Lethal Voltages

[Tom Bavis]

You can use a single bleeder resistor anywhere on the B+ chain, as they're all connected together. As mentioned, it may drop the voltage too much at the "end", depending on the values of series resistors. At 300V and less, I'd use a 100K, 2W resistor for a bleeder. For up to 500V, 150K, 2W. The discharge time (in sec.) to about 1/3 voltage is R*C where R is the bleeder resistance in megohms, C is the total capacitance. Double this time and it'll be about 10% voltage, safe. So let's say you have 150 uF total in the power supply, 150K resistor. It'll take about 22 sec. to drop to 1/3, in 45 sec it will be down to 10%. If you are OK with a slower discharge, use double the resistor value at 1W. And of course use two resistors to cut the discharge time in half. I wouldn't use smaller than a 1W resistor, since smaller ones may not be rated for the voltage.

 

[poconoman]

Hi Tom,

So if I use a single/double 10k 3w resistors, it'll work? I'm assuming it'll take longer to discharge.

 

[jaymanaa]

Hiya,

Not so much lethal from the voltage but usually what would really be worse a lot of time was the reaction. You could fall backwards off your stool and really hurt yourself or also rip open your arm/hand pulling away from the shock and hitting something sharp.

I wear gloves when working on electronics myself. But I can also see why someone would disdain that approach.

Even with the gloves (Later development as I have learned poking at the fire as it burns) I have been shocked so many times in my life I can not remember and every time it has sucked.

Frannie

My Dad always had cuts on his right arm from jerking it back out of TV chassis. He was a little nuts though, I remember him sticking a screwdriver under the HV cup on the picture tubes and judging by the arc if the flyback was putting out. Getting shocked isn't as bad if you are expecting it, but yes, every time I've been hit it sucked. Which hurts worse, AC or DC? I could never decide.

 

[Hajidub]

My Dad always had cuts on his right arm from jerking it back out of TV chassis. He was a little nuts though, I remember him sticking a screwdriver under the HV cup on the picture tubes and judging by the arc if the flyback was putting out. Getting shocked isn't as bad if you are expecting it, but yes, every time I've been hit it sucked. Which hurts worse, AC or DC? I could never decide.

An elderly instructor in my Air Force Tech school used to jolt himself, the way you explained, all the time to "show us the danger." We just thought that's what he did to keep the ticker going.

 

[peterh]

Hey Folks,
I have a question about the voltages and charges stored in tube amps.

I know that when an amp is plugged in and running, the voltages and the current available are LETHAL. What I don't know and was wondering about are the charges stored in the caps when the unit is unplugged.

Can the amount of current and voltage that is stored in a capacitor be lethal even when the unit is unplugged?
I just got hammered by one that I thought I had discharged. Hurt like holy heck.

Break time for sure.
Mark

Well, you know a man's gotta stand for some sparks.
Feeling the surge might want you to urge
for some power to make the heart hower.

Those useless transistors
won't move my elder sisters!

 

[diamondsouled]

There's a reason that the old techs who used to work on the old tube stuff often worked with a 1/2" rubber mat under their chairs. :yes:

It depends on the type of cap as well. Some will hold a charge longer than others and some will recharge all on their own more readily than others.

High voltage caps can even shock/arc without physical contact:

http://ehs.berkeley.edu/hs/129-less...used-by-high-voltage-capacitor-discharge.html

 

[buglegirl]

Jay,

Which hurts worse, AC or DC? I could never decide

I had a friend once who was working on my old Chevy pickup. He was working around the alternator when suddenly he started yelling and trying to pull his hand away. Turns out that he had a watch on that contacted the stud out of the back of the alternator and the case. His watch fused to the stud and he could not pull away without a lot of force.

Left a watch shaped burn on his wrist he still sports to this day. Now I never done anything like that myself but I have to say that looked pretty painful.

Now to AC hurting my sisters husband was working on some Air Conditioning ducting at a work site. He contacted a high voltage line while lifting some duct on his shoulder in a bucket. The line was suppose to be dead but it was not. He fell 15 feet to the ground fracturing his skull. So that was obviously painful as well.

The moral. Can't be safe enough dealing with electricity both small and large AC or DC.

Frannie

 

[cubby01]

Hey Folks,
I have a question about the voltages and charges stored in tube amps.

I know that when an amp is plugged in and running, the voltages and the current available are LETHAL. What I don't know and was wondering about are the charges stored in the caps when the unit is unplugged.

Can the amount of current and voltage that is stored in a capacitor be lethal even when the unit is unplugged?
I just got hammered by one that I thought I had discharged. Hurt like holy heck.

Break time for sure.
Mark

Mark, you got your answers earlier, but you have taught me something. You taught me to never assume that just because someone has been at this hobby and repairing tube stuff for a number of years that they know all the safety rules. Glad you only got a jolt. There was a kid here in KC last year that got killed taking apart a computer power supply. Was unplugged (and I believe out of the computer) and thought to be harmless. His parents said he had taken apart several of them in the past and didn't know they could be dangerous.

 

[jaymanaa]

Jay,



I had a friend once who was working on my old Chevy pickup. He was working around the alternator when suddenly he started yelling and trying to pull his hand away. Turns out that he had a watch on that contacted the stud out of the back of the alternator and the case. His watch fused to the stud and he could not pull away without a lot of force.

Left a watch shaped burn on his wrist he still sports to this day. Now I never done anything like that myself but I have to say that looked pretty painful.

Now to AC hurting my sisters husband was working on some Air Conditioning ducting at a work site. He contacted a high voltage line while lifting some duct on his shoulder in a bucket. The line was suppose to be dead but it was not. He fell 15 feet to the ground fracturing his skull. So that was obviously painful as well.

The moral. Can't be safe enough dealing with electricity both small and large AC or DC.

Frannie

I've never worn a wedding ring at work. There are plenty of stories about them melting right around some poor fellas finger. My scare was a little different. I was up on a mezzanine with a crane swinging a 30hp compressor in to me. When it got close enough I grabbed it, but then it started to swing back out. I let go, but my ring got caught on a head bolt and very nearly pulled me out to a 30 foot drop. :thumbsdn:

 

[diamondsouled]

I've never worn a wedding ring at work. There are plenty of stories about them melting right around some poor fellas finger. My scare was a little different. I was up on a mezzanine with a crane swinging a 30hp compressor in to me. When it got close enough I grabbed it, but then it started to swing back out. I let go, but my ring got caught on a head bolt and very nearly pulled me out to a 30 foot drop. :thumbsdn:

Speaking of wedding rings, my cousins hubby was out bailing hay one day and jumped off the back of a half-ton truck. The half-ton kept moving, along with his ring finger.

 

[dewdude]

Electricity hurts. I to this day have a healthy fear and respect for it.

Tends to be a problem because I'm working with it all the time.

I haven't gotten a really good jolt in a while. The last I remember was when my finger slipped and I hit a 400V B+ feed. Woke me up, that's for sure. The line voltage to the hand doesn't count; I think too many HV shocks have desensitized me. I did more damage banging my hand in to sharp metal pieces than I did that jolt.

But, basically; unless I know its discharged, I never assume. I was told a CRT had been discharged once; it hadn't. The HV caps in front of the flyback still had charge. All I remember is waking up on the floor with my arm feeling like it was on fire. I had numb spots on my hand and arm for quite some time afterwards; I think I have most of the feeling back. I did get zapped with the residual charge from *just* a CRT; that was mostly like a static shock: but left a metal taste in my mouth.

My boss is always giving me crap for being too careful; he's of the "plug it in and see what smokes" mentality. We don't have a variac in the shop; so when I rebuild amps I just have to double check everything and hope.

But still, just one good CRT jolt changed everything. Oddly enough; I still discharge CRTs with a small screwdriver and clip leads. Of course, same person from above also told me to discharge a monitor he forgot to unplug. Kapow!

I won't mention the time I had to power on a monitor that'd been in a flood; ok, I will. I set the thing on a concrete slab; put it on a surge potector and told everyone to clear out.

The green sparks were pretty.

Sent from my SPH-M910 using Tapatalk 2

 

[wa2ise]

Sometimes, when very high B+ voltages are involved, and I don't have a single filter cap of enough voltage rating, I'll series two identical caps when the voltages add up to more than I need. The capacitence of the result is half, BTW. And to be sure the voltage splits evenly, I'll add a 100K resistor in parallel with each cap. This also creates the bleeder mentioned above, thus a safety function for free. You often see this done in computer switching power supplies that use a voltage doubler for 120V or bridge for 240V, and the selector switch for the volts in.

 

[DON73]

Funny (odd) that I'm reading this today. Two nights ago I had a dream in which I turned on my tube amp and when I touched it with my other hand I received an unbelievably real shock. I thought I'd bought the farm! That's one way to get blown away by tubes.
Woke up and my tongue was bleeding where I'd bitten it hard.
I'll have to be super careful now......don't want to bite my tongue again....that hurts.

 

[jaymanaa]

Sometimes, when very high B+ voltages are involved, and I don't have a single filter cap of enough voltage rating, I'll series two identical caps when the voltages add up to more than I need. The capacitence of the result is half, BTW. And to be sure the voltage splits evenly, I'll add a 100K resistor in parallel with each cap. This also creates the bleeder mentioned above, thus a safety function for free. You often see this done in computer switching power supplies that use a voltage doubler for 120V or bridge for 240V, and the selector switch for the volts in.

I did this in my screen drive amp to achieve a safety factor of 100% (1000v when B+ was 500). I didn't want to take ANY chances of the "larger than beer can" 4300uf caps going boom in my living room. I bet one of those things would sound like a stick of dynamite. I still ended up with over 2000uf, so I didn't miss the halved capacitance at all.

 

[Jcricket]

What was weird for me today was what and how it happened. I still haven't figured it out.

I unpluged the amp and I thought I discharged the caps. I took the soldering iron in my left hand and went in to unsolder a resistor. It seemed like I didn't even get to touch the resistor and WHAM, a huge jolt went through my left hand. I dropped a hot soldering iron in my lap too. I didn't get burnt almost at all, but sheesh almighty. Took more than an hour for my hand to stop hurting. Near as I can figure, the soldering iron picked up a charge and let me have it. My chassis is earth ground, but the circuit is not. So, I am still a little befuddled on how I took such a jolt to the chassis.

 

[goldleaf]

What was weird for me today was what and how it happened. I still haven't figured it out.

I unpluged the amp and I thought I discharged the caps. I took the soldering iron in my left hand and went in to unsolder a resistor. It seemed like I didn't even get to touch the resistor and WHAM, a huge jolt went through my left hand. I dropped a hot soldering iron in my lap too. I didn't get burnt almost at all, but sheesh almighty. Took more than an hour for my hand to stop hurting. Near as I can figure, the soldering iron picked up a charge and let me have it. My chassis is earth ground, but the circuit is not. So, I am still a little befuddled on how I took such a jolt to the chassis.

Static can do that too. In winter time we can get visible sparks and they hurt just as bad but are less lethal. Better not to guess though, and play it safe.

 

[Scuzzer]

I unpluged the amp and I thought I discharged the caps.

Don't mess around with it just set up a 10k resistor with alligator clips and hook that to your B+ and ground before you mess with anything. Then the only thing you have to remember is to disconnect it before you fire up the amp again. I've had a few 'What's that smell?" type moments when I forgot to disconnect it. It also never hurts to throw a meter on your B+ before you start touching stuff.

It's a perfectly fine use for those old power supply resistors that you've removed from other amps and would normally throw away. Cracked, drifted resistors from old radios are my preferred sacrificial lambs.

 

[gadget73]

BTDT in regards to getting lit up. I blew a chunk out of the tip on my old iron when something discharged through it. The iron was grounded, and apparently whatever it was I tried to desolder was something above ground. I'm just glad that the soldering iron hit the joint before the roll of solder I was holding in my hand did. I've slipped on meter probes more than once though and gotten a nasty jolt right through the finger. It does not feel good.

 

[ccheath]

I've had a very nasty shock when 10,000volts pass through my fingers, through my body (including through my heart) down to my legs from a black and white tv. Got 200volts shock through my fingers from my fisher 500c during my teen year and later got shock 200v through my fingers again from my current 500c a few years ago. Dealing with electronics, we'll get shock sometimes due to our negligence which is bound to happen sometimes.

 

[bobrown14]

I hear your pain Cricket... What I do when working on tube amp power supplies is have a cement resistor with 2 wire leads soldered to the leads out of the resistor and hold the cement resistor in the middle and touch the 2 soldered leads to the first cap and hold it there for a 5 count... do that again about 10 seconds later, then test the 1st & cap for voltage with DMM, that and always ONLY put one hand in at a time. If I have to solder test circuit with DMM (using 2 hands) ALWAYS test for voltage on the HV caps before going in..

As mentioned ... I ALWAYS assume the circuit is hot.

Just imagine OCD type person with said resistor.. touch first cap... touch first cap again... take voltage reading.... touch first cap again ... and so forth..I've still got jolted, last time was finished amp, I'm good it's making sound, go to pick up the chassis to move to listening room and BAM on my finger cause I accidentally touched one lead on a cap (not btm plate) while holding the chassis. 600V oil cap... a little bang and an ouchy. Dog goes and finds place to hide now when ever the solder iron gets fired up.

Oh yeah another safety thing is try and work on electronics with another person in the house with you, just in case you need them to call the bus. I always get my wife to hang around not to far when I work with power supplies.

Cheers,
Bob