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Author Topic: Old guitar Amp power polarity and live chassis  (Read 16090 times)

Daniel Levi

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Re: Old guitar Amp power polarity and live chassis
« Reply #30 on: June 24, 2016, 02:29:11 pm »

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?

The only thing I could think of (but would only work for steel chassis) is a strong magnet with a piece of wire attached to it, or maybe if you could get one that fitted a contact held on by a c clamp.
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John Roberts {JR}

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Re: Old guitar Amp power polarity and live chassis
« Reply #31 on: June 24, 2016, 03:06:55 pm »

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?

The sleeve of most 1/4" plugs will be bonded to chassis... not sure I trust it to take out a 15-20A breaker.

Wire wrapped around a 1/4" jack nut, but again, not that trustworthy.

Maybe put a 3 wire line cord on the amp when the muso isn't looking.  8)

JR
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Mike Sokol

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Re: Old guitar Amp power polarity and live chassis
« Reply #32 on: June 24, 2016, 03:49:22 pm »

The only thing I could think of (but would only work for steel chassis) is a strong magnet with a piece of wire attached to it, or maybe if you could get one that fitted a contact held on by a c clamp.

I like the idea of a magnet, but I fear that guitar players will think it's magnetizing their amps somehow and changing the tone. 
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Mike Sokol
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Mike Sokol

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Re: Old guitar Amp power polarity and live chassis
« Reply #33 on: June 24, 2016, 03:52:49 pm »

The sleeve of most 1/4" plugs will be bonded to chassis... not sure I trust it to take out a 15-20A breaker.

Wire wrapped around a 1/4" jack nut, but again, not that trustworthy.


I've been toying with the idea of a short 1/4" TS sleeve with an expansion nut. So it plugs in just far enough to grab the inside of the flange, but not so far as to make contact with the spring "tip" contact. Then by spinning a thumbscrew you expand it to make firm contact. A 90 degree bent connector would be the best way to do this.
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Mike Sokol
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Re: Old guitar Amp power polarity and live chassis
« Reply #34 on: June 24, 2016, 04:07:24 pm »

If they only made Banana Plugs in 6.3mm (1/4") diameter in addition to 4mm. That would seem to be a good way to grab a solid chassis connection on most any guitar or bass amplifier.
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John Roberts {JR}

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Re: Old guitar Amp power polarity and live chassis
« Reply #35 on: June 24, 2016, 04:50:03 pm »

a spade lug with 1/4" diameter hole could be attached under the jack nut. Or drill a small hole in the chassis and use a thread cutting screw...

or just convince them to convert to a 3 wire line cord...

JR
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Stephen Swaffer

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Re: Old guitar Amp power polarity and live chassis
« Reply #36 on: June 24, 2016, 05:13:45 pm »

Why not just a short ts to ts extension with a lug soldered to the shell of the ts plug?  You could still use the jack.  For security a long cord wrap style piece of velcro?

Thread cutting screws are not acceptable for bonding by code- but then neither is a 1/4"  ts jack.

Ask them which they would rather be the case-playing their amp with a very minor safety modification, or having a pristine, unmodified amp in their casket with them?
« Last Edit: June 24, 2016, 05:17:22 pm by Stephen Swaffer »
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Stephen Kirby

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Re: Old guitar Amp power polarity and live chassis
« Reply #37 on: June 24, 2016, 06:19:19 pm »

Most old school guitar amps have multiple channels or at least multiple paralleled input jacks.  Hook your ground lead to the sleeve of a plug with the tip disconnected and plug it into an unused input.  Some amps may have a bright cap on an alternate input but if the tip is disconnected that won't be brought into play.

So, having made up one of these, what happens when someone plugs in an amp backwards to the way the ground switch should be set?  Meaning that the hot is on the chassis and the neutral is on the amp's power switch waiting for someone to turn it on.  If the amp is plugged in first and you come along with your ground bond plug, does that instantly trip the breaker or GFCI?  Any possibility of an arc flash as you plug it in?  While holding the metal barrel of the plug?  Or if you plug in the 1/4" ground plug before someone gets a chance to plug the amp in, does the breaker instantly trip?  Requiring you to flip the amp's ground switch and reset the breaker?

Maybe this could have an indicator function?  As simple as a neon lamp in series and a jumper switch.  Plug it in, if the lamp glows, indicating chassis to ground potential, then flip the amps ground switch.  When the amp is out, engage the jumper so that there is now a hard ground.  This could be automated with some diodes but you'd be back to an open ground with component failure.
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Tom Bourke

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Re: Old guitar Amp power polarity and live chassis
« Reply #38 on: June 24, 2016, 07:38:13 pm »

If the guitar player is that worried about having his amp modded or something affecting his tone your already on the losing side.  Having "an odd plug" on his input is going to get you blamed for all kinds of stuff.

Put them on a GFI and show them a picture of some one who got there lips blown off.
If they then really want a safety ground a C clamp with a contact or a good strong alligator clip.
Give them a list of techs you trust to do the mod.
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John Roberts {JR}

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Re: Old guitar Amp power polarity and live chassis
« Reply #39 on: June 25, 2016, 12:44:44 am »

If the guitar player is that worried about having his amp modded or something affecting his tone your already on the losing side.  Having "an odd plug" on his input is going to get you blamed for all kinds of stuff.

Put them on a GFI and show them a picture of some one who got there lips blown off.
If they then really want a safety ground a C clamp with a contact or a good strong alligator clip.
Give them a list of techs you trust to do the mod.
yup... GFCI power drop.. and leave the amps alone.

(or replace the line cord with 3 wire, "and" use the GFCI). 

JR

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Re: Old guitar Amp power polarity and live chassis
« Reply #39 on: June 25, 2016, 12:44:44 am »


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