ProSoundWeb Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: 1 2 [3] 4 5 ... 7   Go Down

Author Topic: Old guitar Amp power polarity and live chassis  (Read 15016 times)

Kevin Graf

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 305
Re: Old guitar Amp power polarity and live chassis
« Reply #20 on: June 22, 2016, 09:47:05 pm »

So what is a bad stinger cap?
If it's open, it won't do anything.
If it's shorted it may do more than trip the GFCI.
Logged
Speedskater

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3354
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: Old guitar Amp power polarity and live chassis
« Reply #21 on: June 22, 2016, 10:42:48 pm »

So what is a bad stinger cap?
If it's open, it won't do anything.
If it's shorted it may do more than trip the GFCI.

If it's shorted it should only trip the GFCI and shut down its own incoming AC power. That's why I recommend one GFCI per backline instrument. You don't want a single misbehaving stage amp to take out your entire backline.
Logged
Mike Sokol
mike@noshockzone.org
www.NoShockZone.org

John Roberts {JR}

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 16332
  • Hickory, Mississippi, USA
    • Resotune
Re: Old guitar Amp power polarity and live chassis
« Reply #22 on: June 22, 2016, 11:49:39 pm »

They now sell "X" and "Y" rated capacitors that are designed so that they will not fail dangerously. Capacitors that self heal, after a transient short circuit fault that fuses open and continues functionally like a capacitor.

Back in the "death" cap day, caps were not that well rated***, so a dangerous shorted cap 'could' happen.

JR 

*** I do not know when they started properly rating caps for use across line voltage, but probably after 3 wire line cords became standard.
Logged
Listen to what a properly "cleared" drum sounds like.   http://circularscience.com/

Jonathan Johnson

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2984
  • Southwest Washington (state, not DC)
Re: Old guitar Amp power polarity and live chassis
« Reply #23 on: June 23, 2016, 12:55:12 pm »

So what is a bad stinger cap?
If it's open, it won't do anything.
If it's shorted it may do more than trip the GFCI.

If it is shorted, it will NOT trip the circuit breaker unless the cabinet is incidentally well-grounded by means other than an equipment grounding conductor (third prong). Remember, stinger caps generally only are present on ungrounded amplifiers.

If it is shorted and it does not trip the circuit breaker by incidental grounding, a performer who makes contact with both the now-hot amplifier chassis (via the interconnect cable and the steel strings on the guitar) and another grounded object (such as by placing their lips on a microphone screen) will complete a circuit and receive a potentially deadly electric shock.

How do they complete the circuit? The microphone screen is connected to the shield of the cable, which in turn connects to the grounded chassis of the console. The EGC from the console connects through the EGC of the building wiring to the service entrance panel, where the EGC is bonded to the neutral conductor. The neutral conductor returns to the utility transformer, completing the circuit.

Now if the stinger cap happens to be shorted, you have a 50/50 (1 in 2) chance of getting a live chassis, because the plug on the power cord is not polarized. So the shorted cap could be between the neutral and the chassis, or it could be between the hot and chassis, depending on which way the power cord is inserted into the receptacle (or which way the on-off-on switch is flipped).

You cannot predict when a stinger cap will fail as a short circuit. Therefore, you do not want to assume that just because a chassis is not live now, it won't be live later on in the show.

While replacing the power cord and eliminating the stinger cap is the safest fix, that's not always feasible. Feasible alternatives include:
  • Connect the amplifier to a GFCI to protect the performer (use a dedicated GFCI so a fault does not de-energize other equipment).
  • Perform continuity and voltage testing to ensure that the chassis (via the stinger cap) is connected to the neutral rather than the hot. This is not without caveat, as you must ensure that the power plug remains properly oriented and the power switch remains in the proper position -- two issues that are very difficult to control without mechanical means of preventing change.

Many performers will resist modification of their amplifiers, whether it's because they believe it will change the tone, or they don't want to change the appearance of the amplifier, or they fear devaluing the amplifier. Proper modifications to power cords are unlikely to change the tone in an undesirable way. The appearance aspect can be mitigated by using period-appropriate materials (such as cloth-jacketed cord) that comply with modern standards. As for valuation, they are placing perceived value over life safety, and that's just plain stupid.

If value is more important than safety, then the amp is only good for looking at, not for actually using -- and there goes the value.
« Last Edit: June 23, 2016, 12:57:17 pm by Jonathan Johnson »
Logged
Stop confusing the issue with facts and logic!

Steve M Smith

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3160
  • Isle of Wight - England
Re: Old guitar Amp power polarity and live chassis
« Reply #24 on: June 24, 2016, 02:51:11 am »

It's hard to believe that someone thought a stinger cap and a switch was a god idea!


Steve.
Logged

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3354
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: Old guitar Amp power polarity and live chassis
« Reply #25 on: June 24, 2016, 06:59:48 am »

While replacing the power cord and eliminating the stinger cap is the safest fix, that's not always feasible. Feasible alternatives include:
  • Connect the amplifier to a GFCI to protect the performer (use a dedicated GFCI so a fault does not de-energize other equipment).
  • Perform continuity and voltage testing to ensure that the chassis (via the stinger cap) is connected to the neutral rather than the hot. This is not without caveat, as you must ensure that the power plug remains properly oriented and the power switch remains in the proper position -- two issues that are very difficult to control without mechanical means of preventing change.

All true, but there's one more option we haven't explored. You could add a temporary EGC connection to the stage amp. See below for the Qube wrist strap grounding plug. By plugging this into any properly wired receptacle and connecting the banana plug ground to the chassis, you're "bonding" the amplifier chassis to the EGC. There's several possible ways to bond this to the guitar chassis including an alligator clip (not recommended since it could pop off) to putting a lug under a chassis screw (will the musician let me do that) to a dummy 1/4" TS  plug connected to the speaker jack (looks crazy, but should work unless it switches the output transformer or speaker connection)
Logged
Mike Sokol
mike@noshockzone.org
www.NoShockZone.org

Peter Kowalczyk

  • Full Member
  • ***
  • Offline Offline
  • Posts: 176
Re: Old guitar Amp Stinger Caps and live chassis
« Reply #26 on: June 24, 2016, 12:54:57 pm »

Does anyone make and deploy Quad Boxes with GFCI receptacles for backline?  Seems simple enough...

P
Logged

Stephen Swaffer

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2204
Re: Old guitar Amp power polarity and live chassis
« Reply #27 on: June 24, 2016, 01:47:47 pm »

All true, but there's one more option we haven't explored. You could add a temporary EGC connection to the stage amp. See below for the Qube wrist strap grounding plug. By plugging this into any properly wired receptacle and connecting the banana plug ground to the chassis, you're "bonding" the amplifier chassis to the EGC. There's several possible ways to bond this to the guitar chassis including an alligator clip (not recommended since it could pop off) to putting a lug under a chassis screw (will the musician let me do that) to a dummy 1/4" TS  plug connected to the speaker jack (looks crazy, but should work unless it switches the output transformer or speaker connection)

I haven't looked at the specs-but I doubt this is actually bonding to the EGC.  An ESD ground is typically through a resister-1 Meg or so.  This allows static to drain off without intentionally grounding a tech.  You don't really want someone working on a tech bench to be solidly grounded in many cases.  Yes, there are other precautions they should take-but there is no need to create a hazard.

*I am using the term "bonded" as it used in the NEC.
Logged
Steve Swaffer

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3354
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: Old guitar Amp power polarity and live chassis
« Reply #28 on: June 24, 2016, 01:58:28 pm »

I haven't looked at the specs-but I doubt this is actually bonding to the EGC.  An ESD ground is typically through a resister-1 Meg or so.  This allows static to drain off without intentionally grounding a tech.  You don't really want someone working on a tech bench to be solidly grounded in many cases.  Yes, there are other precautions they should take-but there is no need to create a hazard.

*I am using the term "bonded" as it used in the NEC.

I've got some of these, and IIRC they're a direct Bond to the EGC. The current limiting resistor for human side is part of the wrist strap connector itself. But I'll check to be sure. 
Logged
Mike Sokol
mike@noshockzone.org
www.NoShockZone.org

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3354
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: Old guitar Amp power polarity and live chassis
« Reply #29 on: June 24, 2016, 02:16:13 pm »

I've got some of these, and IIRC they're a direct Bond to the EGC. The current limiting resistor for human side is part of the wrist strap connector itself. But I'll check to be sure.

I've confirmed with the manufacturer that these are indeed bonded thru. That is, there's a solid brass pin welded to a brass backer plate, and the entire assembly is injection molded with flame retardant plastic. And the hole is sized for a standard banana plug.They're sending me a few to play with, but in the meantime, how do we create a solid yet temporary connection to the guitar amp chassis?
Logged
Mike Sokol
mike@noshockzone.org
www.NoShockZone.org
Pages: 1 2 [3] 4 5 ... 7   Go Up
 


Page created in 0.04 seconds with 22 queries.