I'm getting shocked Electricity grounding with 2-prong?

[mgb_phys]

I never said "ALL STAGES SHOULD USE GFCIs!"

Sorry to break into someone's argument but all stages SHOULD have GFI's.
I'm surprised it isn't a code requirement even for existing public venues - of course I'm not surprised that most stages don't

 

[Averagesupernova]

It's ok MGB. I was just trying to keep Turbo on track with what I was saying. I'm not up on commercial code so I don't know if new construction requires GFCIs in such places or not. To be honest I would be surprised if it didn't. I'm amazed by what Turbo has said about how some of these amps are constructed. Ground and neutral tied together IN the amp? That's just asking for disaster in my book. A GFCI wouldn't run that amp for a millisecond with good reason.

 

[mgb_phys]

I've built CCD electronics with ground and neutral tied in the equipement.
I've also fitted switches to allow the case to float or be earthed.

For very low noise level electronics in odd environments it's trial and error what should be earthed to what, especially when you have conducted, inducted and radiate noise. Of course if you give the occasional astronomer a shock - it just teaches them not to touch the hardware.

 

[berkeman]

Of course if you give the occasional astronomer a shock - it just teaches them not to touch the hardware.

That killed me mgb.

I didn't read the rest of the thread (apologies), but I'm pretty sure the NES in the US requires connecting Earth and Neutral at the panel, and nowhere else. But I could be wrong.

 

[sophiecentaur]

That is incorrect and it is dangerous misinformation. In grounded amps, the ground is tied to neutral. . . . . . . .

I have just got back to read this statement. I am totally amazed at what you say. It can't be UK practice. Can the US be such a lethal place?
Do you realize that the implication of this is that if the Neutral on the feed becomes damaged or disconnected then the equipment load appears in SERIES with the person holding it, connected to the Live, the only path to ground being through the player's body. Almost the whole of the supply volts appear across the player. There is no 'safety net for this, at all, unless someone else happens to touch it first.
How could the Neutral become disconnected? Somebody steps on the plug and snaps off a pin, then plugs it in without looking too closely. Or, someone wrenches the cord and pulls out the Neutral from the pin, inside the plug.

There are other ways of eliminating Hum Loops than deliberately exposing people to mortal danger. Transformers don't cost that much and optical coupling kills Earth Loops stone dead.

 

[turbo]

That's the common practice in US-built amps. It was worse earlier on. The most popular vintage amps here are Fenders. I have rehabilitated, restored and made safety improvements to dozens of them. Tweeds are the touchiest, since most owners want them 100% original. I wouldn't service them if the owners wouldn't let me make safety improvements. The Black-Face series was marginally better, but still didn't come with grounded plugs. Ditto with cord replacement. If an owner wanted me to fine-tune and voice his amp, but wouldn't let me reverse the connections on the fuse holder and replace the power cord with a grounded one, I wouldn't do ANY work on the amp. By the time Fender started making Silver-Face amps, there were some minor improvements in electrical safety, but progress over those 3 decades was very slow.

Still, the convention is to ground the power supply to the chassis, and use chassis ground as part of the signal path, so that the rings (outer conductor) of the guitar cord are at ground potential, as are the strings, bridge, and internal shielding of any guitar plugged into the amp. Things may be different today, since I have been out of the amp-repair business for ~15 years or so, but I'd be surprised if they were.

Edit: I would like to know if British amps are configured any differently, because Marshalls were almost direct copies of the Fender Bassman, at first, and Vox (although single-ended and hot-running) were configured similarly with respect to tying neutral to ground.

 

[sophiecentaur]

Edit: I would like to know if British amps are configured any differently, because Marshalls were almost direct copies of the Fender Bassman, at first, and Vox (although single-ended and hot-running) were configured similarly with respect to tying neutral to ground.

In the UK we have this thing called Type Approval and "they're" very hot on electrical safety. It would really surprise me if you could even import such dangerous kit.
I'm also surprised that someone hasn't had their arses sued, in the US, over the accidents that must have happened.
I just don't understand what's wrong with a Transformer. Would it be such a major increase in the price of something which is, in any case, already over-priced?

 

[turbo]

In the UK we have this thing called Type Approval and "they're" very hot on electrical safety. It would really surprise me if you could even import such dangerous kit.
I'm also surprised that someone hasn't had their arses sued, in the US, over the accidents that must have happened.
I just don't understand what's wrong with a Transformer. Would it be such a major increase in the price of something which is, in any case, already over-priced?

I don't think they have to be imported. I just looked up a schematic of a reissue Marshall amp last revised in 2003. They still seem to follow the conventions established by Fender decades ago. Notice that the power input ground (chassis ground) is the same as the preamp ground, and it is used as the return path for the inputs, which are also at the same ground potential. The option of switching input power polarity in such a system is not dangerous in and of itself for an individual amplifier, but for multiple amplifiers and PA equipment on a stage, it poses safety problems, since various amplifiers, mics, guitars, etc can end up at different ground potentials and the threat of shocks is very real.

http://www.schematicheaven.com/marshallamps/marshall_2203_reissue.pdf

 

[vk6kro]

We use 240 volts in Australia and Australian lives are valuable (and expensive if you wreck one.)... :)
We have this system:

[PLAIN]http://dl.dropbox.com/u/4222062/electrical%20wiring.PNG

Imported Audio Visual equipment has no ground connection but it is in plastic cases and has an isolating power supply. Any exposed metal is isolated from the mains, but is not grounded.

If it is common practice to connect neutral to metal cases of amplifiers in a country where many mains plugs are reversible, I would be wearing rubber gloves.

 

[mgb_phys]

The US wiring codes are pretty much the same.
The problem comes when you have equipment from the 50s/60s that has been re-wired numerous times by generations of musicians/roadies with different levels of skill.

Then there's the stuff that was hastily modified on stage to solve a ground hum in the venue with a couple of extra outlets put in by the owner's brother-in-law.

I worked in the lab in a major university where the world famous physicist had fitted an extra outlet by simply knocking a hole in a wall and plugging an extension cord into a spare socket in another lab. Unfortunately the other lab was a different phase, so the computer and monitor were a mere 415V apart.

 

[Averagesupernova]

I don't see that the schematic of the Marshall amp is strange at all. This is pretty common practice. Neutral is kept isolated from conduit ground as it should be. The fact that signal grounds are tied to conduit ground is a good thing for safety as far as I'm concerned. Why should these grounds be treated any differently than any other metal chassis part of something that plugs into the wall? If the human body routinely touches it and it's metal and part of something that plugs into the wall, it needs to be tied to conduit ground.

 

[mgb_phys]

The fact that signal grounds are tied to conduit ground is a good thing for safety as far as I'm concerned. Why should these grounds be treated any differently than any other metal chassis part of something that plugs into the wall?

Because it can cause mains hum and ground loops.
The correct solution is to have all the signal grounds connected to a separate star ground and connect those together to a single reference point as well as having all the screened leads between instruments only connected at one end.
A real world solution on stage is just to cut the Earth line off the plug.

 

[Averagesupernova]

I never said there was no chance of noise. BTW, I took Turbos word for it that the inputs ground is referenced to conduit ground. However, I don't actually see this on the schematic. I would assume it is the node that is the low side of gain pot, presence pot and volume pot but I don't see it actually referenced on the schematic. I also notice that all output tubes except V4 have the control grids connected to the cathode (conduit ground). This must be a mistake on the schematic?

Edit:

The option of switching input power polarity in such a system is not dangerous in and of itself for an individual amplifier, but for multiple amplifiers and PA equipment on a stage, it poses safety problems, since various amplifiers, mics, guitars, etc can end up at different ground potentials and the threat of shocks is very real.

I don't see it that way. Obviously it's a bad idea to switch the hot and neutral on account of the fuse ending up in the neutral, but if all equipment on stage has a power transformer configured the way the Marshall amp is, it won't make any difference concerning grounds ending up at a different potential. I'd like a schematic posted of an amp that has the neutral and ground tied together in the amp. Turbo, can you provide? :)

 

[sophiecentaur]

We are clearly talking at cross purposes in this thread.
The schematic in the earlier post shows a transformer between the equipment and the 'mains'. That's fine.
But there have been statements about DIRECT connection of the Mains Neutral to parts of the circuit. I still can't believe that anyone would ever do that - at least in modern times. Is there a published schematic of this practice?

 

[vk6kro]

But there have been statements about DIRECT connection of the Mains Neutral to parts of the circuit. I still can't believe that anyone would ever do that - at least in modern times. Is there a published schematic of this practice?

The neutral has to connect somewhere. The hazard comes when it is connected to the exposed metal parts of the device.
This would be madness, especially as the American 2 pin plug comes in an unpolarized version which would give you a 50% chance of getting the mains active on exposed metal surfaces, including microphones.

 

[turbo]

There is no DIRECT reference of neutral and ground inside that Marshall amp. They are referenced by the polarity of the power supply. Sorry if I was unclear on that. On older Fenders, there was no ground lug, and the chassis ground's potential depended on which way the plug was oriented in the outlet. On later amps, there was as polarity switch, with which you could switch the hot and neutral feeds to the power transformer. Also dangerous, because the chassis' potential depended on the polarity chosen with that switch. People who snap off ground lugs and/or use outlets that they have not tested (testers are really cheap!) can cause the same dangerous situation.

Again, if your amp is operating at an opposite polarity of another amp on stage, it is a dangerous situation, because the chassis grounds of those amps are operating at potentials 120V apart. Touch a mic or another guitar or another guitarist, and current will flow.

 

[sophiecentaur]

@vk6kro
The Neutral connects to one end of the primary of the input transformer and the Live connects to the other end. Thereafter, everything is AC coupled; the secondary has a floating AC voltage across it. The only schematic shown on this thread is exactly as I should expect it to be. How do you reckon that a neutral can be connected to any other part of the amp?

@turbo-1
I don't understand what you mean by the word "referenced".
As far as I can see on the schematic, there is noDC connection between incoming supply and the amplifier circuitry. The secondary of the mains transformer appears to be isolated from the mains, as it should be. How can the sense in which it, the secondary, (or, for that matter, the Primary) is connected to the circuit make any difference to the safety of the device? The secondarys of two transformers on two pieces of equipment will be floating and there is never a way of being sure that they are 'in phase' if one end of each is connected to ground. This is not relevant, in any case, because the AC from the secondary will be rectified and all the subsequent potentials will be positive relative to ground.

 

[Averagesupernova]

I think what Turbo is saying is that older Fender amps were what was once called a hot-chassis. This was quite common and acceptable years ago with consumer gear. AM receivers for instance had NO power transformer and simply rectified and filtered the 120 VAC power coming from the power cord. This gave a plate supply voltage around 160 - 170 volts DC. The 'ground' or chassis side of the DC supply could be a direct connection to the hot side of the 120 VAC outlet depending on which way the cord was plugged in. This was not a problem because NO part of the receiever chassis was exposed to the consumer. However, when we try this in guitar amps it can become a problem as Turbo has mentioned since by definition a guitar amp WILL have it's signal ground (sleeve on the 1/4 inch plug) connected to the potentially 'hot' chassis. The reason for a hot-chassis amp or receiver? It's cheap. Eliminate the power transformer.

 

[turbo]

sophie, the chassis ground in an amp is referenced to either the hot or neutral leg of the power supply. That is a purely internal process. If you reverse the polarity of hot and neutral feeds to the power transformer, the system's ground (chassis ground) and the ground state of the connected equipment and musicians will be 120 volts away from the ground-state of the feed power. It's not a simple absence of a ground in the power-supply. The real danger is in the reversal of hot and neutral supplying the power transformer. That reversal results in a chassis ground/system ground that is 120V away from the other guitars, mics, and musicians on the stage. That kind of potential can result in a mouth-to-hand or hand-to-hand shock that will travel right through your heart and stop it.

As others have pointed out, such problems can be averted by the installation of ground-fault interrupters in all outlets on stages, but try to convince the owner of a bar, tavern, music venue to take on that expense, absent codes forcing them to do so. It's like shoveling water uphill.

If a person brought an amp to my open-mic jams and I thought the power cord might be a replacement, I'd have them plug it in and check the ground-state with a cheap multimeter. You'd be surprised how many people are playing through amps that have been "repaired" improperly, endangering their lives.

 

[sophiecentaur]

EDIT @Averagesupernova
I seem to remember that old b/w TVs did the same thing - with even an autotransformer to provide a higher HT. But they were not much of a risk because the only connection to the outside world was via a floating antenna socket and you weren't expected to connect anything external to them.
Pretty naff and, afaik, this shocking habit died once they started to use transistors, in the mid 60s at the latest.

And no one has ever been sued for an accident resulting from this system for guitar amps? I'm equally surprised by that. I know that Rock Musicians have always been a bit naive in the ways of the World but Managers aren't such suckers.

 

[sophiecentaur]

@turbo-1
I see that you are not referring to the schematic shown in the thread. You are referring to what were once called AC/DC systems, I seem to remember.
I think you would be advised not to touch anything like that with a proverbial barge pole, in any professional capacity. Anyone getting a shock from equipment you had 'seen to' would be likely to sue YOUR arse.

Also, a ground fault detector can shut off even when neutral is connected in the right sense. You only need a finite Neutral voltage and a tiny drain to Earth. RCDs are not the solution to making the amp work even if they save lives.

BTW, By "referenced" do you mean 'connected'?

 

[turbo]

I am not referring to "transformerless" amplifiers (which are deadly dangerous) but to amplifiers with separate power transformers and output transformers that are still being manufactured today. The reversal of hot and neutral feeds to the power transformer results in chassis grounds that are referenced to hot instead of to neutral. In a single amp being used in private, that is not much of a problem. On stage with other musicians, it can be a deadly problem because your "ground" (the potential of your bridge, strings and fingers) will be 120V removed from the "ground" of other guitars, mics, etc, on stage.

I don't know of a simpler way to explain this. I should be able to to better, since I wrote the section on electrical safety for a program that I presented to the superintendents of the electrical departments of one of the largest chemical companies in the world. The man who managed the clerical/scheduling aspects of my projects would do a better job, because he had extensive experience with systems on naval vessels that employed ungrounded/floating ground chassis. Grounding is a complex thing in practice.

 

[vk6kro]

I am not referring to "transformerless" amplifiers (which are deadly dangerous) but to amplifiers with separate power transformers and output transformers that are still being manufactured today. The reversal of hot and neutral feeds to the power transformer results in chassis grounds that are referenced to hot instead of to neutral. In a single amp being used in private, that is not much of a problem. On stage with other musicians, it can be a deadly problem because your "ground" (the potential of your bridge, strings and fingers) will be 120V removed from the "ground" of other guitars, mics, etc, on stage.

If the secondary of the transformer is used to produce DC to run an amplifier, then the input connections to the transformer make no difference. The active lead goes to one end of the primary and the neutral goes to the other end. As long as they don't go anywhere else, there is no problem.

Grounding should, and must, be done with the GROUND lead, not the neutral.

 

[turbo]

If the secondary of the transformer is used to produce DC to run an amplifier, then the input connections to the transformer make no difference. The active lead goes to one end of the primary and the neutral goes to the other end. As long as they don't go anywhere else, there is no problem.

Grounding should, and must, be done with the GROUND lead, not the neutral.

And what of the polarity of the DC when the leads to the power transformer are reversed? What happens to the DC produced after rectification? Do you still think that there is "no difference"? Do you believe that the chassis ground will be at the same potential as that of an amp that has not had its power supply reversed? Why?

Please give reasons.

 

[TurtleMeister]

Reversing the hot and neutral at the transformer primary should have no effect on the DC output regardless of whether the chassis is grounded or not. It should be grounded for safety, but even if it's not there should be no dangerous current available between the chassis and ground (as long as the transformer is doing it's job of isolating the primary from the secondary). And there should be no dangerous current available between the chassis of two amplifiers regardless of the polarity of the primary connections. However, there may be a very tiny leakage current due to capacitive coupling between the transformer windings and core.

 

[turbo]

And when the DC in your amp is peaking when the DC in your buddy's amp is at its lowest, that's no problem, right?

The bloviating on this thread has been dreadful. Imagine you can plug in a Blackface Champ (I'm picking the simplest Fender circuit for practicality), and plug one into an outlet, and plug another into the same outlet with the polarity reversed. What happens if you check the chassis grounds of the two amps?

 

[Averagesupernova]

I'm still waiting for a schematic Turbo. Your last 2 posts have thoroughly confused me.

 

[turbo]

I'm still waiting for a schematic Turbo. Your last 2 posts have thoroughly confused me.

You can pick up schematics for Fender and Marshall amps all over the net. Look at the schematics, and imagine what would happen to the rectified DC after the power transformer if the hot and neutral feeds to that transformer were switched. Once you have digested this, you should think about how the ground potential of the amps are referenced, either to hot or neutral. Is anybody picking up on this?

 

[TurtleMeister]

And when the DC in your amp is peaking when the DC in your buddy's amp is at its lowest, that's no problem, right?

What DC are you talking about? The plate suppy?

Imagine you can plug in a Blackface Champ (I'm picking the simplest Fender circuit for practicality), and plug one into an outlet, and plug another into the same outlet with the polarity reversed. What happens if you check the chassis grounds of the two amps?

Do you mean the voltage between the two chassis? If you use a VOM then it should be zero, or close to it. If you use a more sensitive meter then you may read the transformer leakage I mentioned in my previous post. I'm assuming these amps have transformers.

I will take your advise and try to find schematic. But I can't imagine that these things would be wired so dangerously.

 

[Averagesupernova]

You can pick up schematics for Fender and Marshall amps all over the net.

I would really prefer you post a link to something specific you have in mind.

 

[vk6kro]

And what of the polarity of the DC when the leads to the power transformer are reversed? What happens to the DC produced after rectification? Do you still think that there is "no difference"? Do you believe that the chassis ground will be at the same potential as that of an amp that has not had its power supply reversed? Why?

Please give reasons.

The output polarity of any DC supply depends only on the orientation of the rectifiers in the power supply and is not affected by the phasing of the input AC to the primary of the transformer.

Of course the chassis grounds will be at the same potential if they are only joined to the ground wire.
That has nothing to do with the phase of the mains input to the primary of the transformer.

You seem confused about that Marshall amplifier circuit. The switch in the primary of the power transformer is not for reversing the phase of the input AC. It is a double pole switch that switches the Active and Neutral on or off. This is a good safety precaution.

 

[TurtleMeister]

Imagine you can plug in a Blackface Champ (I'm picking the simplest Fender circuit for practicality), and plug one into an outlet, and plug another into the same outlet with the polarity reversed. What happens if you check the chassis grounds of the two amps?

The chassis is isolated from the primary by the transformer. The chassis is not grounded (floating). There is a .047 mfd capacitor connecting one side of the primary to the chassis.

What happens if you check the chassis grounds of the two amps?

Depending on the sensitivity of the meter you may read 120 volts. But it is not dangerous because once a load is placed between the two chassis the voltage will disappear or drop dramatically. According to my calculations (considering the capacitive reactance of two .047 mfd capacitors and the resistance of the human body) there may be around 1 micro amp current flow if you touch the two chassis. I don't know if that's enough to feel, but I'm pretty sure it's harmless.

 

[vk6kro]

That circuit looks like a death trap.

If the switch, the transformer, the fuse holder or the .047 uF capacitor in the primary circuit of that transformer were to fail by shorting to the chassis, the chassis could easily become live at 110 volts.

At least it does have a transformer in it, and probably there aren't many of these around.
Any that are left should have a grounded plug and cord fitted or they should be thrown in a bin with the power cord cut off.

 

[sophiecentaur]

And when the DC in your amp is peaking when the DC in your buddy's amp is at its lowest, that's no problem, right?

The bloviating on this thread has been dreadful. Imagine you can plug in a Blackface Champ (I'm picking the simplest Fender circuit for practicality), and plug one into an outlet, and plug another into the same outlet with the polarity reversed. What happens if you check the chassis grounds of the two amps?

I think you have a real difficulty with electrical theory here, turbo-1. The 'peak' of one man's DC is the same as the 'peak' of another man's DC; apart from a few volts of ripple, (they do have smoothing, don't they, to stop hum) the rectifiers ensure that they are the same and Definitely of the same polarity (else the valves wouldn't be working, would they?). I can't imagine anyone connecting the (floating) HT+ to the Ground / chassis.
Edit: The full wave rectifier means that 'peaks' for both amps will come along at the same time.

You have not told us why the Neutral of the supply needs to be connected to anything other than one end of the secondary. Just refer to the one schematic that has been supplied to us and make your argument in terms of that.
And I hate to carp but could you please tell me what you mean by the word 'referenced'?

And you manage to complain about bloviating! Give us some serious theory. Just because you claim to have lectured to people about your ideas does not justify them in the slightest. I have been paid, for many years, to get these things right and to tell people about it - many cleverer people than I have agreed with what I have had to say. This is such elementary stuff.

 

[TurtleMeister]

That circuit looks like a death trap.

No, it is not as safe as modern circuits because it is not grounded. But you have to remember that back in the 60's many U.S. households did not even have grounded receptacles. If you think that is a death trap then what do you think about this?

I use to collect and restore antique radios and this was very common. The radios were usually housed in a non conductive enclosure but they were still very dangerous. This particular model had a volume control and tuning capacitor that mounted on the chassis. The control shafts protruded from the case. If the plastic knob broke from the shaft then the user could be directly exposed to 120 volts.

When I first read this thread I thought this was the type of circuit that Turbo-1 was referring to. But then when he said the amps had power transformers I became confused about what he was talking about.