In my continuing quest to build my first tube power amplifier, I've come up with yet another question that I couldn't find answered in a previous thread.
So, I essentially isolated everything from the power supply in this old EL84 organ amplifier I'm toying with, and I noticed, after a couple of zaps (I didn't learn the first time), that there appears to be a difference between the chassis ground of the amplifier and the ground of my scope's chassis. A surprising, painful difference that caused me to question the existence of god. Okay, okay, maybe not quite that different. More like 40VAC.
Thinking that this was the responsibility of a single leaky cap between line voltage and the chassis, I unplugged the thing and clipped the cap out, and went back to probing.
Here's a picture of what I've got (my MS Paint skills are to be feared):
So! Even after the suspect cap is gone, the chassis voltage is still there. I got spooked after blowing a 2mm diameter hole in my scope probe by accidentally using it to short line voltage to ground, so I haven't worked on it since yesterday.
Any idea what could be causing this problem? Any idea what could fix it? I've resolved to work with one hand in my pocket from now on, so to avoid further, more deadly, mishaps.
So, I essentially isolated everything from the power supply in this old EL84 organ amplifier I'm toying with, and I noticed, after a couple of zaps (I didn't learn the first time), that there appears to be a difference between the chassis ground of the amplifier and the ground of my scope's chassis. A surprising, painful difference that caused me to question the existence of god. Okay, okay, maybe not quite that different. More like 40VAC.
Thinking that this was the responsibility of a single leaky cap between line voltage and the chassis, I unplugged the thing and clipped the cap out, and went back to probing.
Here's a picture of what I've got (my MS Paint skills are to be feared):
An externally hosted image should be here but it was not working when we last tested it.
So! Even after the suspect cap is gone, the chassis voltage is still there. I got spooked after blowing a 2mm diameter hole in my scope probe by accidentally using it to short line voltage to ground, so I haven't worked on it since yesterday.
Any idea what could be causing this problem? Any idea what could fix it? I've resolved to work with one hand in my pocket from now on, so to avoid further, more deadly, mishaps.
the easiest answer to this question requires you to answer another question, does the power cord have two, or three prongs? if it only has two...buy a three prong cord, because the circuit is looking for any ground to go to, and it sees your scope, and your chassis as the best ground it can find, what you need to do is ground the chassis, to the third prong in your outlet.
I have been shocked countless times, however only once it was due to a faulty ground, I'm amazed in retrospect that I am here today.
-Alex
I have been shocked countless times, however only once it was due to a faulty ground, I'm amazed in retrospect that I am here today.
-Alex
This assumes everything is wired properly as the drawing suggests.
With it unplugged, use a multimeter and check to make sure the primary and secondary windings of the transformer arn't shorted togeather. The amp chassis should be connected to goround by the third prong on a 3 prong plug. Be sure your scopes chassis uses the third prong for chassis grounding as well. Also, there is a metal band that is by tip of of some probes which is grounded. If you accidently bump that on a hot line when metering, it will cause an arc. You may already know but, once you put the caps back in the circuit, remember that they can hold a charge once power is removed. Especially if there is no load on it.
With it unplugged, use a multimeter and check to make sure the primary and secondary windings of the transformer arn't shorted togeather. The amp chassis should be connected to goround by the third prong on a 3 prong plug. Be sure your scopes chassis uses the third prong for chassis grounding as well. Also, there is a metal band that is by tip of of some probes which is grounded. If you accidently bump that on a hot line when metering, it will cause an arc. You may already know but, once you put the caps back in the circuit, remember that they can hold a charge once power is removed. Especially if there is no load on it.
I was smart enough to check whether the primary and secondary windings were shorted before I even turned the thing on - it checks out, my DMM shows no continuity. It does hum a little when it's plugged in, but I expect that's normal.
The power cord is two-prong, unpolarized. Will the fact that the chassis has some voltage on it not cause a current to flow straight into ground if I connect a third prong to it?
If not, tonight I'm going to replace the old brown two-prong power cord with an IEC connector and cable, and install a switch on the AC input. My tube-tech mentor here at work just handed me some AC rated caps, so I can replace the one that was probably leaky anyhow.
And about that ground band.. boy do I know about that. Exactly the part that's got a 2mm hole blasted in it. The next one gets some electrical tape around that band. I do realize that had it been my hand, which was not grounded, I probably would've just gotten a nasty zap, but it's still spooky.
The power cord is two-prong, unpolarized. Will the fact that the chassis has some voltage on it not cause a current to flow straight into ground if I connect a third prong to it?
If not, tonight I'm going to replace the old brown two-prong power cord with an IEC connector and cable, and install a switch on the AC input. My tube-tech mentor here at work just handed me some AC rated caps, so I can replace the one that was probably leaky anyhow.
And about that ground band.. boy do I know about that. Exactly the part that's got a 2mm hole blasted in it. The next one gets some electrical tape around that band. I do realize that had it been my hand, which was not grounded, I probably would've just gotten a nasty zap, but it's still spooky.
Before you do anything further I would say you need to check that there are no connections deliberate or otherwise between the power transformer primary and the chassis.
It sounds suspiciously to me like the insulation in that power transformer is pretty leaky. Check also for the presence of additional death caps (like the one you removed), faulty power switches and possibly even fuse holders.
A three wire ground cord is a great idea, just make sure that you have found and fixed the leakage issue first. Hiding it with a grounded cord will make you safe until the day you find an outlet with a faulty ground connection or until the leakage gets so bad it trips the breaker or GFI in your panel.
I once had a pair of Marantz 9 in for service with faulty power switches that in the off position just happened to short AC line right to the chassis. An "interesting" experience to be sure. An internal plastic bushing wore out and shorted the mechanism to the bat.. Ouch...
Edit: IMHO all work benches in the US/Canada should be protected by a ground fault circuit interuptor (GFI/GFCI) available at most home centers for under $10. Install it in place of the outlet you plug your test equipment, bench power and projects into. I'm sure similar devices are available for the 220V - 240V world as well and highly recommend the installation of same on work bench..
It sounds suspiciously to me like the insulation in that power transformer is pretty leaky. Check also for the presence of additional death caps (like the one you removed), faulty power switches and possibly even fuse holders.
A three wire ground cord is a great idea, just make sure that you have found and fixed the leakage issue first. Hiding it with a grounded cord will make you safe until the day you find an outlet with a faulty ground connection or until the leakage gets so bad it trips the breaker or GFI in your panel.
I once had a pair of Marantz 9 in for service with faulty power switches that in the off position just happened to short AC line right to the chassis. An "interesting" experience to be sure. An internal plastic bushing wore out and shorted the mechanism to the bat.. Ouch...
Edit: IMHO all work benches in the US/Canada should be protected by a ground fault circuit interuptor (GFI/GFCI) available at most home centers for under $10. Install it in place of the outlet you plug your test equipment, bench power and projects into. I'm sure similar devices are available for the 220V - 240V world as well and highly recommend the installation of same on work bench..
There's some stray capacitence between the primary and secondaries of the power transformer. Normal. This capacitence would be on the order of a thousand picofarads, and at 60Hz would not be a shock hazard. It can cause hum in interconnected equipment though. The grounded 3 prong power cord would be a good idea, and sticking your extra hand in your pocket is also a good idea. Long sleeved shirts help some (reduces contact to high voltage to your arm. Rubber shoes help too. You want to avoid taking a shock current across your heart.
Here are some measurements I just took on my lunch break:
Primary-chassis no continuity
primary-primary 2.8 ohm
HV-HV 71.7 ohm
HV-chassis 35.3 ohm (to one lead, CT is connected to chassis)
chassis-HV 38 ohm (to the other lead)
5V-chassis no continuity
chassis-5V no continuity
5V-5V 1.4 ohm
6.3V-chassis 1.1 ohm (to one lead, CT is connected to chassis)
chassis-6.3V 1.3 ohm (to the other lead)
6.3V-6.3V 1.2 ohm
6.3V-HV 35 ohm
It looks like I'm getting resistance where I should, and open connections where appropriate. Could an imperfectly wound HV or 6.3V center tap be causing the chassis to experience this voltage?
Primary-chassis no continuity
primary-primary 2.8 ohm
HV-HV 71.7 ohm
HV-chassis 35.3 ohm (to one lead, CT is connected to chassis)
chassis-HV 38 ohm (to the other lead)
5V-chassis no continuity
chassis-5V no continuity
5V-5V 1.4 ohm
6.3V-chassis 1.1 ohm (to one lead, CT is connected to chassis)
chassis-6.3V 1.3 ohm (to the other lead)
6.3V-6.3V 1.2 ohm
6.3V-HV 35 ohm
It looks like I'm getting resistance where I should, and open connections where appropriate. Could an imperfectly wound HV or 6.3V center tap be causing the chassis to experience this voltage?
Check it out later, this might be a temporary problem. The AC neutral isn't always "ground". This can happen if the power company is experiencing unbalanced loads. That will always shift the neutral above ground. 40V sounds about right for load imbalance.
Last week, I got unjustly accused of tripping the wrong breaker when this guy who was installing a new dishwasher got a zap. The breaker was properly disconnected, but the AC power feeder neutral was hot enough to draw sparks to the real ground. Not the type of crackling sparks that leave burn marks on screw driver blades, since these currents are mostly reactive.
Still, damned unconfortable when it happens.
Last week, I got unjustly accused of tripping the wrong breaker when this guy who was installing a new dishwasher got a zap. The breaker was properly disconnected, but the AC power feeder neutral was hot enough to draw sparks to the real ground. Not the type of crackling sparks that leave burn marks on screw driver blades, since these currents are mostly reactive.
Still, damned unconfortable when it happens.
Sounds like the cap you removed was the death cap. Leave that cap out. Wire in a 3 prong plug first, diagnose the problem after. Hot to fuse to switch to transformer primary. Neutral other side of primary. Ground to chassis. No caps on this side of the transformer.
I wouldn't speculate further on your problem until you get that 3 wire in.
I wouldn't speculate further on your problem until you get that 3 wire in.
ethermion said:
I completly agree
It doesn't really matter that the chassis voltage is running to the ground, although you have to be slightly careful if your house is grounded to the water line, but thats against code now. but my house is still grounded as such, and i've never been shocked while washing my hands
I'll bet the 'stray' voltage will change if you reverse the plug in the wall. It might go away completely or at least go down.
I recommend what others have said- immediately install a three prong plug and connect the earth (green) wire to the chassis.
My son has an older Gibson guitar amp (not tubes unfortunately!). He received mild shocks occasionally when playing (he didn't tell me about it immediately or I would have had him stop using it until I fixed it). He discovered that by reversing the plug in the outlet it was fine.
I cut out the old cord, installed an IEC connector, connected the earth to the chassis and all is well. Do the same for your amp.
I recommend what others have said- immediately install a three prong plug and connect the earth (green) wire to the chassis.
My son has an older Gibson guitar amp (not tubes unfortunately!). He received mild shocks occasionally when playing (he didn't tell me about it immediately or I would have had him stop using it until I fixed it). He discovered that by reversing the plug in the outlet it was fine.
I cut out the old cord, installed an IEC connector, connected the earth to the chassis and all is well. Do the same for your amp.
It can also sometimes help to see which way around the primary leakage is the lowest..you can try the plug-flipping,and notice which way the voltage is the lowest,and which primary wire went to L and N,then when you put the IEC in,connect the transformer primary wires to the line the same way they were with the lowest leakage on the 2-prong plug.That way you've reduced the leakage,AND have the safety ground.
Okay guys, I went to Rat Shack yesterday and picked up a couple of toggle switches and some hookup wire. I also cannibalized a fused IEC socket from an old dead scope (which I have now decided is worth keeping around due to the large number of quality parts inside it).
Assuming I can figure out how to cut holes in this chassis without destroying it or me, I'll have them both installed tonight.
After that, it's power supply *design* questions, and not *safety* questions.
Assuming I can figure out how to cut holes in this chassis without destroying it or me, I'll have them both installed tonight.
After that, it's power supply *design* questions, and not *safety* questions.
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>>"The power cord is two-prong, unpolarized. Will the fact that the chassis has some voltage on it not cause a current to flow straight into ground if I connect a third prong to it?"<<
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As others have posted check for proper ac phase polarity orientation of the primary of the transformer.
Plug the power supply into the wall outlet. energize the power transformer. With a multimeter set on ac volts measure from from the chassis to the equipment ground of the electrical outlet. Note the voltatage reading. Now unplug the power supply rotate the plug 180 degrees and plug it back into the electrical outlet. Again measure the voltage from the chasis to the equipment ground of the electrical outlet. The lowest reading, of the two, is the correct phase polarity oriention for the primary of the transformer.
Basically what you are doing is putting the ac line neutral conductor (the grounded conductor) of the primary of the transformer on the same ground plane as the secondary side of power supply chassis ground.
>>"The power cord is two-prong, unpolarized. Will the fact that the chassis has some voltage on it not cause a current to flow straight into ground if I connect a third prong to it?"<<
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
As others have posted check for proper ac phase polarity orientation of the primary of the transformer.
Plug the power supply into the wall outlet. energize the power transformer. With a multimeter set on ac volts measure from from the chassis to the equipment ground of the electrical outlet. Note the voltatage reading. Now unplug the power supply rotate the plug 180 degrees and plug it back into the electrical outlet. Again measure the voltage from the chasis to the equipment ground of the electrical outlet. The lowest reading, of the two, is the correct phase polarity oriention for the primary of the transformer.
Basically what you are doing is putting the ac line neutral conductor (the grounded conductor) of the primary of the transformer on the same ground plane as the secondary side of power supply chassis ground.
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alexmoose posted:
It doesn't really matter that the chassis voltage is running to the ground, although you have to be slightly careful if your house is grounded to the water line, but thats against code now. but my house is still grounded as such, and i've never been shocked while washing my hands.
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If I could clarify a little.
In the USA the ac electrical service shall be a grounded system, per NEC. The service neutral conductor shall be bonded to the main incoming domestic water line if it is metallic. Also NEC requires a minimum of two grounding electrodes be driven into the earth and also bonded the the service neutral conductor.
Jim
alexmoose posted:
It doesn't really matter that the chassis voltage is running to the ground, although you have to be slightly careful if your house is grounded to the water line, but thats against code now. but my house is still grounded as such, and i've never been shocked while washing my hands.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
If I could clarify a little.
In the USA the ac electrical service shall be a grounded system, per NEC. The service neutral conductor shall be bonded to the main incoming domestic water line if it is metallic. Also NEC requires a minimum of two grounding electrodes be driven into the earth and also bonded the the service neutral conductor.
Jim
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