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Lethal Voltages

Jcricket

Jcricket

not someday, but today
Subscriber
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
 
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There is no exact right answer. Alot of variables. First health. If you have a bad heart, very little current can kill you. Cap shocks tend to be unpleasant but rarely lethal. Point of entrance point of exit. If your touching with one hand and then your other hand completes path, current goes across your body (heart) its the disruption of your natural cardiac pulses. If current enters finger of your hand and leaves different finger of same hand current only goes through your hand. Unpleasant but rarely fatal. Good rule of thumb, keep one hand in your pocket when you're poking around. Be aware that you are not grounded. An isolation transformer isn't a bad idea.
 
USUALLY, the current drawn by the circuit will discharge the capacitors to a safe level pretty quickly when it's turned off. But, if a unit is turned on without tubes, or it's turned off before the tubes have warmed up (and started drawing current), the caps can hold a charge for a LONG time. If you're building or modding something, you may want to add a bleeder resistor to help in such cases (though 100K across 100 uF will take about 20 seconds to drop to 10% of starting voltage). In any case, it's wise to stick a voltmeter probe in before you stick in your fingers...
 
There are definitely times where they can store up voltage, beyond what would normally seem to be a "normal" discharge interval.

One nearly SURE way to leave a bunch of charged caps, is to briefly turn on a solid-state rectified amp- not leaving it on long enough for the preamp and output tubes to heat up and start conducting. Unless there is a bleeder resistor (or some other resistor to ground somewhere in the power supply), there will be nothing to drain the voltage down.

Also, SOME caps will "regenerate" voltage, from energy chemically stored in the electrolyte (like a battery). I've seen as much as 50v "come bacK" after shorting a cap with a test lead.

So, unless the cap has been actively discharged continuously for a LONG time (as in, a shorting lead or parallel resistor to ground connected across it for quite a while), you should NEVER assume a cap, that has been hooked to power, does NOT still have charge on it!

Regards,
Gordon.
 
As Tom B said, the caps will discharge rather quickly when the circuit is drawing current. That's why when I test/tune tube amps I always supply signal to the amp for 10-15 sec after I turn off power to make sure the charge is drained from the caps.
Just my .02
 
Yep, a 1.5 megaohm, 1/2w resistor (or 1M 1w) from B+ to ground, can go a long way toward safety.

Regards,
Gordon.
 
goldleaf said:
Using bleeding resistors on power supply B+ caps seems like a really good idea. :yes:

http://www.diyaudio.com/forums/solid-state/29766-bleeder-resistors-what-values.html
Click to expand...

I'd say it's crazy not to use a bleeder. Depending on what you are dealing with, and your religion concerning power supplies, "bleeders" can be used as a very inexpensive type of "regulator". Scott always had that big white cylinder type R sticking up on the rear of the chassis that would burn the crap out of you. As far as I could ever figure, that was a VERY primitive type of screen regulation. We all know about TX sag, so what if you load the TX down to where it stops sagging. You get a more stable voltage, even though your PT is thinking horrible thoughts about you and your Mother. :D
 
I think there are two major things to remember when thinking about or working on old commercial power supplies. 1, they didn't have silicon devices like we do today, and 2, they were answering to bean counters. These two things are why it's such a simple matter to improve on these supplies for literally a few bucks.

Sorry for the OT Mark. As far as the original question, I would suggest treating an unplugged amp just like a plugged in one in terms of voltage safety. Now if you get EVERYTHING drained down to 0 (which may mean bleeding more than one spot), you can feel pretty safe. When I get an amp in that state for major repairs, I still touch all the potential hot nodes with the back of a knuckle just to put myself at ease.
 
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What a whore I am. Another thing that you can get in the habit of, is putting on your disposable wrist strap before you work on anything, every time. I've been doing this because of my using more and more sand in my amps, but there is a side benefit. If you touch something hot, it will have a easy path to ground through the strap. If using them for safety, I'd use one on each wrist to avoid a heart shock. It just gets down to how safe you need to be. I'm old and have an old heart injury anyhow, so I try to be awfully careful.
 
jaymanaa said:
I think there are two major things to remember when thinking about or working on old commercial power supplies. 1, they didn't have silicon devices like we do today, and 2, they were answering to bean counters. These two things are why it's such a simple matter to improve on these supplies for literally a few bucks.

Sorry for the OT Mark. As far as the original question, I would suggest treating an unplugged amp just like a plugged in one in terms of voltage safety. Now if you get EVERYTHING drained down to 0 (which may mean bleeding more than one spot), you can feel pretty safe. When I get an amp in that state for major repairs, I still touch all the potential hot nodes with the back of a knuckle just to put myself at ease.
Click to expand...

I was trouble shooting my amp. I had the power off and unplugged. I had run one channel without tubes. I went in with the soldering iron to remove the transofrmer leads and WHAM, right through the soldering iron and through my had to the chassis. THat was an hour ago and my hand is still a bit numb.
I thought I had killed the charge on the b+.
I think I am going to name this amp Biter.
 
In the rc car world people build light bulb bleeders to drain unwanted power. I can see that being useful with many electronics applications.
 
Hello everyone,

Noob here and I didn't want to start a new thread.

I want to discharge the quad cap safely on a ST70. I have 10k 3w resistors. Now, one lead goes to a + quad section and the other lead on the chassis. Use a multimeter and if it's at least 10v or lower on all sections of the quad cap, then it's safe. Do I have this correct? Also, any other suggestions or a better alternative is appreciated. What about bleeders? How do I install it on the ST70? Any 'how to' pics?

Thanks
 
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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
 
poconoman said:
Hello everyone,

Noob here and I didn't want to start a new thread.

I want to discharge the quad cap safely on a ST70. I have 10k 3w resistors. Now, one lead goes to a + quad section and the other lead on the chassis. Use a multimeter and if it's at least 10v or lower on all sections of the quad cap, then it's safe. Do I have this correct? Also, any other suggestions or a better alternative is appreciated. What about bleeders? How do I install it on the ST70? Any 'how to' pics?

Thanks
Click to expand...

That should be fine. I would double-check it about 2 minutes or so, after you bleed it the first time, to make sure it hasn't "regenerated" any more voltage.

As for permanent bleeders- a 1 megaohm, 1w resistor (a 1/2w resistor would be OK, but it might run a little hot) from the first can cap section (between the rectifier and the choke), to ground, should be fine.

Regards,
Gordon.
 
One of my earliest memories as a boy was opening the back of our tube TV and poking around out of curiosity. Got the shock of my life. Respected electricity ever since. :D
 
GordonW said:
That should be fine. I would double-check it about 2 minutes or so, after you bleed it the first time, to make sure it hasn't "regenerated" any more voltage.

As for permanent bleeders- a 1 megaohm, 1w resistor (a 1/2w resistor would be OK, but it might run a little hot) from the first can cap section (between the rectifier and the choke), to ground, should be fine.

Regards,
Gordon.
Click to expand...

HELLO Gordon, thanks for replying!

Can I use a resistor on each quad section? IOW, one resistor per section.

Thank you
 
You can, but it could have the effect of lowering voltages at the "end" of the B+ chain. There also isn't a big gain in safety by doing this.


I've taken to discharging caps and then leaving the discharge wire in place. I have a 100k 2w resistor and an alligator clip lead that I use at home. I used to just directly run things to ground but after having a tube rectifier flash over and self-destruct because I forgot to remove the grounding lead I quit doing that.
 
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