in the power, before flicking the switch, put one hand in pocket
f) don't 'drink and drive' with hv

Safety - working with powered chasis
a) don't take the hand out of your pocket !
b) work methodically without haste
c) stop if feeling tired

Safety - finishing work
a) after turning off power wait and confirm psu caps are discharged before leaving the work area
b) physically disconnect the power
c) take hand out of pocket !

Yikes !

[piano3]

Originally Posted by Globulator

I actually had a transistor amplifier (a 50 wpc Pioneer SX838) that kept a 40V charge in each of the big capacitors (80V total) for days.

Days. Fully assembled and working, obviously when off there was no current path, even via the main output transistors.

I added bleeder resistors after I discovered that!!

I had a microphone phantom power circuit that in the haste of constructing to a deadline I forgot to put bleed resistors in; I found the caps still had 35V(out of about 50) after nearly two months. I think this must be an extreme case. They were described as computer grade electrolytics and were 6800 uF.

[M Gregg]

Originally Posted by piano3

I had a microphone phantom power circuit that in the haste of constructing to a deadline I forgot to put bleed resistors in; I found the caps still had 35V(out of about 50) after nearly two months. I think this must be an extreme case. They were described as computer grade electrolytics and were 6800 uF.

For all new to tubes!
So when you are working on or building a project.
Please ensure you fit discharge resistors to your project!

A good safety feature is to fit a 1M ohm 2-3Watt across your HT supply it will give a slow discharge should you have a break and start work on your project assuming you have discharged your HT! It may save you!

The golden rule test before you touch!

Those that already know take precautions!

[Rosinante]

Originally Posted by jackinnj

This exact topic was discussed by Click and Clack on Car Talk two weeks ago.

I know little about electronics. More about cars. Automotive ignition systems used to build around 15,000 to 20,000 volts. Today, 30,000-volt systems are wimpy. 40K-50K-60K and beyond are common. This amount of voltage is just ITCHING to go to ground. It will find the craziest paths. I have personally seen 3" arcs. I have also personally become a conductor, and this voltage will get your attention. Never ever touch an ignition distributor cap or ignition wires while the engine is running.