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Author Topic: Old guitar Amp power polarity and live chassis  (Read 14289 times)

Peter Kowalczyk

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Old guitar Amp power polarity and live chassis
« on: June 20, 2016, 03:10:23 pm »

Onstage last night, a guitarist sets up his vintage Guild combo amp.  It's equipped only with a two-prong, non-detatchable AC mains plug.  After soundcheck, while disconnecting his pedalboard, he gets a zap when touching two 1/4" leads on opposite sides of a pedal, one of which is connected directly to the amp.

A NCVT indicates the amplifier's chassis is live, and a DVM measures 115 VAC from the chassis to various ground points on the pedalboard.

The amp has a three-position power switch - off in the center, on either up or down.  He flips the power switch to the OTHER 'on' position, and the Amp's chassis is now at ground.  I advised him to get the amp serviced as soon as possible.  He plays the show this way with no ill effects.

My understanding is that the two 'On' positions of a three-position power switch swaps the polarity of the AC mains waveform entering the amp's power transformer.  However, it seems there's still a dangerous failure of the AC mains wiring somewhere within the amp's chassis.

Any thoughts on what may be the problem?  How would you have handled this situation?
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John Roberts {JR}

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Re: Old guitar Amp power polarity and live chassis
« Reply #1 on: June 20, 2016, 04:18:20 pm »

"Stinger cap".... old school amps used a capacitor to shunt the chassis to neutral, in the quiet switch position, the other noisy swith position cap coupled the chassis to line.

You may have a faulty (shorted) stinger cap, or just a sensitive VOM. A proper stinger cap should limit the current to below harmful levels.

Have the 2 wire line cord replaced with a proper 3 wire line cord and ground the chassis to safety ground.

JR
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David Buckley

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Re: Old guitar Amp power polarity and live chassis
« Reply #2 on: June 20, 2016, 04:23:10 pm »

The way these old danger boxes work is that the switch connects a capacitor to the chassis to one or other of the pins of the two prong mains socket.  So either, the chassis is connected to neutral through a cap, or connected to hot through a cap.  Get it wrong, one can get a decent size tingle from the chassis (or the guitar!) to something earthed, like the PA company's SM58.

The safest thing is for a guitar tech t replace the old two pronger with a correctly connected three prong plug and cable, with a proper chassis ground, and for him to disconnect the now-redundant capacitor.

The other thing of note is that this arrangement is old, and thus these capacitors are not Y rated capacitors, which a cap between a mains wire and "ground" should be.  Y rated caps are designed to fail safe.  Old caps of uncertain construction and parentage, that's crap shoot territory.  The cap could fail open, or could fail closed.... with possible deadly consequences.

EDIT: Goddam, beaten by JR.
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Frank DeWitt

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Re: Old guitar Amp power polarity and live chassis
« Reply #3 on: June 20, 2016, 06:53:18 pm »

Any thoughts on what may be the problem?  How would you have handled this situation?

Probably won't help in your situation but I work with the same group all the time and there is a hard and fast rule.  No new equipment is introduced on "game day" In our case, Sunday morning Church.  You have a new amp, or peddle board.  Great, congratulations, I look forward to seeing it at our next practice.  For now, it goes back in the car.  BTW This applies to the stuff in the booth as well.  Saves a lot of running around at high speed.
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Bob Leonard

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Re: Old guitar Amp power polarity and live chassis
« Reply #4 on: June 20, 2016, 09:53:51 pm »

The way these old danger boxes work is that the switch connects a capacitor to the chassis to one or other of the pins of the two prong mains socket.  So either, the chassis is connected to neutral through a cap, or connected to hot through a cap.  Get it wrong, one can get a decent size tingle from the chassis (or the guitar!) to something earthed, like the PA company's SM58.

The safest thing is for a guitar tech t replace the old two pronger with a correctly connected three prong plug and cable, with a proper chassis ground, and for him to disconnect the now-redundant capacitor.

The other thing of note is that this arrangement is old, and thus these capacitors are not Y rated capacitors, which a cap between a mains wire and "ground" should be.  Y rated caps are designed to fail safe.  Old caps of uncertain construction and parentage, that's crap shoot territory.  The cap could fail open, or could fail closed.... with possible deadly consequences.

EDIT: Goddam, beaten by JR.

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. ::)





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Tom Bourke

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Re: Old guitar Amp power polarity and live chassis
« Reply #5 on: June 20, 2016, 11:58:16 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 think you have a typo.  Ground to chassis, hot to switch, neutral to other transformer lead is how I know it.
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John Roberts {JR}

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Re: Old guitar Amp power polarity and live chassis
« Reply #6 on: June 21, 2016, 12:04:45 am »

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. ::)
Neither hot nor neutral should be connected to chassis, both are insulated from ground per UL.

Bob are you drunk posting again?

JR
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Steve M Smith

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Re: Old guitar Amp power polarity and live chassis
« Reply #7 on: June 21, 2016, 02:40:34 am »

I think you have a typo.  Ground to chassis, hot to switch, neutral to other transformer lead is how I know it.
That's what I was thinking.

Bob are you drunk posting again?
We know that he knows what he's doing.  As long as he's not drunk soldering, it should be ok!


Steve.
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Jonathan Johnson

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Re: Old guitar Amp power polarity and live chassis
« Reply #8 on: June 21, 2016, 02:05:32 pm »

I think you have a typo.  Ground to chassis, hot to switch, neutral to other transformer lead is how I know it.

Wait. What? Bob made a mistake? Say it ain't so!  :o

(Bob, that's an attempt at humor. I don't mean it as a sarcastic criticism. I may not agree with your style sometimes, but I've never known you to put out bad information. You have my respect.)
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Peter Kowalczyk

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Re: Old guitar Amp power polarity and live chassis
« Reply #9 on: June 21, 2016, 02:33:26 pm »

Thanks All - the concept of a 'stinger cap' is new to me; thanks for the lesson.

Googling 'Stinger Cap', I found Bob's 'Death Cap' (isn't that also the name for a type of toxic mushroom?) thread from 2014 that shows a Fender Deluxe with an 0.047uF cap switchable between line and neutral.  Looking at this arrangement, It seems to me that since the chassis is floating, if the cap is in the line-to-ground position, it will ALWAYS couple line voltage onto the chassis, albeit current-limited*, even if the cap is not faulty.  If the cap became shorted, compromising it's current-limiting effect, then you'd have a possibly deadly situation.  Does this sound correct?

* JR - in that thread, you noted that 0.047 uF would limit current at 120 VAC to 2 mA.  Is this your calculation?

Using i = C * dv/dt,
let dV = peak-to-peak voltage of AC mains waveform, 120V * sqrt(2) * 2 = 340V
let dT = a half period of that waveform, 1 / (2 * 60Hz) = 8.333 ms

(0.047 e-6 uF) * (340 V / .00833 s) = 0.00192 A

... if that's the case, wouldn't this 2 mA be an instantaneous peak, while the max RMS current would be more like 1.4 mA? 

Frank - love the idea of 'no new gear on game day', but (as you anticipated) working as a house engineer with different musos on every gig, its a new backline every night!

This particular amp and musician is on down the road, but I'll see if I can contact him to forward a synopsis of your advice.  He didn't seem too concerned that his amp was at 115V; kinda shrugged it off.  That attitude scares me more than the fault itself.

...And there are far worse things to do drunk than solder; a cocktail is just the thing to make cable 'knitting' more enjoyable  ;-)

Cheers!

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