Grounded

[6267]

I had a friend that uses a Gibson Labs L5. This amp was in a bad way, and had an intermittent power switch. Someone had installed a bungie cord (I never saw this, thankfully it was repaired by someone else, I would have fallen over laughing--or electrocuted) around the power switch's contacts, so that to start the amp you wiggled the bungie. So this musician is showing someone the amp at a gig, and says something like "You just have to treat it like a fine woman" while wiggling this bungie cord in the chassis when *BOOM* he gets the big shock. He always got his amps repaired after that, and he ain't so misogynist either!

[oc disorder]

just thought I'd add a couple of things but firstly I would only offer the option of grounding it as part of the repair or not working on it at all.
In Australia all 110 volt amps imported into Australia have to comply
with the regulations. All the amps I've seen have the "grounding cap" removed and a three wire power cable installed obviously with a ground wire and the appropriate plug fitted.
The cap is often not rated for 230/240 volts anyhow.
Recently all public venues have to have RCD's fitted (earth leakage protection) i.e. any current detected going to ground shuts down the power.
All new houses have to have them fitted but refrigeration is allowed to have a separate supply as for example if something happened while the owner was away the frozen goods will still be ok.
Had a quick look for "electrocuted Musicians" and interestingly... the few that I could find were English.Normally the USA leads the way with litigation
so I'd expect there would be a few precedents in the archives not that I've looked.
Also most repair shops have to carry public liability insurance whether they
trip on a step on the way in or electrocute themselves on the repaired device later. I guess 230 volts has more potential for harm than 110v but of course its the current that stops the heart.
We often have the odd electrocution , drilling into a wall cavity with unknown live wires lurking is the last one I can recall.

Anyway fitting the ground wire on imports didnt seem to affect sales.....!

Just adding there is an exception when the equipment is double insulated.VCR's well now DVD players Digital set top boxes (recieves digital channels and has vid out) some power tools,hair dryers etc.All these have plastic cases/chassis which is virtually
impossible to do in a guitar amp with an unbalanced input.
I think we will see more of wall warts as its much eaiser to make the device with a low voltage input and the wholesaler in the respective country just has to supply a local stepdown power supply to suit.

[6267]

I also just wanted to add that a 3-wire cord'd amp should not have any more hum than a 2-wire amp. If it does, that's just kind of bad design and you could probably pitch to the customer as a safety improvement to clean up the grounding!

[Chuck H]

I am in total agreement with not taking any repairs that could be considered unsafe. Aside from the obvious, I think Enzo hit it with this:

"And I also don;t want to advocate hazy safety concerns to those novice techs lurking"

But... Because it's a valid part of the subject I'm going to play devil's advocate one more time and ask:

What about the people who fancy their amp, table radio, etc. a "museum quality" piece and feel it should be wired absolutely authentic for that reason. Obviously there will be very few of those individuals who DON'T intend to play the unit, and fewer still that actually deserve to be called "museum quality". I'm sure we've all met the guy who is way too proud of his silver face Deluxe Reverb. I actually met a guy using one at a club who refused to get it recapped or retubed because he wanted to keep it as original as possible. It sounded awful. Point is, these guys are out there. Some actually do have museum quality pieces and they want the stock wiring. Who is going to repair their amps?

Chuck

[Enzo]

Those are people who have developed the form without the substance. They hear about keeping things stock, and without really understanding the contexts for that are convinced it is better to have a non-functional amp that is completely original than it is to have a working amp you can use.


Sure there are those guys out there. SO where do we set our own priorities? Do we lower our standards so absolutely no one gets turned away - like that guy? Or do we accept that he won;t want proper work done and we decline his repair? Or do we go the other extreme and set standards so high we can't fix anything? DO we serve the least common denominator or the main market. SHould McDonalds stop selling potatoes because ther is a guy in Kansas alergic to them? Extremes are extremes, they are not typical.


Makes me recall... I used to work in the arcade business. One gameroom operator had our pinballs in his place, and he had one kid who came in a lot and was relly good at it. he could beat most any machine. Put in quarter and play all afternoon if he wanted. His buddies would come watch and they all tried to emulate the guy. The room operator was freaking out. This guy is playing for hours on a quarter, you gotta make the machine harder!!! Never mind that as soon as he stopped playing all his bad-shot friends spent the rest of the night trying to get good on the game. Never mind that the game made more money than the rest of the games in that arcade. All the operator could find to focus on was the one guy who was "beating him." He wanted me to make the machine so hard, that this guys could only win now and then. That of course meant that no one else would ever be able to beat it, and they would never play it. I could show him the cash box, with more quarters in it than the other games. I could show him the meter readings and book keepeing on teh game to demonstrate it did better than most games, but all he could see was this one kid who never had to pay more than a quarter.


There will always be an extreme example, but of all the repairs I face, not many involve making a decision most owners would think controversial. And of those cases, darn few actually put up a stink, and of the ones that do, maybe I get a whacko who wants his amp in silly condition. To him I say, "well, I hope you find someone who can do the work you need, thanks." He doesn't come up often enough to worry about.

[Rick Erickson]

Even if your amp is properly grounded it doesn't mean you are "safe". What it really means is you are now a better conductor, through your guitar strings and guitar cable, for any source of electricity you might come in contact with. The best way, as a performer, to not be a "conductor" is to use a wireless transmitter system. This will at least cut the direct connection between your body and the potential that electrons tend to seek. As for the 2 conductor vs 3 conductor cord argument: I encourage all customers to have a 3 conductor cord installed. I see no advantage in using an over-sized cord however. An 18 AWG cable is fine for most applications. Larger cables may require drilling of the chassis to fit, and this is a bad path to take on a vintage chassis - especially if the customer is finicky about "originality". If the stock cable was 16 AWG or larger, then by all means use the original size cable. I have seen many bad installs of oversize AC cords that did damage to the terminals they were connected to. Big wires don't flex as easily as smaller ones. When in doubt - keep as close to stock as possible.

As for turning down unsafe jobs - I had a customer walk in with a Fender Deville, or Deluxe, Hot Rod or Blues... whatever... combo amp. The amp had been "modified" with gigantic computer grade filter caps in the bottom of the cabinet. The caps were connected to the amps pc board via a harness of speaker-grade MONSTER CABLE! We're talking the gigantic stuff designed for low voltage and high current. Clear soft insulation on large diameter cable. I had to call out my partners and our employee to see this thing. It was a technicians nightmare. We had to explain to the owner of the amp that we could not possibly work on that amp as it was, because of the liability factor. We offered to put it back to stock condition and then troubleshoot the rest of the amp for him. He walked. I think I have pictures of that amp somewhere. I should post them on our website under "bad ideas".

RE

[gbono]

Physiological effects of current density on the human body are shown in the table below. Contrary to popular belief, it is the current - not the voltage - level which is responsible for effects. According to Ohm's Law, of course, a certain voltage is required to cause the necessary currents to flow. Values show vary depending on the body.

Onset Current Level
(mA) Effect
1 Threshold of sensation
8 Mild sensation
10 Painful
13 Cannot let go
21 Muscular paralysis
20 Severe shock
38 Breathing labored
42 Breathing upset
70 Extreme breathing difficulties
90 Ventricular fibrillation
100 Death

[Chuck H]

According to Ohm's Law, of course, a certain voltage is required to cause the necessary currents to flow. Values show vary depending on the body.

And, of course, that all had to be carefully worked out when they developed the various taser (tazer?) systems, didn't it. How do suppose they fine tuned that thing.

Ouch... No add more

AAArrgh... Ok, we're getting closer...

No. Don't test on Perkins this morning. He just drank a Gatoraid.



More seriously, did you see any reference to what kind of voltage ranges it would take to get those currents to flow through a human being? I do know that if current isn't available no amount of voltage matters. But suppose current is ample. I'm sure most people have similar electrical properties. Same PH, approximate electrolyte balance, etc. I'm pretty dense (insert joke) so I mave have lower resistance.

Chuck

[Dave Curtis, dB AudioTech]

As a licensed Master Electrician, I am legally bound to update the old amp in question to current safety standards in the course of the repair. No disclaimer needed. The customer is told this and advised of the potential consequences of keeping the 2 wire cord, and has the choice of me doing the work or not.

"If you bought it to collect it, put it in a glass case and never plug it in"

I don't recall anyone refusing a repair because of it; it's not really that much work.

Check this guy's amp out, and the last question, where it looks very much like he's answering a question from his own shill account. I sent him a word of advise, but it fell on deaf ears, I believe.

[gbono]

Okay you asked the question regarding what is the impedance of human "skin". There are actually three principle layers to what we call our "skin". No medical lecture to follow but assume that the "typical" resistance of the epidermal layer, per cm sq, is 200k ohms at 1 Hz and 200 ohms at 1 MHz. There is whole science dedicated to electrode-electrolyte interfaces and modern tasers use a variety of techniques to overcome the resistance of the epidermal layer without breaking the skin - ouch.

[Enzo]

WHy get carried away with tangents? SKin is dry or moist from moment to moment - that directly affects how electricity outside conducts to tissues inside. SO the questions of some sort of standard are avoiding the main issue of safety.

[Chuck H]

No no, I asked stricktly out of interest about taser weapon developement. (imagine..."So, what do you do for a living?") Not that I plan on making one. But it's an interesting trivia exercise. There must be some science behind it. I'm sure they don't just build stuff and shoot monkeys with it until something works. I'm not trying to put safety in a box.

Chuck

[Enzo]

OK. Voltage matters. A few thousand volts will skip over some of the impediments through your skin a lot easier than say 12v DC will. And I have no idea what freqs the tasers run at.

Likewise the taser is designed to shock and stun you, rather than cause damage.

Plus what gbono said - there is a lot of research already on this.

[cbarrow7625]

A taser gun is designed specifically to send current through the skin & not through the heart (which will kill you). With the electrodes place only a few inches apart & both in the skin, you can put currents through the skin between those points that would kill you if the same current passed though the heart.

Anybody who has watched the Mythbusters knows that taser guns are designed as two stage devices. They put out a large current initially to get the conduction going and then drop rapidly as soon as resistance is detected between the two points. The drop in current is so the thing REALLY doesn't hurt you. If it stayed at the higher levels it would probably burn the skin, etc.

Unfortunately for the guitar amp / microphone grounding issue we don't have either of the protections given by a taser. There is more current available from devices plugged into 120V / 240V wall outlets than is available from from a battery powered hand held device so once the current flows, it stays at that same high level at which it started. Secondly, sometimes when we have these kind of accidents it is because we have one hand on the grounded guitar strings and grab a live microphone (or vice versa). In this scenario, the shortest path between the two sources is right down your arms & through your heart. If enough current passes for long enough (is it 60mA through the heart for death?) you are gone. Luckily, most of the time we are just feeling the tingle through our lips (higher resistance path) and not straight down our limbs and through the heart.
Just because there are a lot of misses doesn't mean the danger isn't there. The current just didn't take the correct path to casue death. How many of us use the "one hand in the back pocket rule" when servicing equipment? I do. It is for just this reason - don't create a path through your heart for the electricity to flow. You can still get shocked & it can hurt a whole helluvalot but you'll be around to tell someone about it.

[GainFreak]

Hi,


My usual grounding arrangement is the AC ground lug going to the chassis near the AC inlet, the first filtering caps ground from the PCB to the same spot and this works without any problems.
In some amps /mainly Marshall/ I've seen the signal ground going to chassis ground through a combo of two diodes, cap and resistor.
My question is what is the purpose of that?
Recently I tried that but found out that the pots that were in contact with the chassis were sensitive to touching. I grounded them to the signal ground and the problem was solved but that means that I actually shorted the combo in question.