# How best to implement three prong cord



## JHarasym (Mar 27, 2007)

I have a 60's Gibson Minutemean GA-20RVT amp. The schematic can be seen here:
http://www.prowessamplifiers.com/schematics/Gibson/Ga_20rvt.html
The neutral wire is connected to the fuse, then to the switch. The switch has two positions, one of which appears to connect to a grounded cap (the death cap?). I understand it's the hot wire that should be fused. Can anyone tell me how best to rewire this to connect a three prong cord?


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## GuitarsCanada (Dec 30, 2005)

Have you read through this one yet? Might give you an idea of whats invloved

http://www.guitarscanada.com/amp-tech-section/32309-vintage-amps-two-prong-plugs-conversions.html


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## JHarasym (Mar 27, 2007)

Yeah, but I was hoping someone with more experience would look over the schematic and weigh in. My reading is: if the cap connected to S1B is removed, then S1B is rendered superfluous. The hot wire from the new cord would then be connected to the fuse, and the neutral wire would connect to the transformer (which amounts to reversing the original connections), and the ground is connected to the chassis. 

When reversing the hot and neutral leads do I need to consider the lamp?


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## greco (Jul 15, 2007)

Hi John:

I had a look at the circuit and I don't understand why there are 2 rotary switches. 
However, my schematic reading skills are much too limited to offer any comments of value.
Hopefully one of the amp techs will see this and help you.
Also, I'm not sure about the answer to your question re: the pilot light, but I think (am guessing) it would still work.

Looking forward to following this thread.

All the best

Cheers

Dave


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## WCGill (Mar 27, 2009)

Yes you have it right. Best way is to switch only the hot. You can leave the cap hooked up to the neutral if you want. You can also use S1B to switch the neutral as well as the hot but you have to make sure that it is disconnected from S1A so that polarity won't flip as it's currently wired. Easiest way is just switch the hot, disconnect the cap, not using S1B.


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## greco (Jul 15, 2007)

WCGill said:


> Yes you have it right. Best way is to switch only the hot. You can leave the cap hooked up to the neutral if you want. You can also use S1B to switch the neutral as well as the hot but you have to make sure that it is disconnected from S1A so that polarity won't flip as it's currently wired.* Easiest way is just switch the hot, disconnect the cap, not using S1B*.


And will the neon (I think) pilot light still work ?
I assume that this is to be assumed from your post...I just hate assuming anything.

Cheers

Dave


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## WCGill (Mar 27, 2009)

Yes it should Dave. AC has no polarity so there shouldn't be any difference which goes where. Sorry, forgot to address that in original post.


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## JHarasym (Mar 27, 2007)

greco said:


> I had a look at the circuit and I don't understand why there are 2 rotary switches.


S1A and S1B are integrated in a single rotary switch body (so, double pole/double throw?). The switch has three positions, one Off and two On. From the schematic it appears the difference in the On positions is reversed polarity. In both On positions the "death cap" is connected. I'm unclear about the dangers of leaving the cap as is - it seems most posts recommend taking it out.


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## WCGill (Mar 27, 2009)

The danger the cap poses is if in the reversed polarity position, ie, hot to the cap, that the cap shorts and you have the chassis electrified. The switch must be rewired to prevent this so you might as well take the cap out at the same time.


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## greco (Jul 15, 2007)

WCGill said:


> .......that the *cap shorts* and you have the chassis electrified.


You have the chassis electrified and you could *cook your shorts*
(good way to remember this...for folks with a warped sense of humour and reasoning... like mine).

On a more academic perspective now...Why was this specific circuit (with the double rotary switch) designed this way?

Cheers

Dave


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## WCGill (Mar 27, 2009)

Probably to use just one switch for on/off/ground reverse, and I'm sure Gibson wanted to differentiate itself from Fender too. I've also seen DPDT toggles also in this application.


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## JHarasym (Mar 27, 2007)

My initial analysis of the switch looks wrong - polarity is the same in both switch positions. The only effect is to select where the grounding cap connects, hot or neutral. So am I right to assume once the cap is removed I can leave the switch as-is (ie. not worry about removing the wiring connected to S1B)?


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## jb welder (Sep 14, 2010)

You are correct, disconnect the cap and you are good.


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## JHarasym (Mar 27, 2007)

Done and done - thanks guys!


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## jb welder (Sep 14, 2010)

Not meaning to confuse things but I will add a bit about polarity. The polarity switch only ever switches which side of the line the polarity cap connects to. It would be pointless to switch the connections to the transformer as AC has no polarity. We have "hot" and "neutral" sides only because one side of the AC in your house electrical panel is connected to a ground rod outside your house. The side connected to the ground rod is then called "neutral". So in a sense, the AC is not polarized but your electrical wiring is.
Now we have an AC source which is referenced to ground. For safety, we want the fuse and switch on the hot side, and we want to avoid a "death cap" situation where a cap is connected from "hot" to chassis and failure of that cap could allow voltage from the hot to get to the chassis.
As WCGill mentioned, it's ok to have that noise suppression cap connected from neutral to chassis. But there is still potential for danger if you get somewhere that has the AC outlets miswired or ungrounded.
There are now special caps available for line to ground applications, called class Y capacitors. These can't short or leak from hot to chassis if they fail. So there are now modern amps being built which have polarity switches and what appear to be "death caps", but they are special caps designed to be safe in these applications.


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## JHarasym (Mar 27, 2007)

jb welder said:


> one side of the AC in your house electrical panel is connected to a ground rod outside your house. The side connected to the ground rod is then called "neutral".


Interesting - where does the bare "ground" conductor in the house wiring terminate?



jb welder said:


> There are now special caps available for line to ground applications, called class Y capacitors. These can't short or leak from hot to chassis if they fail. So there are now modern amps being built which have polarity switches and what appear to be "death caps", but they are special caps designed to be safe in these applications.


How does one identify a class Y cap? I'd hate to remove one inadvertently. Would it make sense to replace the death cap with a class Y cap?


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## greco (Jul 15, 2007)

JHarasym said:


> Interesting - where does the bare "ground" conductor in the house wiring terminate?


It is often a large gauge copper wire clamped to the cold water supply pipe where it comes into the house. 
Look near your water meter.
I'm not sure if this is the same/consistent in newer homes, but I suspect that it is.
Others will correct me if it varies.

Cheers

Dave


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## jb welder (Sep 14, 2010)

At our house the big ground wire from the electrical panel runs outside to a long ground rod pounded into the ground. As greco mentioned, sometimes it is run to the cold water pipe. Not sure what the current codes require, a lot of plumbing uses plastic pipe now.

As far as the class Y caps, yes you can use them in place of the "death cap", usually if needed for noise suppression or hum issues. If your amp has more hum after removing the death cap then it may be a solution. For identification, the schematics sometimes (but not always) have a warning to only replace with same type and safety rating. The caps themselves have markings to show the different approvals that are required. A few pics and more info on this page:
http://www.justradios.com/safetytips.html


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## JHarasym (Mar 27, 2007)

jb welder said:


> one side of the AC in your house electrical panel is connected to a ground rod outside your house. The side connected to the ground rod is then called "neutral".





jb welder said:


> At our house the big ground wire from the electrical panel runs outside to a long ground rod pounded into the ground.


So both the neutral and ground conductors in the house AC circuit are connected to a rod in the earth, which essentially means to each other? What's the point of having the third conductor then (sorry, I'm confused by this, as I am about most things electrical).


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## greco (Jul 15, 2007)

JHarasym said:


> .....sorry, I'm confused by this, as I am about most things electrical.


There is a club for this. The members number in the thousands and it is free to join. 
I am a long-standing member and have a brief application form that I could send to you.




JHarasym said:


> So both the neutral and ground conductors in the house AC circuit are connected to a rod in the earth, which essentially means to each other? What's the point of having the third conductor then


 I looked up the information below and I now understand this to a much greater extent after reading /looking at the following:

*Electrical Ground*

When the subject of electricity comes up, you will often hear about electrical grounding, or just ground. For example, an electrical generator will say, "Be sure to attach to an earth ground before using," or an appliance might warn, "Do not use without an appropriate ground."
It turns out that the power company uses the Earth as one of the wires in the power system. The planet is a good conductor, and it's huge, so it makes a handy return path for electrons. "Ground" in the power-distribution grid is literally the ground that's all around you when you are walking outside. It is the dirt, rocks, groundwater and so on.
If you look at a utility pole, you'll probably be able to spot a bare wire coming down the side of the pole. This connects the aerial ground wire directly to ground. Every utility pole on the planet has a bare wire like this. If you ever watch the power company install a new pole, you will see that the end of that bare wire is stapled in a coil to the base of the pole. That coil is in direct contact with the earth once the pole is installed, and is buried 6 to 10 feet (2 to 3 meters) underground. If you examine a pole carefully, you will see that the ground wire running between poles are attached to this direct connection to ground.
Similarly, near the power meter in your house or apartment there is a 6-foot (2-meter) long copper rod driven into the ground. The ground plugs and all the neutral plugs of every outlet in your house connect to this rod. 









There are two basic types of grounds in any electrical or electronic system - *earth ground* and* signal*
ground. 
The first thing most people learn about electricity is that current won’t flow unless it can travel in a
closed loop. This simple fact is often overlooked when a system is developed (i.e. drawn) using the ground symbol.
By a stroke of the pen one avoids having to draw the return paths of most of the current loops in the
system. The “ground” turns into an apparently infinite current sink, so that any current that flows into it
is gone and forgotten. Forgotten it may be, but it’s not gone. *It must return to its source to create the*
*closed loop required by any current flow.*
The use of the ground symbol also causes many developers to “abuse” the chassis ground by using it as
the primary system ground. This can cause vast amounts of electrical noise to be injected into the control
electronics, resulting in systems that fail constantly after they are installed in the field.


Chassis and Earth Grounds
The chassis or “earth” ground is used as a protection against electrical shock. The earth is not a
“zero-resistance” surface, so earth ground potential can vary. However, circuits are almost always
connected to earth ground for prevention of shock hazards.
In a standard 3-wire single-phase AC power system, *the white wire is earth-grounded at the service*
*entrance.* If a load circuit has a metal enclosure or chassis, and if the black wire develops a short to the
enclosure, there will be a shock hazard to operating personnel if the enclosure is not also earth-grounded. If
the enclosure is earth-grounded, then a short results in a blown fuse instead of creating a “hot” enclosure.
The earth-ground wire should not carry any current (except in the case of a fault), so it should be at
earth-ground potential along its entire length. *The “neutral” white wire, on the other hand, may be*
*several volts off of earth-ground, depending upon the gauge and length of the wire as well as the level of*
*current flowing through it*


Cheers

Dave


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## JHarasym (Mar 27, 2007)

Hahaha - thanks Dave, sign me up!

This will require some reading methinks.


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## JHarasym (Mar 27, 2007)

OK, let me take a stab at this:
In normal operation the device's (eg. amplifier) current flows in on the hot conductor and out on the neutral conductor, and then to ground outside the house. The third conductor is connected to the amplifier chassis in case of a fault which inadvertently shorts the hot lead to the chassis, so the current can flow to ground through the wire, rather than through the human. There should also be ground connections for components directly to the neutral wire (ie. not through the chassis), to minimize noise.

Can someone explain why an amp's fuse would blow?


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## jb welder (Sep 14, 2010)

JHarasym said:


> Can someone explain why an amp's fuse would blow?


 I think you are referring to this statement "If
the enclosure is earth-grounded, then a short results in a blown fuse instead of creating a “hot” enclosure". Perhaps they should have said "the units fuse or the outlet's circuit breaker will blow". A short results in high current. Excessive high current causes fuses to blow. So depending on where in the amp the short occurs, it's fuse may blow. If, for example, the AC cord hot lead in the chassis (before the fuse) comes loose and touches the chassis, the circuit breaker in the electrical panel will blow. If the fault in the amp is after the fuse, then the amps fuse will probably blow before the breaker pops. 

As far as your other question: "So both the neutral and ground conductors in the house AC circuit are connected to a rod in the earth, which essentially means to each other? What's the point of having the third conductor then?" It's basically an extra fail safe precaution. Polarized 2 prong plugs with a wider blade for the neutral were only a partial solution because outlets could still be miswired fairly easily. They decided a third wire would be safer and would not normally carry current, unlike the neutral. There is a good explanation here: http://amasci.com/amateur/whygnd.html


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## JHarasym (Mar 27, 2007)

Wow, I feel smarter now - I actually think I understand this. Thanks!


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## jb welder (Sep 14, 2010)

Glad it helped. Sometimes an explanation of the "why" makes things easier to understand. Did for me anyway.


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