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Author Topic: Death cap  (Read 27816 times)

Bob Leonard

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Death cap
« on: December 13, 2014, 09:45:17 AM »

I have been following the thread concerning the electrocution of the guitar playing lead singer in Argentina with interest. So much interest that I felt a separate thread specific to guitar amplifier circuits may be in order. I too have been bitten more times than I can count during the early years, and the fault was always improper grounding. Testing was simplistic back then and was usually performed by holding the strings of your guitar and tapping the energized mic. If you were shocked, check the grounds (or flip the A/C cord) for everything until you weren't shocked anymore.
 
Today we have much better grounding circuits in modern amplifiers and in the venues where we perform. That may be all well and good but the possibility of shock and electrocution still exists, but is often taken for granted.
 
I have often referred to the "death" cap found in the circuit of  many older manufacturers guitar amplifiers. Not only is this capacitor prone to failure in 50 year old amplifiers, but the cap is often tied to a switch whose purpose is to alter the ground path. My cure for this issue has been to remove the death cap and disable this portion of the AC circuit. That includes removal of the 2 conductor AC plug and replacing that plug/cable with a quality #12 or #14 grounded cable whose ground lead is soldered to the chassis of the amplifier. Soldered, not attached under a screw head or bolt.
 
John and Mike both have knowledge concerning shock hazards far beyond that of my own, combined with some great idea's to help eliminate this hazard. Attached below is the schematic and layout for one of the most common guitar amplifiers in the world. Note the death cap, and note the input circuit for the guitar itself. Seeing the circuit, keeping in mind the guitar is also a part of that circuit, and following the guidelines below, what are your thoughts and suggestions.
 
1. The guitar can not be altered.
2. The amplifier can have the death cap removed, but can not be altered in any other way.
3. Tone can not be effected.
4. A stomp box sized unit is preferred IF the device is placed in line with the guitar output cable.
 
I am willing to test any of the designed or suggested circuits and devices in real life conditions as needed.
 
 
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John Roberts {JR}

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Re: Death cap
« Reply #1 on: December 13, 2014, 11:58:27 AM »

I have been following the thread concerning the electrocution of the guitar playing lead singer in Argentina with interest. So much interest that I felt a separate thread specific to guitar amplifier circuits may be in order. I too have been bitten more times than I can count during the early years, and the fault was always improper grounding. Testing was simplistic back then and was usually performed by holding the strings of your guitar and tapping the energized mic. If you were shocked, check the grounds (or flip the A/C cord) for everything until you weren't shocked anymore.
Bob, thank you for doing this. As you know I am very interested in making things safer "and" good tone.
Quote

 
 
1. The guitar can not be altered.
https://www.gearslutz.com/board/geekslutz-forum/125202-tired-getting-shocked-gigs.html Here is a link to a 2007 thread about modifying the guitar internally.

I do appreciate that many will not tolerate modifying classic guitars.
Quote


2. The amplifier can have the death cap removed, but can not be altered in any other way.
Let me suggest another option. Instead of hard grounding the guitar amp chassis to a 3 wire line cord, instead use the 3 wire line cord, but connect the stinger cap (switch end) to that safety ground. Now the chassis is floating except for the capacitor reactance (exactly like before) .047uf will limit 120 VAC 60Hz current to around 2 mA.  Then only use GFCI protected outlets to power the amp to protect the user from internal amp faults. 

This way the musician is protected from a mic with 120v on it's ground. While the amp should work exactly like before.
Quote

3. Tone can not be effected.
amen
Quote

4. A stomp box sized unit is preferred IF the device is placed in line with the guitar output cable.
 
I will still talk to my friend James Brown who owns a pedal company (amptweaker.com). James was actually a well known amp designer at Peavey (he did the EVH 5150 among others), so his pedals probably don't suck. The series cap ground mod inside the guitar is not in the audio path, so does not affect tone, I'm afraid breaking or messing with ground at the amp input need to break both circuits, so the amp and pedals are also protected from damage.   
Quote

I am willing to test any of the designed or suggested circuits and devices in real life conditions as needed.

If it isn't too much work, I would love for you to try my alternate stinger cap to safety ground mod, while leaving the chassis only cap coupled. In my judgment in combination with GFCI this should provide complete human safety protection from all anticipated threats.  Note: The stinger cap still needs to be 600V for universal mains voltage safety.

JR

PS: In the case of a RPBG the GFCI will still protect internal amp leakage, and the player is only exposed to 2 mA of current from the now hot safety ground.
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Greg_Cameron

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Re: Death cap
« Reply #2 on: December 13, 2014, 02:41:25 PM »

In my judgment in combination with GFCI this should provide complete human safety protection from all anticipated threats.


Maybe installing a GFCI plug on the cord like all portable hair dryers have would be the call.


Greg
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John Roberts {JR}

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Re: Death cap
« Reply #3 on: December 13, 2014, 04:07:21 PM »


Maybe installing a GFCI plug on the cord like all portable hair dryers have would be the call.
Like these guys sell...  While they do not sell direct.

http://www.towermfg.com/distribution.htm 

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Greg
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Bob Leonard

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Re: Death cap
« Reply #4 on: December 13, 2014, 06:07:48 PM »

John,
I'm putting a Twin Reverb up on the bench the week before xmas, maybe even Monday night, so I'll try that mod to the death cap.

Greg,
A GFCI would work but not if plugged into another GFCI. I tried that and the amp won't power on.


Keep in mind that Fender is not exclusive to this type circuit, so a universal fix, probably outboard, is the way to go.
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John Roberts {JR}

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Re: Death cap
« Reply #5 on: December 13, 2014, 06:27:35 PM »

John,
I'm putting a Twin Reverb up on the bench the week before xmas, maybe even Monday night, so I'll try that mod to the death cap.
Great thanx
Quote
Greg,
A GFCI would work but not if plugged into another GFCI. I tried that and the amp won't power on.
I am not aware of any problem from series GFCI. That said the stinger cap if switched to hot might imbalance a very sensitive GFCI while .047uF should not be enough current to trip a typical GFCI.

Quote

Keep in mind that Fender is not exclusive to this type circuit, so a universal fix, probably outboard, is the way to go.
JR
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Mike Sokol

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Re: Death cap
« Reply #6 on: December 13, 2014, 09:49:32 PM »

I do appreciate that many will not tolerate modifying classic guitars.  Let me suggest another option. Instead of hard grounding the guitar amp chassis to a 3 wire line cord, instead use the 3 wire line cord, but connect the stinger cap (switch end) to that safety ground. Now the chassis is floating except for the capacitor reactance (exactly like before) .047uf will limit 120 VAC 60Hz current to around 2 mA.  Then only use GFCI protected outlets to power the amp to protect the user from internal amp faults.

I agree with the logic of adding  a .047 cap in the EGC path that's powered by a GFCI. However, while it could certainly be added inside the guitar amp itself, I believe it would be far more universal if it was mounted inside the back-line GFCI outlet rather than inside the guitar amp. Any old stage amp with a stinger cap should have it removed and a proper 3-wire power cord installed instead of the 2-banger power cord.

If you think about it, this new stinger capacitor could mounted between the incoming AC power line ground and the ground screw of the GFCI. As we've discussed before, a GFCI doesn't require a EGC connection to operate, and can be used in a non-grounded wiring situation as long as it's marked "No Equipment Ground".

This external "stinger cap" will provide GFCI trip protection in case the amplifier chassis becomes energized by some internal hot-to-chassis fault. And it will limit musician shock to a few mA from an energized mic while holding a "grounded" guitar. However, I'm suggesting a 47K ohm drain resistor in parallel with the capacitor may help "pull down" any DC floating bias of the guitar amplifier. That would add only another 2.5 mA current worse case, so the combination of the resistor/capacitor would allow a max fault current through the musician of less than 5 mA (in the USA) and less than 10 mA (in 230-volt mains countries) using the same cap/resistor values. I could be totally wrong, and JR's suggestion of the .047 stinger cap alone may be all that's required. But, I have a gut feel that the additional parallel resistor may reduce amplifier noise pickup from the environment. That would be one really interesting thing for you to test.

If this external capacitor/GFCI combination works, then we should run the idea by the code monkey's on Mike Holt's forum to see if there's any existing exceptions that would make this code compliant as is. In theory it should be code compliant, but getting UL approval would be another challenge. 
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Stephen Swaffer

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Re: Death cap
« Reply #7 on: December 14, 2014, 12:15:54 AM »


 That includes removal of the 2 conductor AC plug and replacing that plug/cable with a quality #12 or #14 grounded cable whose ground lead is soldered to the chassis of the amplifier. Soldered, not attached under a screw head or bolt.
 

Bob,

I cut my teeth experimenting with my dad's Eico VTVM and misc Heathkit tube equipment -think I even had a small Allied audio amp (why my dad let me use gear with one side tied to the AC line with non polarized plugs on metal shelving still puzzles me)

That said, I appreciate the value of a soldered connection (especially for a signal ground)-the other perspective is that as an electrician joints that rely entirely on solder are not considered suitable for grounding-under fault conditions the solder has a nasty habit of melting.  It would be best to use a joint that is mechanically sound, then solder it if you want to keep a screw from coming loose.  This may well be your habit,  but I think it is important for others to follow.

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John Roberts {JR}

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Re: Death cap
« Reply #8 on: December 14, 2014, 10:31:09 AM »

I am not optimistic about UL blessing an outlet strip that lifts the ground bond, but with the GFCI they might allow it with a skull and cross bones marking.

I like the idea of sensing for ground current and breaking all 3 lines... not as cheap and easy but might protect against more potential hazards.

JR

+1 to SS point, safety grounds need to be mechanical while any stinger cap will not encounter enough current to melt solder.
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Kevin Graf

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Re: Death cap
« Reply #9 on: December 14, 2014, 10:33:52 AM »

Without getting into modifying classic equipment, my thoughts are:

a] Both the switch and the fuse should be on the Hot conductor.
b] The Safety Ground/PE/EGC should connect to the chassis near where the power cord enters the chassis.
c] The connection needs to be very robust. It may need to carry 150 Amps or more until the circuit breaker trips.  Some say that a simple solder connection is not enough while in the next breath they say that a nut & bolt connection needs to be vibration proof.
d] The circuit/power supply common (ground) should connect to the chassis near the input jacks (not near the Safety Ground).
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Speedskater

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Re: Death cap
« Reply #9 on: December 14, 2014, 10:33:52 AM »


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