ProSoundWeb Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: 1 ... 3 4 [5] 6 7   Go Down

Author Topic: Old guitar Amp power polarity and live chassis  (Read 14115 times)

Stephen Kirby

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3003
Re: Old guitar Amp power polarity and live chassis
« Reply #40 on: June 25, 2016, 07:36:51 pm »

I think based on Mike's earlier advice, I'm just gonna put the GFCIs in the drops and if I see a vintage amp, run up there with an NCVT and make sure the chassis isn't hot before the guy trips it and ignoring the shock he got (which they're probably used to) blames me for having things that cut the power off on him.
Logged

Jeff Bankston

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2227
Re: Old guitar Amp power polarity and live chassis
« Reply #41 on: June 26, 2016, 02:42:26 am »

« Last Edit: June 26, 2016, 04:11:39 pm by Jeff Bankston »
Logged

John Roberts {JR}

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 16235
  • Hickory, Mississippi, USA
    • Resotune
Re: Old guitar Amp power polarity and live chassis
« Reply #42 on: June 26, 2016, 11:54:52 am »

I watched these 2 videos last night on death caps.

http://www.youtube.com/watch?v=GFxMeswY5kw

http://www.youtube.com/watch?v=xS_5K5YEYv8
One took me to a movie clip, the other a mildly inaccurate discussion of electricity (AC voltage/current will not charge up the body's capacitance then stop, etc).

3 wire line cords and GFCI on backline are your friend.

I still prefer our stinger cap GFCI for absolute human safety, to protect against a good ground on the guitar becoming the ground path for a hot microphone (rare but could happen).  Of course a cap coupled safety ground is not UL approved so only a mental exercise at this point. 

Some protect at the guitar by using a stinger cap or fuse in series with the guitar ground, but musicians dislike modifying guitars even more than modifying amps.

I guess this stinger ground could be done in series with the guitar cord but the 0.47uF Y rated cap is pretty large. I like the cap to be big enough to trip and open a GFCI in the mains path.

JR
Logged
On the internet people tell you everything "they" know, not the answer to "your" question.....  http://circularscience.com/

Jeff Bankston

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2227
Re: Old guitar Amp power polarity and live chassis
« Reply #43 on: June 26, 2016, 04:14:29 pm »

One took me to a movie clip,
JR
I changed the link to the link that should have posted. I dont know how that movie link got there.
Logged

Bob Leonard

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 6807
  • Boston, MA USA
Re: Old guitar Amp power polarity and live chassis
« Reply #44 on: June 27, 2016, 07:53:53 pm »

Which is why they're called death caps. Cut the cap out, wire with a 3 prong switch, and solder the neutral to the chassis. This is one of our labster's Fender amps that I rebuilt. The ground is on the far left beside the power transformer. I do them all the time and no one's called to say they've been shocked. ::)






I'm so ashamed.

Cut out the cap, wire the neutral and hot to the transformer routing the hot leg through the fuse and switch. Solder the ground wire to the chassis.
Logged
BOSTON STRONG........
Proud Vietnam Veteran

I did a gig for Otis Elevator once. Like every job, it had it's ups and downs.

John Roberts {JR}

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 16235
  • Hickory, Mississippi, USA
    • Resotune
Re: Old guitar Amp power polarity and live chassis
« Reply #45 on: June 27, 2016, 08:15:35 pm »

don't worry Bob we got your back, just like people get my back when I'm wrong.... This community is pretty deep.

JR
Logged
On the internet people tell you everything "they" know, not the answer to "your" question.....  http://circularscience.com/

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3341
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: Old guitar Amp power polarity and live chassis
« Reply #46 on: July 01, 2016, 02:14:23 pm »

Prostat sent me a pair of QUBE grounding plugs to experiment with. They did confirm that there's no current limiting resistors... it's a machined chunk of brass with holes for banana plugs, then coated in a fireproof plastic shell. If we can come up with a banana plug to amp-chassis connection that makes sense, then this would seem to be a reasonable way to add a temporary safety ground to old backline amps.
Logged
Mike Sokol
mike@noshockzone.org
www.NoShockZone.org

Jonathan Johnson

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2953
  • Southwest Washington (state, not DC)
Re: Old guitar Amp power polarity and live chassis
« Reply #47 on: July 01, 2016, 02:59:33 pm »

Prostat sent me a pair of QUBE grounding plugs to experiment with. They did confirm that there's no current limiting resistors... it's a machined chunk of brass with holes for banana plugs, then coated in a fireproof plastic shell. If we can come up with a banana plug to amp-chassis connection that makes sense, then this would seem to be a reasonable way to add a temporary safety ground to old backline amps.

Problem I see is that this takes up one more receptacle. What would be ideal is a sort of reverse cheater plug, that passes all three prongs through and provides a wire or terminal connected to ground for grounding older, ungrounded devices.

Why all three prongs? Because you might want to plug a grounded device in there anyway, if using the plug to provide a ground for something else.

Then again, I guess it wouldn't be much different from using a cube tap. How hard would it be to modify a cube tap? There are some out there that are riveted rather than comolded so it could be doable.
Logged
Stop confusing the issue with facts and logic!

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3341
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: Old guitar Amp power polarity and live chassis
« Reply #48 on: July 01, 2016, 03:04:37 pm »

Problem I see is that this takes up one more receptacle. What would be ideal is a sort of reverse cheater plug, that passes all three prongs through and provides a wire or terminal connected to ground for grounding older, ungrounded devices.

Why all three prongs? Because you might want to plug a grounded device in there anyway, if using the plug to provide a ground for something else.

Then again, I guess it wouldn't be much different from using a cube tap. How hard would it be to modify a cube tap? There are some out there that are riveted rather than comolded so it could be doable.

The reason I'm considering something like the QUBE is that it's UL rated for grounding, and looks like it grounds stuff. I can easily hang a green wire out of a plug I bought at Lowes, but there's going to be some AHJ that questions how you wired it. In this case it should be obvious that the ONLY thing it can connect to is the EGC contact in the NEMA receptacle. Note that one of these QUBE's will feed "grounds" to three different circuits.
Logged
Mike Sokol
mike@noshockzone.org
www.NoShockZone.org

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3341
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: Old guitar Amp power polarity and live chassis
« Reply #49 on: July 02, 2016, 07:01:28 am »

A slight twist to this scenario involves a modern double-insulated guitar amp without a dedicated ground pin. This was a Roland Cube amp with a wall-wart power supply. Obviously, the insulation failed inside the wall-wart and energized the chassis of the amplifier. The guitar player said he got one hell of a shock from his guitar when he touched the mic with his lips. This was in a church that experienced a large lightning strike the month before which took out most of the sound system. I'm guessing it flashed over inside of the wall-wart but didn't blow up the amplifier.
Logged
Mike Sokol
mike@noshockzone.org
www.NoShockZone.org
Pages: 1 ... 3 4 [5] 6 7   Go Up
 


Page created in 0.084 seconds with 24 queries.