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

[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.

 

[vk6kro]

Yes, pretty bad.

I started to fix a radio like that once, without noticing it was transformerless "AC/DC". They were never common here.

I attached an aligator clip from a signal generator to the chassis and the tip of it it got turned to molten metal while I was holding it. There was a huge flash.
Fortunately, the signal generator was well grounded.

Never made that mistake again, but good grounding gave me the chance to learn from it.

That transformer amplifier has one capacitor, a 1960's paper capacitor probably, between you and sudden death. Even that stupid game Russian Roulette gives you a better chance of survival than that.

 

[turbo]

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'?

Neutral is not connected to the secondary, but to one end of the primary. That's the only connection neutral makes in the amp.

Re "referenced': Chassis ground is NOT necessarily mains (supply) ground. In a properly-wired amp with a working grounded power cord, plugged into a properly-polarized receptacle, it will be at mains ground, but there are many ways to sabotage that. In the simplest case, like a little Fender Champ, chassis ground can be referenced to Hot or to Neutral depending on which sockets the Hot and Neutral prongs are plugged into.

 

[Averagesupernova]

Neutral is not connected to the secondary, but to one end of the primary. That's the only connection neutral makes in the amp.

I can't speak for him, but I think he meant primary, not secondary.

In the simplest case, like a little Fender Champ, chassis ground can be referenced to Hot or to Neutral depending on which sockets the Hot and Neutral prongs are plugged into.

[/QUOTE]

Here we go again. If the Fender Champ you refer to is the same as the schematic posted in this thread (post #68) then you are WAY off. The only thing wrong with that schematic is that there is no 3rd wire ground hooked to the chassis of the amp. It is no different than using a 'cheater'. If that amp is left as the schematic shows it, it will never matter how the cord is plugged in as long as there is no accidental short from either of the wires in the cord. I am still waiting for a schematic of a guitar amp that shows the neutral conductor from the cord hooked to the chassis ground (which hooks to the sleeve of the input jack).

 

[TurtleMeister]

That transformer amplifier has one capacitor, a 1960's paper capacitor probably, between you and sudden death. Even that stupid game Russian Roulette gives you a better chance of survival than that.

Yeah, those paper capacitors are very unreliable. One of the first things you do when restoring an old radio (after replacing the power cord) is to replace all the electrolytic and paper capacitors. However, in the case of the Fender amp shown in schematic of post #68, my guess would be that the .047 mfd capacitor is a ceramic. Ceramics are very reliable. I don't think I've ever had one to fail. The value and voltage rating seem about right for a ceramic.

 

[turbo]

I have tried to keep it simple and explain how polarity changes the potential of the chassis ground. Without much success, I might add.

Here is an illustrative schematic of the type of circuits Fender was using in the 60s and seventies. Just pay attention to the bottom of the schematic. See that switch labeled "ground switch"? Fender included that so that you could tie the chassis ground either to the hot or neutral leg. This type of wiring is illegal in much of the world, but it is still used in some high-end studio amps in the US. It does much the same thing as reversing the plug of an un-polarized power cord (both lugs the same size).

http://www.schematicheaven.com/fenderamps/super_reverb_ab568_schem.pdf

 

[TurtleMeister]

That is called a floating ground. It was used on some of the old AC/DC tube type radios to help reduce the "hot-chassis" safety problem. The capacitor value is large enough to provide a path for audio but low enough to prevent electrocution. I did not know it was illegal.

Edit: The chassis is already a floating ground even without the capacitor.

 

[turbo]

According to Dave Funk (manufacturer of Thunderfunk Bass Amps) it is illegal to include a switch to flip the ground reference from neutral to hot in European markets. His amps are quite high-end, so I'm pretty certain that he has explored the legal aspects.

 

[TurtleMeister]

So I assume the reason for this is that the capacitor will allow enough current to pass to cause a shock? A .047 mfd capacitor connected directly to 120 volts at 60 hz will pass about 2ma. It takes about 10ma to kill.

 

[Averagesupernova]

@Turbo:

I don't think that the 'ground switch' is 'referencing' anything when operating correctly. Notice the .047 uF 600 V cap is labeled UL. My guess is that it is a pretty bullet proof cap as caps go. Of course, nothing is failure proof. It looks to me like it accomplishes the same thing (more crude though) that the hum balance control does on the Marshall amp. I assume by 'reference' you mean that whatever is being referenced is at the same voltage as whatever it is being referenced to with the ability to source/sink current. Well that is where the difference is. A .047 uF cap has an Xc of about 56,500 ohms at 60 hertz. It won't pass any more current at 60 Hz than about 2 mA into a dead short.
-
I will say that it most certainly isn't perfectly safe and I wouldn't be surprised if it would trip a GFCI under some conditions. I'm not sure what the threshold is on the typical GFCI, I know it is pretty low, but I also know that it is common practice to use caps like that in modern present day equipment. Take a look at post #45 made by VK6KRO. The schematics in that have caps tied to ground the same way. If you look in Digi-Keys catalog you will find caps spec'd for just that purpose. With a 'cheater' adapter, you could probably make anything built today just as unsafe. At this point I am going to assume there aren't ANY guitar amps out there that use a power supply transformer that have the neutral tied DIRECTLY to signal ground. You haven't posted any schematic that shows it even though I've requested it several times.
-
Edit: Turtle, you posted a whole hour ago and I didn't see it??!??!??!??!??

 

[turbo]

At this point I am going to assume there aren't ANY guitar amps out there that use a power supply transformer that have the neutral tied DIRECTLY to signal ground. You haven't posted any schematic that shows it even though I've requested it several times.

Hot and Neutral are connected to the primaries of the power transformer. Using ground selector switch, it is possible to reference the chassis ground to either of them. In ungrounded amps you can accomplish the same thing by reversing the plug in the socket.

 

[sophiecentaur]

Neutral is not connected to the secondary, but to one end of the primary. That's the only connection neutral makes in the amp.

Re "referenced': Chassis ground is NOT necessarily mains (supply) ground. In a properly-wired amp with a working grounded power cord, plugged into a properly-polarized receptacle, it will be at mains ground, but there are many ways to sabotage that. In the simplest case, like a little Fender Champ, chassis ground can be referenced to Hot or to Neutral depending on which sockets the Hot and Neutral prongs are plugged into.

Yes - for "secondary" read "primary" - me being sloppy. (sorry)

Are you actually reading the comments that we are all making?

Do you mean "connected to"? You still don't make it clear. You won't get a shock by a "reference" - you will only get a shock when there is a high enough Potential Difference between your finger and Earth.
If the chassis is not connected to ground, the Potential with respect to the Earth can be absolutely anything and is quite independent of the way round you plug the mains connector in. There is NO DC connection to the mains supply so there is no defined DC potential. The potential wrt Earth is only established once you choose to connect (any) part of the amplifier circuit to Earth. It is not and cannot be affected by the polarity of the mains connection. If you think that it is then perhaps you could give a coherent explanation - including something to do with Kirchoff's laws - which are the bottom line for any of these circuit problems.

 

[Averagesupernova]

Hot and Neutral are connected to the primaries of the power transformer. Using ground selector switch, it is possible to reference the chassis ground to either of them. In ungrounded amps you can accomplish the same thing by reversing the plug in the socket.

So far everything posted except the Marshall is ungrounded including the one with the switch. Maybe I'm confused by what you call grounded and ungrounded. Do you mean to say that an amp like the one in post #68 you can do what you call 'reference' to either the hot or neutral by reversing the plug? That's obvious. Aside from the switch, there is no difference. They are both what you call 'grounded' or 'ungrounded' or whatever.

 

[TurtleMeister]

Edit: Turtle, you posted a whole hour ago and I didn't see it??!??!??!??!??

I've had that happen also. Not in this thread, but recently. Could be a bug.

Well, I think I have a good understanding of what the deal is with the capacitor and switch. I don't think it is as much of a safety hazard as turbo-1 has made it out to be. When properly selected, a capacitor can be as reliable and safe as a transformer. Yes, it could pass enough current to give you a shock. But not enough to be dangerous. And as Averagesupernova pointed out, the part is UL listed. If safety is a major concern then you could always just replace with a three prong plug and ground the chassis.

 

[turbo]

Please wander over to a decent book-store and read Chapter 21 of Dave Funk's "Tube Amp Workbook" entitled "Ground Loops, Noise, and Safety". I can't copy his copyrighted materials and he gets into great detail about how you can get into some dangerous, perhaps fatal situations with guitar amps. Disregard the value of the capacitor in the ground-switch circuit. There are situations in which an improperly-configured guitar amp can nail you.

 

[sophiecentaur]

Please wander over to a decent book-store and read Chapter 21 of Dave Funk's "Tube Amp Workbook" entitled "Ground Loops, Noise, and Safety". I can't copy his copyrighted materials and he gets into great detail about how you can get into some dangerous, perhaps fatal situations with guitar amps. Disregard the value of the capacitor in the ground-switch circuit. There are situations in which an improperly-configured guitar amp can nail you.

Guitar amps - even valve amps are not a special, reserved, part of space / science. The normal laws governing how electricity works apply for guitar amps too. I would much rather look at a recognised publication about AC theory than a book about particular, niche, equipment - they can be a bit quirky at times.

Except for 'AC/DC' circuits, which should never have been allowed in the first place, the sense in which the primary of the mains transformer is connected has no bearing on the safety. It may have a bearing on the Hum, however, and it could be worth swapping - or swapping the secondary polarity, even. Reducing hum can be achieved as in the Marshall amp circuit in the earlier post, using a separate LV winding with a balancing pot to chassis. (Again, having no effect on safety).

I really wish you could explain (imagine you are talking to a complete idiot!) what you mean by "referenced". It really doesn't mean a thing to me, I'm afraid.

 

[TurtleMeister]

Now that I understand the purpose and function of this capacitor I can relate to turbo-1's use of the word "reference". It may not be a good word to use, but I cannot think of a better one.

An effective way of reducing hum in an amplifier is to ground the chassis. Back in the 60's many U.S. households still had the old two prong unpolarized receptacles. One was the neutral and the other was the hot. The neutral was connected to ground at the transformer and/or service panel (so neutral was ground). The guitar amps of that era, had a floating ground chassis. Without a real ground, the amps would have a problem with hum. So a capacitor was installed between the chassis and the primary neutral/ground. The value of this capacitor was chosen so that it would provide enough of an AC path to ground to reduce the hum problem but not allow enough AC current to pass that would create a safety hazard (should it be connected to hot instead of neutral/ground). But since the power plugs of that day were not polarized, the user had to manually flip the plug to find the position that connected the capacitor to the neutral/ground (produced the least amount of hum). Or if the amp had a ground switch he could just flip the switch.

So when turbo used the word "reference" he was referring to the capacitor coupling between the chassis and neutral/ground or between the chassis and hot (depending on the plug or switch position).

Did I get that right turbo?

 

[sophiecentaur]

Now that I understand the purpose and function of this capacitor I can relate to turbo-1's use of the word "reference". It may not be a good word to use, but I cannot think of a better one.

I agree with your analysis of why they put the capacitor where they do. Isn't it usually described as 'coupling'? But where does the "referencing" come into it and where does it become a matter of 'safety'?
There is still no theoretical justification for those statements that turbo is making. The reactance of a 0.047uF capacitor at 50Hz is about j0.8MΩ. That won't let any more than a mild 'jolt' of current through you, even if your wet feet were planted on a copper floor at Earth potential. How can it be lethal?
I thought these fora were expected to vaguely scientifically rigorous so that the uninformed reader could rely a bit on what appears here. AC electricity not a mystical force and it obeys all the normal rules - so let's use them where we can.

Of course, we are relying on the integrity of those capacitors to provide isolation!

 

[Averagesupernova]

Of course, we are relying on the integrity of those capacitors to provide isolation!

That is the first thought that comes to mind of course. However, how many other single things are we relying on to provide isolation? Transformer windings, power switches, indicator lights, cord insulation, the list goes on and on. That is what engineering is all about. Making a determination of how important it is for a specific part to not fail and design/select that part or system around that criteria. There hasn't been any serious whining in this thread about those parts now has there? I think it has been recognized that there is a potential safety issue in that part of the circuit with that cap, but the recognizer has failed to see the numerous other potential safety issues within the amp and many other accepted systems that are no less serious than the cap.

 

[sophiecentaur]

Agreed.
It's just a very good thing that AC/DC is no longer with us. My Dad scared me about that in the 60s and it has lived with me 'even unto today'.

 

[Averagesupernova]

I am curious to see what the authors of the book recommended have to say. I don't feel like buying one, but I do have an interest in older technology and it just might be worth it.

 

[sophiecentaur]

Just don't touch the chassis!

 

[turbo]

So when turbo used the word "reference" he was referring to the capacitor coupling between the chassis and neutral/ground or between the chassis and hot (depending on the plug or switch position).

Did I get that right turbo?

Yes, that is correct. A chassis ground is NOT necessarily at the ground potential of the AC supply wiring. It can easily be at the same potential as the hot leg.

 

[turbo]

I am curious to see what the authors of the book recommended have to say. I don't feel like buying one, but I do have an interest in older technology and it just might be worth it.

Dave Funk's book is very popular among amp repairmen, and he addresses the many mistakes that Fender made over the years in their power supply configuration, including putting the fuse on the neutral leg, instead of before the power switch on the hot leg. Unfortunately, Marshall and other amp-makers often copied Fender's mistakes, so dangerous guitar amps are all over the place. If you're interested in older guitar amps, take a look at Dave's book. You will buy it - it's that good.

 

[sophiecentaur]

Yes, that is correct. A chassis ground is NOT necessarily at the ground potential of the AC supply wiring. It can easily be at the same potential as the hot leg.

Yes- it could be at the same potential but, with almost j1MΩ in series, how is it going to kill you - except if it causes you to fall off a ladder in surprise?

An isolated / insulated chassis could find itself at any potential - so can a human body.

 

[dlgoff]

You might not be killed, but if you have ever had sweaty hands on the guitar strings and touch your wet lips to a grounded microphone, you might wish you had died.

 

[turbo]

Yes- it could be at the same potential but, with almost j1MΩ in series, how is it going to kill you - except if it causes you to fall off a ladder in surprise?

An isolated / insulated chassis could find itself at any potential - so can a human body.

If the amp's chassis is at 120V (not neutral) and you're playing guitar, your strings and you will be at 120V, too. Touch a properly grounded mic, and your body becomes the path to ground. Musicians have died from this, so grounding/polarity is not trivial. It is likely that the OP was in a similar situation.

 

[TurtleMeister]

Turbo, there is an important distinction that you have not made here. If musicians have died from the use of one of these amplifiers (that we have shown schematics for in this thread), it was not because of their design. It is more likely that a fatality would have been the result of a component failure. Or even more likely, someone who did not know what they were doing making internal modifications to it. If you disagree then tell me under what circumstances someone could be killed from one of these amps solely because of their design.

I'm not advocating that these amps are as safe as modern amps. They're not. But they are not going to kill you because of a design flaw. The improved safety standards of today's equipment simply means that your chances of personal injury is decreased when something goes wrong. If you drive a car without seat belts you are more likely to be killed if you are in an accident. But the lack of having seat belts does not cause the accident.

There's a Barns & Noble in a shopping center not far from my home. I will try to remember to check out that book you mentioned the next time I'm there. I'm not really into guitar amps but I am into old tube type equipment.

 

[TurtleMeister]

It is likely that the OP was in a similar situation.

No, the op was about a piece of equipment with a grounded three prong plug where the ground prong had fell off. Actually, I find it very unlikely that the prong would just fall off. It's more likely that someone cut it off. I've seen people do that when they want to use a two wire extension cord with a three wire grounded device. They're in a hurry and don't have time to go out and purchase the proper cord. Anyway, the point is that the op situation is not a case of improperly designed equipment. It's a case of improper and dangerous user modification. The simple solution is to replace the plug, which someone had already suggested.

 

[vk6kro]

I agree with your analysis of why they put the capacitor where they do. Isn't it usually described as 'coupling'? But where does the "referencing" come into it and where does it become a matter of 'safety'?
There is still no theoretical justification for those statements that turbo is making. The reactance of a 0.047uF capacitor at 50Hz is about j0.8MΩ. That won't let any more than a mild 'jolt' of current through you, even if your wet feet were planted on a copper floor at Earth potential. How can it be lethal?
I thought these fora were expected to vaguely scientifically rigorous so that the uninformed reader could rely a bit on what appears here. AC electricity not a mystical force and it obeys all the normal rules - so let's use them where we can.

Of course, we are relying on the integrity of those capacitors to provide isolation!

I make that reactance about 67.76 K and the current from 250 volts would be 3.7 mA rms, neglecting body resistance.
So, you would feel it, but it might not kill you.

But, that is talking about a relatively safe amplifier as long as no components in the primary of the transformer fail. The bad ones are where they connect one side of the mains to the chassis and attach a reversible plug to the power cord.

Incidentally, the original poster of this thread was last seen on April 27th. We all agree that it is bad to electrocute people and we can't correct the bad mistakes of the past, so maybe it is time to wind this one up?

 

[turbo]

Turbo, there is an important distinction that you have not made here. If musicians have died from the use of one of these amplifiers (that we have shown schematics for in this thread), it was not because of their design. It is more likely that a fatality would have been the result of a component failure. Or even more likely, someone who did not know what they were doing making internal modifications to it. If you disagree then tell me under what circumstances someone could be killed from one of these amps solely because of their design.

There are many older Fender amps (and others) that could easily end up with 120V Hot on the chassis through improper modification, internal failure, etc. When a guitarist using such an amp touches another instrument or a mic that is properly grounded, their body becomes the easiest path from hot to ground. Mouth to hand, hand to hand... both can put the heart in the path and it doesn't take much to stop a heart.