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Author Topic: Old guitar Amp power polarity and live chassis  (Read 14265 times)

John Roberts {JR}

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Re: Old guitar Amp power polarity and live chassis
« Reply #10 on: June 21, 2016, 03:00:13 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? 
I calculated the effective impedance of 0.047uF @ 60Hz (56.4k ohm). At 120 Vrms = 2.1mA

The exact value is academic since low single digit mA is well under human hazard threshold, unless you fall off a ladder.
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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!
Yes musicians tend to take such shocks casually, they shouldn't.

JR
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Jonathan Johnson

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Re: Old guitar Amp power polarity and live chassis
« Reply #11 on: June 21, 2016, 03:05:56 pm »

Yes musicians tend to take such shocks casually, they shouldn't.

Because you cannot predict when a minor shock will turn deadly. (Not "if." When. A shock is an indication of failure, and failures do not go away by themselves reliably. They tend to become worse.)
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John Roberts {JR}

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Re: Old guitar Amp power polarity and live chassis
« Reply #12 on: June 21, 2016, 04:39:29 pm »

Because you cannot predict when a minor shock will turn deadly. (Not "if." When. A shock is an indication of failure, and failures do not go away by themselves reliably. They tend to become worse.)
That is the insidious thing about stinger caps, the low mA shocks are generally harmless (if stinger cap is not shorted), so they teach musicians that all shocks are harmless......

BZZZT today most small shock are evidence of larger problems.

JR
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Mike Sokol

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Re: Old guitar Amp power polarity and live chassis
« Reply #13 on: June 21, 2016, 06:02:33 pm »

Back to the original question of what to do if this shows up on a gig. A GFCI on the backline is the best way to protect musicians from themselves. If there's a short in the stinger cap and the switch is in the LINE position, the GFCI will trip if the musician conducts chassis current to anything grounded such as a microphone.
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John Roberts {JR}

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Re: Old guitar Amp power polarity and live chassis
« Reply #14 on: June 21, 2016, 09:16:33 pm »

Back to the original question of what to do if this shows up on a gig. A GFCI on the backline is the best way to protect musicians from themselves. If there's a short in the stinger cap and the switch is in the LINE position, the GFCI will trip if the musician conducts chassis current to anything grounded such as a microphone.
FWIW from a 0.047uF stinger cap I calculated 2 mA which is well below the 5 mA GFCI threshold.

I still very much support using GFCI for backline.

JR
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Stephen Kirby

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Re: Old guitar Amp power polarity and live chassis
« Reply #15 on: June 22, 2016, 02:18:08 pm »

So that I understand this correctly, even a bad stinger cap will only trip a GFCI if they turn the amp on with the ground switch the wrong way?  Flipping the ground switch and resetting the GFCI will allow the amp to come on and work?

I've wanted to put a GFCI in my backline stringer for outdoor shows, but have been concerned about getting into it with the local blues nazis and their vintage amps.

The music in the park thing I did last week was off of the city buildings that had GFCI Edisons.  Got an outdoor gig on the 10th with a metal trailer stage and it's off of a CS of questionable origins.  Last year it had the right voltage and passed a three light tester after my distro.  This year people I don't know the people who will be playing very well and I don't know how many old school amps will be there.
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John Roberts {JR}

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Re: Old guitar Amp power polarity and live chassis
« Reply #16 on: June 22, 2016, 02:50:56 pm »

So that I understand this correctly, even a bad stinger cap will only trip a GFCI if they turn the amp on with the ground switch the wrong way?  Flipping the ground switch and resetting the GFCI will allow the amp to come on and work?
correctamundo
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I've wanted to put a GFCI in my backline stringer for outdoor shows, but have been concerned about getting into it with the local blues nazis and their vintage amps.
I will repeat the advice to use multiple GFCI drops instead of running the entire backline through one GFCI, since leakage currents could combine to more than 5mA causing nuisance trips. You mostly need GFCI for guitar players who sing.

JR
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The music in the park thing I did last week was off of the city buildings that had GFCI Edisons.  Got an outdoor gig on the 10th with a metal trailer stage and it's off of a CS of questionable origins.  Last year it had the right voltage and passed a three light tester after my distro.  This year people I don't know the people who will be playing very well and I don't know how many old school amps will be there.
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Mike Sokol

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Re: Old guitar Amp power polarity and live chassis
« Reply #17 on: June 22, 2016, 04:33:57 pm »

So that I understand this correctly, even a bad stinger cap will only trip a GFCI if they turn the amp on with the ground switch the wrong way?  Flipping the ground switch and resetting the GFCI will allow the amp to come on and work?

As JR said, that's correct. However, it won't trip immediately. It needs leakage current to sense the leakage current, so you'll have to have the switch in the "wrong" position PLUS touch the guitar and the mic at the same time. I would suggest a one-hand test for this. Touch the tuning keys with your thumb and the mic with your pinky finger at the same time. If you feel a tingle, reverse the switch and test again. Even if you don't do this test, a 2 or 3 mA shock to the lips can be pretty startling, so a one-hand test is best. And no, I don't recommend this test on non-GFCI circuits since it's possible to take a hand-to-foot shock if you're standing on a damp stage. We really should have a separate GFCI for each back-line instrument power drop, especially in festival situations where there's not a lot of testing and fixing time.
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Stephen Kirby

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Re: Old guitar Amp power polarity and live chassis
« Reply #18 on: June 22, 2016, 05:19:50 pm »

As JR said, that's correct. However, it won't trip immediately. It needs leakage current to sense the leakage current, so you'll have to have the switch in the "wrong" position PLUS touch the guitar and the mic at the same time. I would suggest a one-hand test for this. Touch the tuning keys with your thumb and the mic with your pinky finger at the same time. If you feel a tingle, reverse the switch and test again. Even if you don't do this test, a 2 or 3 mA shock to the lips can be pretty startling, so a one-hand test is best. And no, I don't recommend this test on non-GFCI circuits since it's possible to take a hand-to-foot shock if you're standing on a damp stage. We really should have a separate GFCI for each back-line instrument power drop, especially in festival situations where there's not a lot of testing and fixing time.
Thanks gentlemen.

Since I grew up with such amps I'll relate our old "non-contact" way of determining the plug polarity.  Plug a cable into the input of the amp put don't plug it into a guitar, turn the amp up to a moderate volume.  Using a drumstick or something non-conductive turn the amp on and let it warm up.  Again using the drumstick, flip the ground switch back and forth to see which way produces less hum.  That kept us bare footed Hawaiians safe on concrete garage floors.   ;)

Would an NVCT on the amp's chassis suffice?  It would be relatively quick while moving mics around to waive one at any vintage style amps on stage.  It would be faster than metering the chassis to the quad box.

And yes, I've been "kissed" by a microphone more than once in my life.  After the first few times you learn not to cuss since whatever comes out of your mouth goes right into the PA.   ::)

Project for the next weekend.  Put a GFCI in each box on the stringer.
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Mike Sokol

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Re: Old guitar Amp power polarity and live chassis
« Reply #19 on: June 22, 2016, 06:03:28 pm »

Would an NCVT on the amp's chassis suffice?  It would be relatively quick while moving mics around to waive one at any vintage style amps on stage.  It would be faster than metering the chassis to the quad box.

Yes, a NCT should work perfectly to tell you if the "neutral/line" switch is in the "correct" position. I would still plug it into a GFCI protected outlet.
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