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

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

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

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

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

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- 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

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

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 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

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.

turbo

Probably a good idea. I made it a campaign of mine to make every amp that I serviced as safe as possible, and took offense to people who minimized the risk of shock, and that seems to have ticked off a few people. Like Dave Funk, I have dissected amp designs and have refused to service amps if the owners did not let me modify them to current safety standards (fuse on the hot leg before the power switch, 3-prong plug securely grounded to chassis, etc, and NO "ground switch" allowing the chassis to be referenced to either hot or neutral). I was a small operator, but was not about to allow people to be put at risk because I ignored the mistakes of the amp designers. I highlighted Fender stuff (and Marshall) because that was the bulk of my business, but you could find some pretty "interesting" configurations in Gibson, Gretsch, and Supro amps, too, that were worse than Fender's missteps. Get into some of those amps, that are true point-to-point amps (no tag-board or circuit-board) and you've got to learn to "read" the circuit without a map.

Averagesupernova

I wouldn't kill this yet quite simply because it is not resolved. I figured 120 volts and not 250. Not sure if any of those came in a 250 volt option. I also cannot imagine ANY guitar amp design with a true hot-chassis (no power transformer). So lets throw that one out of the discussion. I am going to try to have a look at the book and post based on my findings.