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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Bob Leonard on December 13, 2014, 09:45:17 AM

Title: Death cap
Post by: Bob Leonard 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.
 
 
Title: Re: Death cap
Post by: John Roberts {JR} 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 (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.
Title: Re: Death cap
Post by: Greg_Cameron 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
Title: Re: Death cap
Post by: John Roberts {JR} 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   (http://www.towermfg.com/distribution.htm)

Quote
Greg
Title: Re: Death cap
Post by: Bob Leonard 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.
Title: Re: Death cap
Post by: John Roberts {JR} 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
Title: Re: Death cap
Post by: Mike Sokol 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. 
Title: Re: Death cap
Post by: Stephen Swaffer 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.

Title: Re: Death cap
Post by: John Roberts {JR} 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.
Title: Re: Death cap
Post by: Kevin Graf 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).
Title: Re: Death cap
Post by: John Roberts {JR} on December 14, 2014, 10:40:12 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).
I guess I will be repeating myself a lot in this thread.  :(

The hazard I am trying to protect against is when a guitar player gets between two mains power drops (like back line and FOH) and one of those two drops has an energized safety ground (like RPBG).

That has happened and the musician was killed by modern properly ground bonded equipment.

I concede this is an obscure fault, but that's why it's not simple. 

JR
Title: Re: Death cap
Post by: Kevin Graf on December 14, 2014, 10:44:42 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

Bill Whitlock writes aabout a plug-in balanced isolation transformer:

Bill Whitlock, 9/4/2012 Overview of Audio System Grounding & Interfacing page 203

The “Ground Lift” Feature
Legally Disconnects Safety Ground … BUT
Because this unit has the required GFCI on its output outlets, it is legally
allowed to interrupt safety ground. Paradoxically, the same can’t be done
when a premises AC outlet is replaced with a GFCI type. NEC says that, if a
safety ground is available to the outlet, it MUST be connected … even if the
outlet is a GFCI protected one. Although this unit claims to eliminate ground
loops via the “LIFT” switch, there’s a catch ...


He never explains just what the catch is.

http://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf
Title: Re: Death cap
Post by: John Roberts {JR} on December 14, 2014, 11:57:27 AM
An isolation power transformer is very expensive.

JR
Title: Re: Death cap
Post by: Ned Ward on December 14, 2014, 01:22:00 PM
Bob - thanks for starting this post on what is a critical mod/upgrade to older amps. I've had the following done on all my Fenders, and believe something similar was done on my Showman...:

http://www.mylespaul.com/forums/squawk-box/103068-death-cap-2-prong-power-cords-caution.html

The only question to add to the mix - what about the courtesy AC outlet on the back? May be good to include that in wiring solutions (or if there's a strong reason not to, explain).

The ground switch left over leaves a great way to add if needed any changes without marring a vintage amp - no need to drill another hole in the chassis. While I haven't done this, I could see that being far more beneficial than drilling ugly holes every which where. Mine stay in for cosmetics only.
Title: Re: Death cap
Post by: Bob Leonard on December 14, 2014, 09:09:37 PM
Any amps I've done for you Ned have been modified as needed, but the reason I don't disconnect or modify the accessory plug is because the only way to get a 3 prong cable into it would be to break off the ground. You're correct though, this is also an area that should be addressed. I think I'll work with the suggestions from the thread here and make some changes when I get the Twin Reverb on the bench next week. Maybe JR would even send me a schematic of the change he's proposing (HINT).

I finished a Pro last week, and man does it sound good. You need one, you are getting sleepy, repeat after me, Ned needs a Pro Reverb.........
Title: Re: Death cap
Post by: Mike Sokol on December 14, 2014, 10:42:56 PM
I finished a Pro last week, and man does it sound good. You need one, you are getting sleepy, repeat after me, Ned needs a Pro Reverb.........

Does this help?
Title: Re: Death cap
Post by: Stephen Swaffer on December 14, 2014, 11:15:03 PM

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.


Cost could still likely be comparable to a good surge protector-which begs the question which would you be willing to spend more money on-protecting the guitar player or his gear?
Title: Re: Death cap
Post by: Keith Broughton on December 15, 2014, 09:45:49 AM


The only question to add to the mix - what about the courtesy AC outlet on the back? May be good to include that in wiring solutions (or if there's a strong reason not to, explain).


Disconnect from AC and leave it in for cosmetic reasons.
Title: Re: Death cap
Post by: John Roberts {JR} on December 15, 2014, 11:05:16 AM
Any amps I've done for you Ned have been modified as needed, but the reason I don't disconnect or modify the accessory plug is because the only way to get a 3 prong cable into it would be to break off the ground. You're correct though, this is also an area that should be addressed. I think I'll work with the suggestions from the thread here and make some changes when I get the Twin Reverb on the bench next week. Maybe JR would even send me a schematic of the change he's proposing (HINT).

I finished a Pro last week, and man does it sound good. You need one, you are getting sleepy, repeat after me, Ned needs a Pro Reverb.........
Bob: here is a mark up of what I am proposing for use in combination with a GFCI power source.

My speculation is that connecting the chassis to ground through a stinger cap "should" protect human from death from RPBG and external hot ground hazards. It does not protect against an internal HV PS fault inside chassis.

BTW is that amp UL approved? Ungrounded chassis (2-wire)  products generally use higher insulation standards inside transformer primary, etc. 

JR
Title: Re: Death cap
Post by: Keith Broughton on December 15, 2014, 02:17:45 PM
Been looking at the schematics ...what exactly does the stinger cap do and is it required when doing a 3 wire grounded AC cable mod?
Title: Re: Death cap
Post by: Ned Ward on December 15, 2014, 02:31:39 PM
Any amps I've done for you Ned have been modified as needed, but the reason I don't disconnect or modify the accessory plug is because the only way to get a 3 prong cable into it would be to break off the ground. You're correct though, this is also an area that should be addressed. I think I'll work with the suggestions from the thread here and make some changes when I get the Twin Reverb on the bench next week. Maybe JR would even send me a schematic of the change he's proposing (HINT).

I finished a Pro last week, and man does it sound good. You need one, you are getting sleepy, repeat after me, Ned needs a Pro Reverb.........


I do need a Pro Reverb, and a Deluxe Reverb, and a Dr. Z Maz 18 or 38, an orange Gretsch, and a QSC KW181... all in good time. With 5 Fenders in the stable, it's a challenge just to get them all used on a regular basis! We're playing at Saint Rocke Thursday (150 person bar/club) so the Tremolux will do for that show, but we have another outdoor show in April, and for that, nothing but the Showman will do.
Title: Re: Death cap
Post by: John Roberts {JR} on December 15, 2014, 03:12:05 PM
Been looking at the schematics ...what exactly does the stinger cap do and is it required when doing a 3 wire grounded AC cable mod?
What the stinger cap is "supposed" to do is ground the chassis to neutral which is nominally 0V to reduce hum pick-up by the circuitry, and connected guitar.

A relatively safe 3 wire conversion is to bond the mains cord ground wire directly to the chassis. Most (all) modern UL approved guitar amps are wired this way.

I know of at least one musician who was killed when he got between two modern UL approved ground bonded guitar amps. One of those two amps was plugged into a miswired (RPBG) outlet and he was killed by the electrical shock when he grabbed a guitar that had it's ground energized while still holding the other properly grounded guitar.

The stinger cap is sized so that if in the wrong switch position, putting hot on the chassis through the stinger cap, the shock hazard is still low enough to not be lethal (thus "stinger" cap, not "sure death" cap). A faulty shorted stinger cap could indeed present a deadly hazard and these caps are specified at 600V breakdown and often use special caps designed for that purpose.   

I am trying to take a step back from ground bonding the chassis and using a GFCI to protect against a typical mains power fault, then isolating the chassis through the old school stinger cap sized such that even a RPBG outlet is not a deadly hazard, or likewise from a hot mic coming from a rouge FOH with RPBG.

This is not a typical application so would be considered an exception to typical UL wiring practices. 

JR
Title: Re: Death cap
Post by: Steve M Smith on December 15, 2014, 03:21:35 PM
an orange Gretsch

The orange Gretches are so last year. All the cool kids are playing them with amber maple finishes now!

Been looking at the schematics ...what exactly does the stinger cap do and is it required when doing a 3 wire grounded AC cable mod?

It is the work of the devil when used with a two pin power plug.  Theoretically, the capacitor is not required with a normal grounded chassis but if I understand correctly, the purpose of this discussion is to see if there is a safer arrangement which would pose less of a danger to a guitarist simultaneously touching his guitar and a microphone which has become live.

EDIT:  Whilst editing my post to answer this, John has posted a much better answer.


Steve.
Title: Re: Death cap
Post by: Mike Sokol on December 15, 2014, 05:09:08 PM
Just remember that while we're on the subject of 2-wire guitar amps "grounded" with a "death cap", there are also a few old tube amps that had AC-DC non-transformer power supplies. Just like old AC-DC tube radios, all the filament voltages added up to 120-volts, and one side of the incoming line was bonded to the chassis. So if you used a non-polarized power plug, there was a 50/50 chance that the strings of your guitar were now hard-bonded to the hot side of the power line. I have one of these amps in my closet which I've considered rebuilding with a 120-120 volt isolation input transformer and a proper 3-wire grounded power plug. But except for powering it up on the bench, I would NEVER use something like this on stage or in the studio. Way too much chance that something would go wrong and kill somebody, maybe even ME.  ??? 
Title: Re: Death cap
Post by: John Roberts {JR} on December 15, 2014, 05:22:56 PM
Just remember that while we're on the subject of 2-wire guitar amps "grounded" with a "death cap", there are also a few old tube amps that had AC-DC non-transformer power supplies. Just like old AC-DC tube radios, all the filament voltages added up to 120-volts, and one side of the incoming line was bonded to the chassis. So if you used a non-polarized power plug, there was a 50/50 chance that the strings of your guitar were now hard-bonded to the hot side of the power line. I have one of these amps in my closet which I've considered rebuilding with a 120-120 volt isolation input transformer and a proper 3-wire grounded power plug. But except for powering it up on the bench, I would NEVER use something like this on stage or in the studio. Way too much chance that something would go wrong and kill somebody, maybe even ME.  ???

Yup, hot chassis TV sets did not have exposed metal parts that users could touch. A hot chassis guitar amp that telescopes the chassis ground out to the player seems dangerous indeed. I wonder if there could be a high impedance signal isolation transformer used in series with the guitar output/amp input to segregate the power ground from the guitar. Lead guitar pickups are pretty high impedance so I am not optimistic.

Perhaps use a wireless guitar pick-up   :o .

JR

PS: I remember getting shocks while messing with an old school (hot chassis radio) while standing bare foot on the basement floor. I had one old beauty that was about the size of a small juke box, and the front hinged out to reveal a turntable for playing records (78). It also had a magic eye for tuning... I wish I still had that... probably ended up in a landfill after I dissected it. I vaguely recall it had an electromagnet in the speaker... yes it hummed. That's how you knew it was ready.  8)

 
Title: Re: Death cap
Post by: Steve M Smith on December 15, 2014, 05:35:40 PM
Just like old AC-DC tube radios, all the filament voltages added up to 120-volts

We also had AC/DC radios and TVs here in the UK.  Our heater voltages had to add up to 240v (or about 200v with a series resistor).

I vaguely recall it had an electromagnet in the speaker... yes it hummed.

I had a radio like that.  The speaker electromagnet was wired in place of a smoothing choke.


Steve.
Title: Re: Death cap
Post by: Mike Sokol on December 15, 2014, 05:56:51 PM
I had a radio like that.  The speaker electromagnet was wired in place of a smoothing choke.

Yeah, I swapped a permanent magnet speaker into one of these old radios and simply connected the electromagnet wires together. Hummed like crazy until I figured out the speaker's electromagnet was also the power supply choke. But hey, I was probably 14 years at the time so it was a good learning experience. 

The point is, anytime you're playing with old/tube gear, make sure you understand how it works before you start modifying or using it. Things can get dangerous in a hurry if you do something wrong.
Title: Re: Death cap
Post by: Ned Ward on December 15, 2014, 06:57:31 PM
Amen to that. That's why I leave any amp work to competent folks like Bob or my amp tech at Harbor Music.
Title: Re: Death cap
Post by: John Roberts {JR} on December 16, 2014, 01:30:32 PM
My dad was a recording engineer for RCA records when he died in the 1950's so we always had a decent sound system in the house but only mono at the time he died. One large speaker mounted at floor level in a corner of the living room angled up to fill the whole house from only a few watts. Before he worked at RCA he was chief engineer for Muzak, so I was the only house on the block with a Muzak receiver. :-)

 I found some early demo stereo 1/4" magnetic tapes in my did's stuff but his collection was mostly mono and '78s with only a few very early 33 1/3 LP. By the time I started messing with hifi I built a cheapo turntable based on a lafayette platter mechanism bolted to a cut out piece of wood and a plastic tone arm. I recall my first stereo amp, a small 4 tube job (probably a couple watts), and recall seeing the gas inside the tubes light up and dance around to the music.

Dad's  idea of an old amp was a 10W (?) western electric 19" rack unit in the basement using several sections and about 6' tall...It may have powered the home system at one time, but by the time I can recall he had a more modern tube unit, smaller than a bead box, but not by much.

Old amps were really big and heavy, The CS800 was a high powered light weight amp compared to truly old iron.

JR   
Title: Re: Death cap
Post by: Dennis Wiggins on December 16, 2014, 03:04:40 PM
Here's an attempt to explain the "Death Cap".  FYI only...

https://www.youtube.com/watch?v=MMTN3B-zClo

-Dennis
Title: Re: Death cap
Post by: Dennis Wiggins on December 16, 2014, 03:16:31 PM
Here's an attempt to explain the purpose of the "Death Cap".  FYI only...

https://www.youtube.com/watch?v=MMTN3B-zClo

-Dennis
Title: Re: Death cap
Post by: Deric Craig on December 17, 2014, 12:53:43 AM
Here's an attempt to explain the "Death Cap".  FYI only...

https://www.youtube.com/watch?v=MMTN3B-zClo

-Dennis

I took the time to view the entire video. There are several points made in the video presentation that I take exception to, for instance his comments about the "death cap" being connected to the "hot" side of the line to reduce hum...
Uh, Ok...

And I would prefer to have him refer to the chassis as a "common" instead of "ground" especially since "ground" does not exist in these amp examples. Any of us who do amp tech work often refer to the chassis connections as ground, but to me, it is just a possible point of confusion for a lay person.

I would wager this tech guy has not ever held a guitar connected to an amp with this cap that was not in a favorable switch position, and brushed his lips against a grounded vocal mic ...

This gentleman has some resemblance to a guy who, at one point in time, was on the fire investigation team that I was involved with. The man I had to periodically work with, was a lead county electrical inspector plus taught classes at a 'unnamed' university in electrical. More than once I butted heads with this guy on his "conclusions" on cause and origin. Sometimes individuals are not as knowledgeable as they like to perceive themselves. I could share some examples but choose to decline at this point.

I read through a decent amount of the comments below the video. I did cringe upon reading quite a few posted comments thanking this man for "debunking myths" about the death cap.
Oh boy...
I did see a few comments from some who supported the removal and mitigation of the potential issues with the cap.

There will be more comments on this video. Yes, the internet has a wide variety of information. Some of it is actually good.
Title: Re: Death cap
Post by: John Sabine on December 17, 2014, 08:56:46 AM

Yup, hot chassis TV sets did not have exposed metal parts that users could touch. A hot chassis guitar amp that telescopes the chassis ground out to the player seems dangerous indeed. I wonder if there could be a high impedance signal isolation transformer used in series with the guitar output/amp input to segregate the power ground from the guitar. Lead guitar pickups are pretty high impedance so I am not optimistic.

Perhaps use a wireless guitar pick-up   :o .

JR

PS: I remember getting shocks while messing with an old school (hot chassis radio) while standing bare foot on the basement floor. I had one old beauty that was about the size of a small juke box, and the front hinged out to reveal a turntable for playing records (78). It also had a magic eye for tuning... I wish I still had that... probably ended up in a landfill after I dissected it. I vaguely recall it had an electromagnet in the speaker... yes it hummed. That's how you knew it was ready.  8)

 Prisons had a problem with hot chassis radios on cell blocks. Prisoners would disassemble their radios and remove the speakers create home made headphones. The problem would come when they would sit on the metal toilets with their home made headphones on. If they hadn't completely insulated themselves from the electrical circuit in the headphones they would be electrocuted. Some old speakers would use the speaker basket as the ground connection so if they were touching the basket they were contacting the circuit.
 The owner of a radio station that I worked at decided to make an on air monitor using an old desktop radio. He soldered a patch cable directly to the speaker terminals of the radio and then plugged it into the patch bay for the main console. The radio was an old hot chassis design and as a result this caused a great deal of excitement as well as a large flash and a loud bang! Amazingly the console wasn't destroyed.


Sent from my iPad using Tapatalk
Title: Re: Death cap
Post by: Art Welter on December 17, 2014, 10:54:46 AM
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.
 
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.
Bob,

All good, and makes me realize I probably should have done your mod on the Nordmende Isabella amp I just sold yesterday.  The guy that bought it was very aware of the potential shock hazard problems with vintage tube gear. I had labeled the "hot" side of the non-polarized two pin AC plug, and he already knew "the little guy gets you", so I don't think he will have problems.

I'm sure any solder connection you made would be robust enough to last another 50 years, but having experienced too many "cold solder" joints that look OK, but have infinite resistance, my only addition to your instructions would be the ground lead should have a mechanical screw or bolt connection to the chassis in addition to solder, what my dad (R.I.P.) would have called a "belt and suspenders" approach to safety. In addition, the ground lead should have a strain relief within a few inches of the chassis connection so there is no possibility of it flopping around in transit, which can eventually lead to the wire breaking at the connection point.

All that said, I still feel a moral ambiguity about altering classic gear, fortunately I sold my last ungrounded tube amp before seeing your post so won't have to struggle with my conscience  ;).

P.S. I would also suggest modifying point #3 to read "Tone must not be affected.

On a semi-related subject, what brand and value volume and tone potentiometers for Gibson humbuckers,  P90s, and Melody Maker lipstick style pickups are least prone to "scratchy pot" syndrome?

Art
Title: Re: Death cap
Post by: Bob Leonard on December 17, 2014, 11:08:10 PM
CTS pots are the only way to go. Gibson historic are CTS, but they get a premium for them. Take a look at MOJO Tone for these pots.

http://www.mojotone.com/amp-parts/potentiometers-CTS
Title: Re: Death cap
Post by: Bob Leonard on December 17, 2014, 11:20:29 PM
I took the time to view the entire video. There are several points made in the video presentation that I take exception to, for instance his comments about the "death cap" being connected to the "hot" side of the line to reduce hum...
Uh, Ok...

And I would prefer to have him refer to the chassis as a "common" instead of "ground" especially since "ground" does not exist in these amp examples. Any of us who do amp tech work often refer to the chassis connections as ground, but to me, it is just a possible point of confusion for a lay person.

I would wager this tech guy has not ever held a guitar connected to an amp with this cap that was not in a favorable switch position, and brushed his lips against a grounded vocal mic ...

This gentleman has some resemblance to a guy who, at one point in time, was on the fire investigation team that I was involved with. The man I had to periodically work with, was a lead county electrical inspector plus taught classes at a 'unnamed' university in electrical. More than once I butted heads with this guy on his "conclusions" on cause and origin. Sometimes individuals are not as knowledgeable as they like to perceive themselves. I could share some examples but choose to decline at this point.

I read through a decent amount of the comments below the video. I did cringe upon reading quite a few posted comments thanking this man for "debunking myths" about the death cap.
Oh boy...
I did see a few comments from some who supported the removal and mitigation of the potential issues with the cap.

There will be more comments on this video. Yes, the internet has a wide variety of information. Some of it is actually good.

The problem is that he ran all of his tests within the amplifiers. At no point did he reference another ground outside the chassis, or did he reverse the power cable. Lastly, the purpose of the cap was not to reduce hum, but to provide a path to ground for the AC source. The switch on a Fender amplifier allows the EU to choose that path with the result being less hum if the path is correct. The amplifiers he showed do not have this option and the resulting hum will not change regardless of the cap being there or not UNLESS he turns the 2 prong plug 180 degrees. This is a bad explanation that I've seen before.
Title: Re: Death cap
Post by: Ned Ward on December 18, 2014, 12:02:43 AM
Bob,

All good, and makes me realize I probably should have done your mod on the Nordmende Isabella amp I just sold yesterday.  The guy that bought it was very aware of the potential shock hazard problems with vintage tube gear. I had labeled the "hot" side of the non-polarized two pin AC plug, and he already knew "the little guy gets you", so I don't think he will have problems.

I'm sure any solder connection you made would be robust enough to last another 50 years, but having experienced too many "cold solder" joints that look OK, but have infinite resistance, my only addition to your instructions would be the ground lead should have a mechanical screw or bolt connection to the chassis in addition to solder, what my dad (R.I.P.) would have called a "belt and suspenders" approach to safety. In addition, the ground lead should have a strain relief within a few inches of the chassis connection so there is no possibility of it flopping around in transit, which can eventually lead to the wire breaking at the connection point.

All that said, I still feel a moral ambiguity about altering classic gear, fortunately I sold my last ungrounded tube amp before seeing your post so won't have to struggle with my conscience  ;) .

P.S. I would also suggest modifying point #3 to read "Tone must not be affected.

On a semi-related subject, what brand and value volume and tone potentiometers for Gibson humbuckers,  P90s, and Melody Maker lipstick style pickups are least prone to "scratchy pot" syndrome?

Art


Art -


While I agree that altering classic gear makes me not feel great, since I play these regularly, adding a grounded three prong cord and changing the filter caps are an absolute must for me. My amp tech used to save the parts for me (2 prong cord, original filter caps) in case I wanted to sell the amps with the originals to put back in, but I play these old '65 Fenders regularly and will be playing my '65 Tremolux tomorrow. Will probably take that Bogen CHA-33 PA head in over break to have him go over it and replace the power cord (frayed at strain relief) and ensure it's grounded properly.


Yeah, it takes away a little of the vintage value, but I'll be around longer to enjoy it...




[EDIT] Forgot to add - for those that want to keep the possibility of 100% authentic down the road, just store the parts and if you decide to part with the amp, the buyer can choose to put them back in. The filter caps and AC plug are some of the least destructive "mods" I can think of.[/EDIT]
Title: Re: Death cap
Post by: Steve M Smith on December 18, 2014, 02:40:30 AM
While I agree that altering classic gear makes me not feel great, since I play these regularly, adding a grounded three prong cord and changing the filter caps are an absolute must for me.
It would be a must for me too (although in the UK, there is no need).  These amplifiers were made to be used, not to sit on a shelf looking pretty, so a minor modification to the power input to allow them to continue to be used safely, as long as it is done properly, should not be a problem.

A collector might not be interested in an amplifier modified in that way but I don't care what collectors think and don't buy anything thinking about its future value, just its current usefulness.


Steve.
Title: Re: Death cap
Post by: Bob Leonard on December 18, 2014, 07:31:14 AM
True vintage pieces that are collected and not used couldn't be used but in very few instances. Dry caps do not a good sound make, so these collectors aren't interested in tone as much as they are interested in having a 100% original amplifier. I really don't know anyone who collects old amps just to look at, and in the vintage market having new caps and a 3 prong cord doesn't lessen the value of the amp. That would be why a great sounding 65' or 66' Deluxe Reverb that is in great cosmetic shape, and that has been re-capped and overhauled, will have a resale value north of $3K.
Title: Re: Death cap
Post by: Mike Sokol on December 18, 2014, 08:29:51 AM
Here's an attempt to explain the "Death Cap".  FYI only...
https://www.youtube.com/watch?v=MMTN3B-zClo
I finally took the time to watch the entire video and you're correct. It's a bad attempt. This guy really has NO idea what the line cap does or even how to use the switch. I think that part of the problem is that he doesn't understand the difference between hum and buzz. Hum is a 60 Hz (or 50 Hz) sine wave with little or no harmonics, and usually caused by ground loop currents that sneak into a gain stage. There's also a 120 Hz hum (yes, an octave up) with a bit more harmonics caused by open power supply capacitors, but that's a different thread. Buzz is a raspy sounding noise which is mostly  upper harmonics of a 60 (or 50) Hz line frequency.

The idea of the"death cap" was to provide a shunting path to earth for these high frequency harmonics that show up on your guitar just from being in the room full of power wiring. You can observe this harmonic energy by touching the tip of your guitar cable while plugged into a guitar amp. Of course, with a properly "grounded" amp using a 3-wire power cord that energy goes directly to earth. But old-school guitar amps with 2-wire power cords needed a way to establish this earthing connection without the 50-50 chance of death from a flipped power cord with a hard neutral-to-chassis connection. 

So this death cap with a switch allowed you to find the line side of the incoming power easily without flipping a non-polarized power plug. The switch position that hums/buzzes/shocks the least means your amp chassis is now connected to the neutral side of the power line via this cap, which of course forms a 6-dB/Octave low-pass filter. And even if you get the switch reversed and connected to the "hot" line, the impedance at 60 Hz is high enough that it only provides a few mA of source current, not enough to kill you. Of course, if this old capacitor shorts out, then that becomes a low-impedance connection with line current available to electrocute you if flipped to the hot side.

So these line caps weren't really dangerous UNLESS they got leaky or shorted out. Then they became VERY dangerous.     
Title: Re: Death cap
Post by: Art Welter on December 18, 2014, 08:37:36 AM
CTS pots are the only way to go. Gibson historic are CTS, but they get a premium for them.

True vintage pieces that are collected and not used couldn't be used but in very few instances. Dry caps do not a good sound make, so these collectors aren't interested in tone as much as they are interested in having a 100% original amplifier.
Bob,

I must have been lucky with vintage tube amps, only one out of six needed caps replaced, and that was one of the "newer" ones, probably mid 1960s.

Bourns makes pots that are rated for a half million cycles, what are the CTS pots rated for?

Art
Title: Re: Death cap
Post by: Art Welter on December 18, 2014, 08:43:51 AM
The switch position that hums/buzzes/shocks the least means your amp chassis is now connected to the neutral side of the power line via this cap, which of course forms a 6-dB/Octave low-pass filter. 

Of course, some guys would call a capacitor circuit that cuts lows a high pass filter  ;)
Title: Re: Death cap
Post by: Mike Sokol on December 18, 2014, 09:10:54 AM
Of course, some guys would call a capacitor circuit that cuts lows a high pass filter  ;)
I guess it's technically a high-pass shunting filter since it shorts the higher frequency energy to earth. But the final effect on the audio signal is like a low-pass/high-cut filter since it makes the high frequency buzz go away in the speaker.

This is a tough room, isn't it?  :o
Title: Re: Death cap
Post by: John Roberts {JR} on December 18, 2014, 09:34:44 AM
I wouldn't call it a filter at all. It has a capacitor but no characteristic resistance or inductance to form a filter pole with.

It is a cap coupled shunt.

JR
Title: Re: Death cap
Post by: Mike Sokol on December 18, 2014, 11:02:02 AM
I wouldn't call it a filter at all. It has a capacitor but no characteristic resistance or inductance to form a filter pole with.

It is a cap coupled shunt.

JR

I would argue that there IS an internal resistance/inductance to form a filter pole with, but it's a completely unpredictable value because it's formed by all the various parallel resistance leakage paths inside a guitar amp itself.

However, if a human makes contact with an external hot chassis, then they're introducing perhaps 1,500 ohms of their own body resistance into the fault circuit and it becomes a true single-order filter.

But to avoid confusion and for practical reasons I agree with your definition of a capacitor coupled shunt. That's really what it is...  ;D
Title: Re: Death cap
Post by: Ned Ward on December 18, 2014, 11:57:03 AM
Bob,

I must have been lucky with vintage tube amps, only one out of six needed caps replaced, and that was one of the "newer" ones, probably mid 1960s.

Bourns makes pots that are rated for a half million cycles, what are the CTS pots rated for?

Art


Art - on my 3 1965 Fenders, I had the filter caps replaced on all. While the Tremolux caps appeared fine and had no bulging or bubbling, I felt safer with F&T caps under the doghouse. If those vintage filter caps blow, they can take out a lot more important mojo downstream, including the transformers. Better safe than sorry...
Title: Re: Death cap
Post by: Jonathan Johnson on December 18, 2014, 03:01:37 PM
So this death cap with a switch allowed you to find the line side of the incoming power easily without flipping a non-polarized power plug. The switch position that hums/buzzes/shocks the least means your amp chassis is now connected to the neutral side of the power line via this cap, which of course forms a 6-dB/Octave low-pass filter. And even if you get the switch reversed and connected to the "hot" line, the impedance at 60 Hz is high enough that it only provides a few mA of source current, not enough to kill you. Of course, if this old capacitor shorts out, then that becomes a low-impedance connection with line current available to electrocute you if flipped to the hot side.

So these line caps weren't really dangerous UNLESS they got leaky or shorted out. Then they became VERY dangerous.   

So what you're saying is I should put a capacitor in my reverse polarity bootleg ground?  ;D
Title: Re: Death cap
Post by: Jonathan Johnson on December 18, 2014, 03:23:19 PM
2. The amplifier can have the death cap removed, but can not be altered in any other way.

Given that there are musicians who refuse to alter their amplifiers in any way that affects either tone or appearance, I assume that you mean that replacing the two-wire cord with a three-wire cord is not an option.

How about making up a simple adapter, kind of like a 3-prong to 2-prong cheater plug, but going the other way?

You would have a 3-prong male plug, out of which would be a ground wire, a hot, and a neutral. The hot and neutral would go into a 2-prong female cord-end receptacle, while the ground wire would be long enough to attach to some convenient point on the amplifier chassis. The amplifier would plug into the 2-prong receptacle, and the 3-prong plug would plug into the power source.

This would give the amplifier chassis a solid ground connection without modification, thereby preserving historical accuracy and monetary value. However, I don't know how that would affect the tone; that is outside of my scope of experience.

Some potential issues if the death cap is not removed:
Title: Re: Death cap
Post by: Mike Sokol on December 18, 2014, 03:47:08 PM
So what you're saying is I should put a capacitor in my reverse polarity bootleg ground?  ;D

Actually, that is one possible shock-blocker possibility. JR and I have discussed what would happen if you used a GFCI outlet but instead of bonding the ground screw to the incoming EGC, you put the "stinger cap" in series with the earthing path. Since the UL and NEC allow you to install a GFCI on a non-grounded power feed, then maybe they won't care if you put a capacitor in the ground line. I've also suggested putting a 47K resistor in parallel with the cap just to pull the chassis voltage close to earth potential so it doesn't set off a NCVT during a stage check.
Title: Re: Death cap
Post by: Bob Leonard on December 19, 2014, 07:37:00 PM
Given that there are musicians who refuse to alter their amplifiers in any way that affects either tone or appearance, I assume that you mean that replacing the two-wire cord with a three-wire cord is not an option.

How about making up a simple adapter, kind of like a 3-prong to 2-prong cheater plug, but going the other way?

You would have a 3-prong male plug, out of which would be a ground wire, a hot, and a neutral. The hot and neutral would go into a 2-prong female cord-end receptacle, while the ground wire would be long enough to attach to some convenient point on the amplifier chassis. The amplifier would plug into the 2-prong receptacle, and the 3-prong plug would plug into the power source.

This would give the amplifier chassis a solid ground connection without modification, thereby preserving historical accuracy and monetary value. However, I don't know how that would affect the tone; that is outside of my scope of experience.

Some potential issues if the death cap is not removed:
  • If the non-polarized plug is oriented correctly (cap between neutral and chassis), you have a bond of sorts between the neutral and ground. This could result in some return current on the ground wire, which could induce hum and also cause a GFCI to trip. If the cap is shorted, the current and noise will likely be greater.
  • If the non-polarized plug is inverted (cap between hot and chassis), you have a bond of sorts between hot and ground. You may induce current in the ground wire, creating noise and imbalancing the load on a GFCI. If the cap has shorted, sparks might fly and your guitar player may never allow you to touch his stuff ever again.


I should have been more precise. Removing the death will also include the replacement of the 2 prong plug with a correctly wired and grounded 3 prong chord of suitable length and gauge.

Title: Re: Death cap
Post by: Bob Leonard on December 19, 2014, 07:48:41 PM
Bob,

I must have been lucky with vintage tube amps, only one out of six needed caps replaced, and that was one of the "newer" ones, probably mid 1960s.

Bourns makes pots that are rated for a half million cycles, what are the CTS pots rated for?

Art

Art,
CTS pots are rated about the same. The deal with the CTS pots is that they are the pots Fender and Gibson used as OEM hardware pretty much from day one. That has induced a cult following of sorts.

You must be a very lucky person indeed. I have never seen a 50 year old electrolytic that didn't need replacement for one reason or another. Outward appearance aside, electrolytic or not, caps don't last forever. Even the coupling caps in old amps should be checked for back voltage, and electrolytic caps should be measured and tested if you don't intend on replacing them. Tone caps aren't the real issue, and many people can live with lousy tone. It's the power supply filter caps, bias caps and coupling caps, that can and will cause the most grief as Ned already mentioned. I can do an entire amp using quality caps for about $75 in parts, and guarantee there will be a huge difference in tone and performance when done. So, to each his own, but you might want to take a second look at those 50 year old caps.
Title: Re: Death cap
Post by: Ned Ward on December 21, 2014, 01:37:15 PM
Just took my Bogen Challenger to my amp guy yesterday; Chris Fleming from the Fender Custom shop was there, and we talked mods as he loves these old PA's as well. Besides replacing the AC cord for a better non-frayed on and inspecting the grounding, the amp tech will change out the filter caps and swap the old Amphenol mic inputs for 1/4" inputs. Of course, I'll keep all these original parts in a baggie in case someone wants it original... may also look into replacing the 70 volt speaker outlets with a 1/4" speaker out and a impedance selector, per Chris's advice. Or may just get a good heavy duty speaker cable with spade ends to screw on to either 4 or 8 ohms depending on the cab.
Title: Re: Death cap
Post by: Mike Sokol on December 21, 2014, 03:37:12 PM
I'm ordering some .047/600-V capacitors for my Stinger-GFCI experiments, and see a lot of NOS (New Old Stock) available. However, I know that paper capacitors will eventually dry out, so I want something that's as reliable as possible since it's a safety issue. What's your experience with caps like I've posted below. They could be 20 years old already, but these are sealed "dipped" capacitors. I've never seen one of these fail from age, but lots of you have seen way more old amplifiers than I have. 

Opinions, please...
Title: Re: Death cap
Post by: John Roberts {JR} on December 21, 2014, 04:27:12 PM
I would pay $15 for a cap even if I had your money... 8)

DIgikey has .047 uF 10% 630VAC 2000VDC metalized polypropylene... $3.67 each

http://www.digikey.com/product-search/en?pv1292=56&FV=fff40002%2Cfff80010%2C340021&k=X+caps&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25 (http://www.digikey.com/product-search/en?pv1292=56&FV=fff40002%2Cfff80010%2C340021&k=X+caps&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25)

(http://media.digikey.com/photos/Cornell%20Dubilier%20Photos/940C%20SERIES%2046L,22,22.5,23,23.5D_sml.jpg)

JR
Title: Re: Death cap
Post by: John Roberts {JR} on December 21, 2014, 04:35:41 PM
The orange drops from digikey are only 200VAC 600DC

http://www.digikey.com/product-search/en?pv1293=35&FV=ffec605a%2C340021&k=orange+drop&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25 (http://www.digikey.com/product-search/en?pv1293=35&FV=ffec605a%2C340021&k=orange+drop&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25)

(http://media.digikey.com/Photos/Cornell%20Dubilier%20Photos/715P47356LD3_sml.JPG)

$3.53 in onsey twosey...  But for 240VAC countries not up to the task IMO.

JR
Title: Re: Death cap
Post by: Frank Koenig on December 21, 2014, 05:04:34 PM
I know there are film capacitors made for applications where leakage would result in a shock hazard. When I designed electrosurgical generators we were required (by FDA, or it could have been VDE or TUV, can't remember) to use such a highly pedigreed cap as the series capacitor in the high-pass filter in front of the probe that gets stuck into the patient. They're usually used in line filters and such and are in a prismatic, radial-lead, through-hole package that is plastered with agency approval marks, TUV, UL, etc. You probably know the ones.

I'm having trouble finding an example right now but I'll keep looking.

--Frank
Title: Re: Death cap
Post by: Frank Koenig on December 21, 2014, 05:17:28 PM
I'm having trouble finding an example right now but I'll keep looking.

Like this: http://www.kemet.com/docfinder?Partnumber=R413F12200000M (http://www.kemet.com/docfinder?Partnumber=R413F12200000M)
Title: Re: Death cap
Post by: John Roberts {JR} on December 21, 2014, 05:23:38 PM
I know there are film capacitors made for applications where leakage would result in a shock hazard. When I designed electrosurgical generators we were required (by FDA, or it could have been VDE or TUV, can't remember) to use such a highly pedigreed cap as the series capacitor in the high-pass filter in front of the probe that gets stuck into the patient. They're usually used in line filters and such and are in a prismatic, radial-lead, through-hole package that is plastered with agency approval marks, TUV, UL, etc. You probably know the ones.

I'm having trouble finding an example right now but I'll keep looking.

--Frank
Yup I couldn't find one either they were called something like X caps or Y caps for mains power applications.

OK we want a Y cap for line to chassis.

Y caps are metalized film so they are self healing in response to punch throughs.

Here is a proper "Y" cap.

http://www.digikey.com/product-search/en?s=19609&FV=fff40002%2Cfff80010%2Cfffc018f%2C340021&k=Y+capacitor&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25 (http://www.digikey.com/product-search/en?s=19609&FV=fff40002%2Cfff80010%2Cfffc018f%2C340021&k=Y+capacitor&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25)

(http://media.digikey.com/photos/Kemet%20Photos/PHE850EB5470MB14R17_sml.jpg)

http://www.kemet.com/Lists/Filestore/EvoxRifaRFIandSMD.pdf (http://www.kemet.com/Lists/Filestore/EvoxRifaRFIandSMD.pdf)

Here is a brochure about X and Y caps...  I didn't look hard enough before

BTW  300VAC, 1250VDC and only $0.97 for onsey/twosey.

JR

[edit] this is surely over engineered for the application, but at <$1 looks OK [/edit]
Title: Re: Death cap
Post by: Mike Sokol on December 21, 2014, 05:32:02 PM
The orange drops from digikey are only 200VAC 600DC

$3.53 in onsey twosey...  But for 240VAC countries not up to the task IMO.

JR

Since this will be a USA experiment, I'm not worrying about 240-volts AC right now. If this works as envisioned, then a different design will be needed for the UK.
Title: Re: Death cap
Post by: Mike Sokol on December 21, 2014, 05:46:14 PM

BTW  300VAC, 1250VDC and only $0.97 for onsey/twosey.


Hey, only 31 cents each in 5K quantities. I'm going to order of bag of these for my experiments.
Title: Re: Death cap
Post by: John Roberts {JR} on December 21, 2014, 05:57:54 PM
Hey, only 31 cents each in 5K quantities. I'm going to order of bag of these for my experiments.

The right part for the job,,,,,,  the X caps are designed for use across the line without catching fire... UL is picky about stuff like that.

JR
Title: Re: Death cap
Post by: Mike Sokol on December 21, 2014, 07:30:18 PM
The right part for the job,,,,,,  the X caps are designed for use across the line without catching fire... UL is picky about stuff like that.

JR

The thing about this application is that the stinger cap really won't be across the line or even line to chassis. Most of the time it should have very close to zero volts across it, especially if I add a 47K resistor in parallel. But if the guitarist contacts a hot mic, then it will see 120-vots AC and need to have low leakage to protect the musician. Of course, it will have around 2 mA current through it from the 60 Hz AC, and maybe a few more mA from the parallel resistor. So the 5 mA or so of current through the guitarist at this point should be enough to get his or her attention, but not enough to be dangerous. But because this is outside of the GFCI sense path, it won't trip the GFCI. And even if it did the mini-shock hazard would still be there because there's no contact in the EGC path that can be disconnected. That's not a bad thing, just how it will operate.   
Title: Re: Death cap
Post by: Bob Leonard on December 23, 2014, 09:03:32 AM
These will work fine.

http://www.mojotone.com/amp-parts/capacitor-type-film-and-foil
Title: Re: Death cap
Post by: Mike Sokol on December 23, 2014, 09:26:26 AM
I'm going to kludge together a Stinger-GFCI as soon as my caps arrive and will mount the test circuit in a plastic box with a plastic cover plate and a nylon cover screw. That's because plugging this into a RPBG outlet will energize any of the normally grounded metal parts. So for starters it will look something like one of these boxes I use in my No~Shock~Zone clinics. However, I'm not happy with plastic boxes and nylon screws for actual back-line AC drops. I'm wondering if there's a more rugged/waterproof box that would be stage worthy yet electrically isolated from the incoming EGC. Any ideas on insulated boxes and covers that would survive stage usage? 
Title: Re: Death cap
Post by: Mark Cadwallader on December 23, 2014, 09:55:12 AM
O. A. Windsor makes a non-conductive box that is stage tough, but pricey. Mark C.
Title: Re: Death cap
Post by: Art Welter on December 23, 2014, 10:13:05 AM
Art,
CTS pots are rated about the same. The deal with the CTS pots is that they are the pots Fender and Gibson used as OEM hardware pretty much from day one. That has induced a cult following of sorts.

You must be a very lucky person indeed. I have never seen a 50 year old electrolytic that didn't need replacement for one reason or another. Outward appearance aside, electrolytic or not, caps don't last forever.I can do an entire amp using quality caps for about $75 in parts, and guarantee there will be a huge difference in tone and performance when done. So, to each his own, but you might want to take a second look at those 50 year old caps.
Bob,

I don't think it is luck, more likely the caps used on the old Rauland, Bogen, Telefunken, and Nordmende Isabella just  seem to be much longer lived than the cheaper ones used on the Harmony amp, one of two actual old "guitar amps" I owned. It needed recapping while the much older amps performed with no noise, and all had great "tone", due to the better transformers and caps, etc.
If I like an amp's tone, I don't mess with it unless the PSU caps are not up to the filtration task.

I sold the last of the old tube amps recently, and it will probably be another decade or three before I have to replace the caps on the Epiphone Valve Jr. and ART Tube mic pre amps, the only tube gear I have anymore. Fortunately, the small caps those amps use will cost a lot less than $75 to replace.

Art
Title: Re: Death cap
Post by: Art Welter on December 23, 2014, 10:21:54 AM
I'm wondering if there's a more rugged/waterproof box that would be stage worthy yet electrically isolated from the incoming EGC. Any ideas on insulated boxes and covers that would survive stage usage?
Mike,

"Waterproof" stage boxes really do not exist as far as I've seen, but these Hubble units can withstand a fork lift driving over them with no damage. They are expensive but worth every penny. The right angle plug arrangement makes plugging in 4 "wall warts possible, and they have all four outlets hard wired with buss bars inside, so you only need to wire input and through wires, unless you want to split them for multiple circuits.

I recently sold a system using them, other than replacing one lost screw, the 8 units required zero maintenance since 1997.
They are also available with 20 amp Edison outlets also.
Title: Re: Death cap
Post by: John Roberts {JR} on December 23, 2014, 11:29:12 AM
The thing about this application is that the stinger cap really won't be across the line or even line to chassis. Most of the time it should have very close to zero volts across it, especially if I add a 47K resistor in parallel. But if the guitarist contacts a hot mic, then it will see 120-vots AC and need to have low leakage to protect the musician. Of course, it will have around 2 mA current through it from the 60 Hz AC, and maybe a few more mA from the parallel resistor. So the 5 mA or so of current through the guitarist at this point should be enough to get his or her attention, but not enough to be dangerous. But because this is outside of the GFCI sense path, it won't trip the GFCI. And even if it did the mini-shock hazard would still be there because there's no contact in the EGC path that can be disconnected. That's not a bad thing, just how it will operate.

Breakdown voltage ratings for a cap is not about typical voltage but worst case voltage transiently. While a human will generate a voltage divider with such a cap there will be an instantaneous exposure to full voltage due to capacitance in the human body, not to mention the possibility of exposure to full voltage (like when touching mic to guitar strings like experienced musicians often do). We definitely do not want a cap that could punch through the dielectric and short. The specification of metalized film dielectric for this application is because they are self healing so do not generally fail as dead short. Note: static hits are another possible exposure to worry about, again the metalized film should be more robust. 

These are arguably ever-engineered for this application's typical exposure but not IMO for worst case, and at < $1 why not use a cap designed to shrug off permanent connection across the mains voltage?

I rest my case....  8)  (merry christmas).

JR
Title: Re: Death cap
Post by: Mike Sokol on December 23, 2014, 12:06:11 PM
These are arguably over-engineered for this application's typical exposure but not IMO for worst case, and at < $1 why not use a cap designed to shrug off permanent connection across the mains voltage?

I rest my case....  8)  (merry christmas).

JR

You are correct, which is why I ordered ten of them for my experiment. Should be fun...
Title: Re: Death cap
Post by: Mike Sokol on December 23, 2014, 04:16:59 PM
O. A. Windsor makes a non-conductive box that is stage tough, but pricey. Mark C.

Thanks.... I've put in a request to O A Windsor to send me a few sample boxes for the build. These things look like they should survive some serious kicking around on stage.
Title: Re: Death cap
Post by: Bob Leonard on December 27, 2014, 10:13:14 AM
I've been doing a certain amount of tinkering lately, time permitting with the holidays and all. I'm not against applying a fix to the line in side of the amp, and a standard GFI will work to that point. What I have found that concerns me is that there is still enough residual charge left in the PS capacitors to kill you, even after tripping the GFI. I tested this against a few amplifiers and as suspected the charge will last up to 5 seconds, maybe longer.
Title: Re: Death cap
Post by: John Roberts {JR} on December 27, 2014, 11:38:44 AM
I've been doing a certain amount of tinkering lately, time permitting with the holidays and all. I'm not against applying a fix to the line in side of the amp, and a standard GFI will work to that point. What I have found that concerns me is that there is still enough residual charge left in the PS capacitors to kill you, even after tripping the GFI. I tested this against a few amplifiers and as suspected the charge will last up to 5 seconds, maybe longer.

That argues in favor of the capacitor coupled ground inside the guitar itself. There is no perfect solution.

My friend who designs guitar pedals and amps says he has heard of people using transformer isolated guitar inputs. I am uncertain about the high impedance guitar pickup tone being unaffected. Guitar pickups can output decent voltage and are sensitive to capacitance. The only good thing in favor of transformer coupling is that the lead guitar LF response does not go very low, but nether side is low impedance so not an obvious audio transformer application. 

They do make transformer direct boxes but these are usually used with bass guitars that have lower source impedance, for lead guitar active direct boxes are preferred.  Another downside to just hanging a DB transformer in front of a guitar amp is that they also scrub off several dB of voltage gain (intended for mic inputs).

My suggested modifications were to protect against a hot external safety ground. To protect against the amp itself you want to bond the chassis to safety ground, which is back were we started from and how modern guitar amps are manufactured today (but that still can kill people in the case of a power wiring fault.).

Do your own calculus... are you more likely to encounter a live mic, or an amp meltdown?

JR

PS: a relay that breaks the guitar cable circuit (both leads) when ground current is detected seems promising while not very practical. These things would all be easier to build into a new guitar amp design, while it might be difficult to get musicians to pay the cost for such improvements. 
Title: Re: Death cap
Post by: Mike Sokol on December 27, 2014, 11:58:35 AM

These things would all be easier to build into a new guitar amp design, while it might be difficult to get musicians to pay the cost for such improvements.

The real problem is a culture of performers accepting being shocked. If they demanded a shock-proof guitar and were willing to pay for it, then this thread wouldn't be necessary. So in addition to coming up with an affordable and practical anti-shock solution for new backline amplifiers, we also need to educate performers that NO shock is normal for a modern sound system and backline gear. Of course, antique music gear is highly collected and played (plus it sounds great  :D) so an affordable retrofit is needed. But again, without education that any shock can be dangerous and should be avoided, there won't be any push from musicians for a shock-safe solution. Still, a practical design is needed that won't mess with the sound of the amplifiers. I think we're on the right track here...  8)     
Title: Re: Death cap
Post by: Stephen Swaffer on December 27, 2014, 12:03:59 PM
.A capacitor discharge is different than a line voltage shock in that it is a DC current vs AC.  The danger thresh hold is different, and while I am sure a cap charged to 150 volts carries a painful "bite", I am not sure how lethal it is in and of itself (the reaction to the "bite" might be more hazardous) .
Title: Re: Death cap
Post by: Bob Leonard on December 28, 2014, 05:19:51 AM
The B+ side of these amplifiers is usually in a range above 400V @ 1.5 to 3 amps. There's going to be more than a bite.

JR/Mike,

Looking back, and I mean way back, the path to ground through a PA system was generally more direct. The PA's of the day from Bogan, Shure, Marshall, etc. usually had the mic jacks tied to chassis ground. Combine that with AC plugs that had no ground lug and your chances of being shocked was 50/50. In those days one hand on the strings and a wet finger or tongue to the mic was the test, regardless of the acts level.

Today I don't feel the need to tongue test every mic I lean into as electronics and isolated microphone circuits are the norm. I also understand that new equipment designs don't guarantee you won't be shocked, and I have been. My most recent event was just over a year ago at a club in Somerville, MA. I was standing on a Persian rug, leaned over to check the mic level and was knocked on my ass. I spent the next half hour chasing the ground path until finally I removed the rug, and that is what cured the problem.

My idea would be a GFCI for the AC side and for the guitar side of the amp. I'll keep it simple and suggest an off the shelf GFI in the guitar cable side after all of the effects and just before the input jack. If I get time I'll jury rig a box this week and run some tests.
Title: Re: Death cap
Post by: John Roberts {JR} on December 28, 2014, 11:01:25 AM
The B+ side of these amplifiers is usually in a range above 400V @ 1.5 to 3 amps. There's going to be more than a bite.

JR/Mike,

Looking back, and I mean way back, the path to ground through a PA system was generally more direct. The PA's of the day from Bogan, Shure, Marshall, etc. usually had the mic jacks tied to chassis ground. Combine that with AC plugs that had no ground lug and your chances of being shocked was 50/50. In those days one hand on the strings and a wet finger or tongue to the mic was the test, regardless of the acts level.

Today I don't feel the need to tongue test every mic I lean into as electronics and isolated microphone circuits are the norm. I also understand that new equipment designs don't guarantee you won't be shocked, and I have been. My most recent event was just over a year ago at a club in Somerville, MA. I was standing on a Persian rug, leaned over to check the mic level and was knocked on my ass. I spent the next half hour chasing the ground path until finally I removed the rug, and that is what cured the problem.
Not sure I understand.. Static build up from walking on the rug? Damp rug grounded? After living in the south for a few decades I kind of forget all about static, but there was one milk cooler at Walmart that would give a decent static shock when grabbing the metal handle. Probably an open safety ground and an unintentional van de graf generator going on. 
Quote
My idea would be a GFCI for the AC side and for the guitar side of the amp. I'll keep it simple and suggest an off the shelf GFI in the guitar cable side after all of the effects and just before the input jack. If I get time I'll jury rig a box this week and run some tests.
I'm not sure I follow but let us know what you come up with and be careful testing it (don't hurt yourself or any amps in the process. )

JR

PS: I still advocate using stock GFCI for back line, but lean toward shunting FOH safety ground to back line safety ground through a few diodes. Now I need to figure out how to avoid shocks while plugging into the safety gadget.  8)
Title: Re: Death cap
Post by: Scott Holtzman on December 28, 2014, 09:46:20 PM
Not sure I understand.. Static build up from walking on the rug? Damp rug grounded? After living in the south for a few decades I kind of forget all about static, but there was one milk cooler at Walmart that would give a decent static shock when grabbing the metal handle. Probably an open safety ground and an unintentional van de graf generator going on.  I'm not sure I follow but let us know what you come up with and be careful testing it (don't hurt yourself or any amps in the process. )

JR

PS: I still advocate using stock GFCI for back line, but lean toward shunting FOH safety ground to back line safety ground through a few diodes. Now I need to figure out how to avoid shocks while plugging into the safety gadget.  8)

I am going to toss another one at you.  Cheap chinese charge plugged in convenience outlet on power bar at FOH.  Ground connect on DI to iPod.  Connect USB charger, plug in watch phone and wire catch on fire.  So many permutations.

I had almost forgotten about that.

Title: Re: Death cap
Post by: John Roberts {JR} on December 29, 2014, 10:12:57 AM
I am going to toss another one at you.  Cheap chinese charge plugged in convenience outlet on power bar at FOH.  Ground connect on DI to iPod.  Connect USB charger, plug in watch phone and wire catch on fire.  So many permutations.

I had almost forgotten about that.
Generally using modern UL approved gear should prevent such hazards. If you buy non-UL products that plug into mains power you put yourself and those around you in harm's way.

JR