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Title: Old guitar Amp power polarity and live chassis
Post by: Peter Kowalczyk 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?
Title: Re: Old guitar Amp power polarity and live chassis
Post by: John Roberts {JR} 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
Title: Re: Old guitar Amp power polarity and live chassis
Post by: David Buckley 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Frank DeWitt 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Bob Leonard 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. ::)





Title: Re: Old guitar Amp power polarity and live chassis
Post by: Tom Bourke 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: John Roberts {JR} 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
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Steve M Smith 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Jonathan Johnson 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.)
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Peter Kowalczyk 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!

Title: Re: Old guitar Amp power polarity and live chassis
Post by: John Roberts {JR} 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.
Quote
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
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Jonathan Johnson 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.)
Title: Re: Old guitar Amp power polarity and live chassis
Post by: John Roberts {JR} 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
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Mike Sokol 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: John Roberts {JR} 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
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Stephen Kirby 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: John Roberts {JR} 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
Quote
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
Quote
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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Mike Sokol 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Stephen Kirby 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Mike Sokol 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Kevin Graf 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Mike Sokol 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: John Roberts {JR} 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Jonathan Johnson 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:

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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Steve M Smith 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Mike Sokol 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)
Title: Re: Old guitar Amp Stinger Caps and live chassis
Post by: Peter Kowalczyk on June 24, 2016, 12:54:57 pm
Does anyone make and deploy Quad Boxes with GFCI receptacles for backline?  Seems simple enough...

P
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Stephen Swaffer 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Mike Sokol 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. 
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Mike Sokol 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?
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Daniel Levi 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: John Roberts {JR} 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
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Mike Sokol 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. 
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Mike Sokol 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Mike Sokol 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: John Roberts {JR} 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
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Stephen Swaffer 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?
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Stephen Kirby 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Tom Bourke 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: John Roberts {JR} 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

Title: Re: Old guitar Amp power polarity and live chassis
Post by: Stephen Kirby 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Jeff Bankston on June 26, 2016, 02:42:26 am
I watched these 2 videos last night on death caps.

http://www.youtube.com/watch?v=MMTN3B-zClo

http://www.youtube.com/watch?v=xS_5K5YEYv8
Title: Re: Old guitar Amp power polarity and live chassis
Post by: John Roberts {JR} 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
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Jeff Bankston 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Bob Leonard 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: John Roberts {JR} 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
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Mike Sokol 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Jonathan Johnson 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Mike Sokol 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Mike Sokol 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.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Kevin Maxwell on July 02, 2016, 10:06:46 am
I didnít read all of the posts in this thread so this may have already been covered. I did an American Cancer Society Relay for Life once where the guitar player in one of the bands had an interesting problem with his amp. He had it modified to have a 3 prong grounded cord. But he still had the switch wired to flip the hot and neutral. The only power we had had a GFCI breaker on it and it took us a little bit to realize that he was the one that popped the main circuit feeding everything with his amp. I think he did it twice as we were setting the mics for them. I donít know if this amp had the cap that is being mentioned or if this was just the weird way it was wired.     
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Mike Sokol on July 02, 2016, 02:17:15 pm
I didnít read all of the posts in this thread so this may have already been covered. I did an American Cancer Society Relay for Life once where the guitar player in one of the bands had an interesting problem with his amp. He had it modified to have a 3 prong grounded cord. But he still had the switch wired to flip the hot and neutral. The only power we had had a GFCI breaker on it and it took us a little bit to realize that he was the one that popped the main circuit feeding everything with his amp. I think he did it twice as we were setting the mics for them. I donít know if this amp had the cap that is being mentioned or if this was just the weird way it was wired.   

Most likely he still had the stinger cap installed, so flipping the switch one way would be fine since the cap was connected between neutral and ground, while flipping it the other way would place the cap between hot and ground. Those old stinger caps tend to get leaky, so if it exceeded 6 mA of current it would certainly trip any GFCI upstream of it. He just needs to cut the stinger cap out of the circuit and it should work fine.

BTW: This is why I think that there should be a separate GFCI for each backline amp. If one stage amp looses power, then it's not that big a deal. However, taking down the entire PA system due to one GFCI far upstream tripping is what gives GFCI's a bad name.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Jeff Bankston on July 03, 2016, 03:33:40 am
I will be picking up a 1972 Fender Twin Reverb in a few weeks that I'm buying from a friend. It has a 3 wire cord someone installed but he doesnt know anything about the cap so I will be removing it if it's still in place. I'll also replace the PS caps if they are old. I'm also going to remove the master volume circut. Now I will be able to do pig snorts again through a mic while taping on the reverb tank like use to do on the other geetar players Fender Twin Reverbs amps.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Bob Leonard on July 06, 2016, 08:10:29 am
Questions of the day.

Q: What parts of what circuit(s) will be replaced, and what components added, after you remove the master volume control and it's associated components.

A: Trade secret, find out yourself.

Q: When removing the death cap do you, A) bypass the switch and leave it in the chassis to fill the hole, or B) Just remove the death cap?

Answer - A. Bypass the switch and leave it in the chassis to fill the hole and give the appearance the amp is still unmodified.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Geoff Doane on July 06, 2016, 10:43:30 am

Answer - A. Bypass the switch and leave it in the chassis to fill the hole and give the appearance the amp is still unmodified.

It now becomes a placebo, and the guitar player can place it in whichever position he feels gives him better tone (or less noise, or more gain, or....).  :)

GTD
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Mike Sokol on July 06, 2016, 02:03:14 pm
It now becomes a placebo, and the guitar player can place it in whichever position he feels gives him better tone (or less noise, or more gain, or....).  :)

GTD

I like it. One of my studio engineering buddies used to place a 1/3 octave equalizer back at the producer's desk so that the "producer" could tweak the EQ. But the equalizer wasn't actually patched into the mixing path. This made the producer feel like he had ears and was providing real input, and let the engineer do his job without the producer yelling at him for more jellybeans or whatever toher wacky nonsense words that producers like to use.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Stephen Kirby on July 06, 2016, 09:00:46 pm
When I rebuilt my DR I took the switch out and snapped a feed through grommet in the hole to plug it.  Although any guitar player worth their salt can reach around the top of a Fender amp and manage the power and standby switches without looking, I didn't see any need for an extra switch back there.  Because I'd replaced the bad OT with an Allen part, any pretense of originality was gone anyway.

As far as cutting out the master volume, it's easy enough to get an AB763 schematic and follow it.  What may be more involved is fixing the wire dress so that the amp is stable.  My SF to BF conversion went well and tweaking the dress from some on-line pics came out fine.  I've heard that some amps can be real bears though.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Jeff Bankston on July 07, 2016, 05:00:45 am
When I rebuilt my DR I took the switch out and snapped a feed through grommet in the hole to plug it.  Although any guitar player worth their salt can reach around the top of a Fender amp and manage the power and standby switches without looking, I didn't see any need for an extra switch back there.  Because I'd replaced the bad OT with an Allen part, any pretense of originality was gone anyway.

As far as cutting out the master volume, it's easy enough to get an AB763 schematic and follow it.  What may be more involved is fixing the wire dress so that the amp is stable.  My SF to BF conversion went well and tweaking the dress from some on-line pics came out fine.  I've heard that some amps can be real bears though.
The only schematic I could find with the MV removed mod is this one. Is this the same circuit as the 1972 Fender twin reverb ? I plan to replace the MV pot with a rotoray switch and use it for the standby switch. It will have the V knob on it. The faceplate is well worn as is the grill cloth.

                   
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Bob Leonard on July 07, 2016, 06:37:16 pm
Jeff,
I'll suggest that you find a silver face layout and schematic that doesn't include the master volume circuit. Follow the schematic/layout closely and you'll find any number of components that will have to be changed. The fun will start when you find that after CBS components were substituted on a whim depending on what was in the bin that day.

The change you show above is typical and not actual. You will need to treat the amp as a one of a kind working from the power supply to the speaker. And if the amp is a high output version the transformers will need to be changed. I've done more than 50 and maybe as many as 100 MV conversions, and every one was a new experience. Go forth and be careful my son.

Steve,
Reverting back to AB763 is more than just a few components, but I'll bet you knew that. The PI, BIAS balance (if there is one), pre amp tube negative feedback voltages and power supply all have to be changed, removed or rebuilt.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Stephen Kirby on July 07, 2016, 09:10:23 pm
Steve,
Reverting back to AB763 is more than just a few components, but I'll bet you knew that. The PI, BIAS balance (if there is one), pre amp tube negative feedback voltages and power supply all have to be changed, removed or rebuilt.
Yeah, I pretty much gutted the amp and rebuilt the main portion from the ground up.  Spent some time on TGP with Aiken and Allen and added several tweaks like selected carbon film in the signal path with metal film elsewhere, orange drops, poly and silver mica caps.  Was a labor of love but I'm not really interested in doing it again.  Lot of work.  Voltages are slightly different because of the Allen/Vibrolux PT in it as well.  Because of the better OT it's about 28W at clip but the new PT is wound for modern 120 input instead of 110 so B+ is where it originally should be and the amp is slightly softer than an unmolested or perfectly restored one.  It's a player, not a museum piece.  I only have 3 amps.  The Fuchs ODS, this, and a MiniBrute.  Pretty much covers any flavors that I like since I don't really play rock any more and have any need for authentic Marshall/Hiwatt/Orange types of sounds.  Stomp boxes work fine for the times I need it.  There are cool things out there like a JMI AC30 or Watkins Dominator that get noises you can't get any other way but if you can't work with a good black face Fender, more practice is in order.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Bob Leonard on July 07, 2016, 10:03:10 pm
Just for you Steve.

http://www.ebay.com/sch/oldgibsonguy/m.html?item=232004846513&ssPageName=STRK%3AMESE%3AIT&rt=nc&_trksid=p2047675.l2562
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Jeff Bankston on July 11, 2016, 05:21:41 am
..........
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Stephen Swaffer on July 13, 2016, 11:20:25 pm
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.

Answering another post reminded me I wanted to comment on this.  The QUBE (while apparently a very useful device) does not "ground" anything.  It provides a way to "bond" amplifiers/gear/whatever to the building grounding system. 

On the surface a pendantic observation-but the difference between grounding and bonding is key to understanding what needs to happen for safety sake.  Perhaps a topic for Code Study, if and when we get that off the ground?  (Sorry-that was intentional :)-caught it after I typed it but gonna leave it!)
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Mike Sokol on July 14, 2016, 06:24:11 am
On the surface a pendantic observation-but the difference between grounding and bonding is key to understanding what needs to happen for safety sake.

Yes, I'm well aware of the difference. And you'll note that I normally talk about bonding to the EGC (Equipment Grounding Conductor) rather than "the ground". It can get confusing since the word "ground" is used for a lot of different things. However, in the context of guitar amps the common vernacular is to "ground" the amp, or "ground" the musician.
Title: Re: Old guitar Amp power polarity and live chassis
Post by: Jeff Bankston on August 02, 2016, 11:47:38 pm
I did a serial number search on my Fender Twin Reverb and its a 1972. Serial number is A44759. Now to find a non master schematic. I took the death cap out. It has a 3 wire cord with the green soldered to the chassis and new Sprauge PS caps. The grill is not the original doesnt fit very well. 4 screws hold it on.