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Three prong vs two.

Neutral is the return path for hot. The ground blade on a three prong plug usually connects to the chassis and is a safety ground. Many source components and preamps have IEC inlets or three prong cords, but the ground isn't bonded to the chassis. Sometimes it is and the signal ground isn't connected directly to the chassis. Some have the chassis and signal bonded to the ground wire.

My point is this, check the service manual and see how the AC was originally wired. Even if none of the components in a system have the third wire, it still may be perfectly safe. The ever increasing "upgrade" power cord market has led to more and more components using IEC inlets. This doesn't mean the ground connection is even used, but multiple pieces with three wire can lead to ground loops.

Here's an example of how strange and confusing it can be:
A neighbor had a B&K EX-442 amp and a B&K Pro-10MC preamp. Both of these the later Sonata series. Both have IEC inlets on the amp and preamp's external supply. In theory, using a three prong cord for both should result in a ground loop when they are connected by patch cables, but this was not the case. In reality, only the power amp chassis and external supply chassis were grounded, while the preamp chassis wasn't. In order to keep all of the chassis at the same ground potential, we ran a wire from the preamp chassis to a ground post on the power conditioner.

Knowing the grounding scheme for both signal and power inside any unit is important when adding a three wire to something originally supplied with two wire. Best not to try to redesign things unless you know what you're doing. For a six to nine foot power cord, 18ga is more than adequate with any piece drawing 1000VA or less at maximum.
 
WaynerN said:
Other hazards to avoid is replace blown fuses with a different type or higher size then specified, operating equipment with frayed cords, operating equipment where it is exposed to the elements or sources of heat or water. Many older garages do not have GFCI, so if you are running a stereo out there on concrete and your stereo has a metal cabinet and faceplate, you might want to install a GFCI outlet. The GFCI is really the true hero here.
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GFCI are secondary protection. I wouldn't personally stake my life on one always operating correctly. There are also plenty of other vectors not considered above. Bonded cable TV/Sat coax, another metal chassis device that has PE, etc. Grabbing any one of those with one hand and your metal-chassis non-bonded faulty device with the other would be far more effective at stopping your heart than being barefoot on damp concrete.
 
Ok, lets go to equal sized 2 prong plugs: the machine was DC in nature and any incidental AC forced neutral to the frame which matched the DC ground. IT was not double insulated. As a result, some current is present on metal surfaces. (mutants like me can sense it if my old pioneer is plugged in 'backwards') Internal resistance - accidental or on purpose - kept available shock current to a minimum (does not apply to older tube equipment)

This applied to appliances, stereos, tvs etc

Then later came polarized plugs. It forced the neutral to any frame AND in the case of connected electrical components, it kept any ground reference voltage (of which there will always be some) the same. reduced, hum, noise, danger, cholesterol etc

In the case of 3 prong, the round pole is always the ground and assuming the house is wired in a way where it does not burst into flames, it forces polarization. BUT, because wires have resistance, there oft ends up a voltage difference (and AC hum) between neutral and ground, one that wont trip a GFI, but your AF portion of the tuner will gladly amplify.

I am not sure that 3 round poles refer to or what kind of setup this was, but I would not replace power cords with different numbered, or purposed conductors unless you REALLY understand the subject material. I dont think a safety problem will arise (unless you seriously screw up wiring the conductors from hot to ground etc) but you can and will add hummmmmmmmm
 
biscuithead said:
Zip cord speaker wire : a wire only has two ends, it won't complain about about how it is used;).

Two wire cord: just make sure the hot and neutral go where they should go, if it matters. The wire gauge needs to be the appropriate size for the FLA (full load amperage) .

It is true that adding a three prong cord to a piece that didn't originally have a three prong cord, then bonding the ground to chassis, could cause a hum. Depends on the piece.
My personal opinion: stick with OEM. If a cord needs to be replaced because of damage, do a like for like replacement.

I have a drawer full of old 14awg Electric skillet power cords that I've picked up at thrifts over the years. I love the insulation material ( don't know what it is ), it is very pliable. I use these usually on old tube gear that the cord is damaged. Works great.
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I've read this whole thread and the differing opinions.
The wrong opinions, or trusting the wrong opinion in this instance could result in some really bad stuff. This is why we have licensed experts in the world. Trust nothing in this thread and take your equipment to a professional and pay them to make it right. Schematics are your friend, the UL is your friend , IEC and NEC and the NFPA exist for a reason. If you have to ask about a power cord, then please don't trust the "expertise" of an online audio forum.
Take great care and good luck!
 
JP said:
GFCI are secondary protection. I wouldn't personally stake my life on one always operating correctly. There are also plenty of other vectors not considered above. Bonded cable TV/Sat coax, another metal chassis device that has PE, etc. Grabbing any one of those with one hand and your metal-chassis non-bonded faulty device with the other would be far more effective at stopping your heart than being barefoot on damp concrete.
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You can get electrocuted with a 3-prong grounded plug. Say your standing on some wet cement and your working on a power tool and accidentally slice the hot. You are now toast. If you had a GFCI, it would pop out the mini-breaker on the GFCI, and you would be fine.

One of the reasons why many if not most yard tools are now battery operated or gas powered.
 
This came up on another thread.
The only mains equipment which should have a 2 wire power cord must be double insulated internally. This means that if a fault occurs the equipment is designed such that no connection can possibly occur between a mains conductor (either live or neutral - hot or cold for the US) and any conductive enclosure.
All other devices that are mains fed and have a conductive enclosure must have an earth connection bonded to the case using a dedicated fixing (mounting bolts are not allowed to be used).

With regard to our hobby, when restoring old equipment that was originally fitted with a 2 core mains lead, the following points should be considered:-
1. Is the equipment really double insulated? If not, it should be earthed.
2. It is a good idea to replace single insulated (zip wire) with a double insulated power chord as it has a second insulating layer over the conductors.
3. If you earth a piece of equipment for safety and you get a hum loop, there are ways to get out of this by isolating signal grounds from mains earth.
4. Never take a chance on cable types. Always use mains voltage specified cable.
5. Any mains connection within an enclosure must be shrouded to prevent inadvertent contact by humans.
6. If you are not sure, don't do it- get advice. Never take a chance on electrical safety.
7. Electrical standards have changes over the years, with good reason.
8. It is worth having a restored unit tested for electrical safety (Portable Appliance Test). This takes a couple of minutes with a PAT test set.
 
correct me if I am wrong, but it would appear to my reading that some are claiming that electrical 'objects' which contain a metal housing/case/what have you are not bonded to ground. This is only true IF AND ONLY IF the thing that uses electricity inside is utterly isolated from the case, such that a foreseeable fault with itself cannot present voltage to the housing/case. AND it has a 2 prong polarized plug. if it has a 3 prong plug, the earth conductor *IS MOST CERTAINLY* conected to the box/case/housing. OR has no one ever installed an appliance?

Take the case of power drills with 2 prongs. I have 5-6 of them around, mostly being dewalts etc. They have double insulated cords AND the motor and switch are electrically isolated via the use of plastic, wood, feathers, whatever from any exposed metal parts.

Your stereos, which the OP is about, from days of old MOST CERTAINLY DO have a direct path from one conductor of the power cord to exposed metal silver faces or chassis inside. You should have verified this the day you got your first unit and DMM and made threatening moves towards it with a screwdriver.

As stated before in this thread by me, and in hundreds of threads by others, if you flip the non-polarized plug, you will go, when measuring the chassis, from 3-6ish volts to about half the mains voltage. various components, of the same era, when interconnected via RCA jacks or DIN plugs or whatever can and will have a mix-match of these states. Its not that much of a safety factor, unless you decide to take your stereo bathing, but it is often a noise issue.
 
quaddriver said:
correct me if I am wrong, but it would appear to my reading that some are claiming that electrical 'objects' which contain a metal housing/case/what have you are not bonded to ground. This is only true IF AND ONLY IF the thing that uses electricity inside is utterly isolated from the case, such that a foreseeable fault with itself cannot present voltage to the housing/case. AND it has a 2 prong polarized plug. if it has a 3 prong plug, the earth conductor *IS MOST CERTAINLY* conected to the box/case/housing. OR has no one ever installed an appliance?

Take the case of power drills with 2 prongs. I have 5-6 of them around, mostly being dewalts etc. They have double insulated cords AND the motor and switch are electrically isolated via the use of plastic, wood, feathers, whatever from any exposed metal parts.

Your stereos, which the OP is about, from days of old MOST CERTAINLY DO have a direct path from one conductor of the power cord to exposed metal silver faces or chassis inside. You should have verified this the day you got your first unit and DMM and made threatening moves towards it with a screwdriver.

As stated before in this thread by me, and in hundreds of threads by others, if you flip the non-polarized plug, you will go, when measuring the chassis, from 3-6ish volts to about half the mains voltage. various components, of the same era, when interconnected via RCA jacks or DIN plugs or whatever can and will have a mix-match of these states. Its not that much of a safety factor, unless you decide to take your stereo bathing, but it is often a noise issue.
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Put an ohm meter on the neutral of the gear and the metal cabinet. The neutral should not be bonded to the metal cabinet, as it is a current carrying conductor. The cabinet should (if properly designed) be isolated from the primary power. The line (120v) should go directly to a fuse, then on to the power on button.

In Minnesota, There is a saying for ice fisherman that there is no such thing as perfectly safe ice. The same concept can be said for electronics of any kind even the most safely designed products, under unusual circumstances, can become faulty. That is why the GFCI was invented. It measures outgoing and incoming current, and if that number doesn't match, its a trip of its internal breaker.

I have many pieces of vintage equipment in my house. Marantz, Dynaco and even some older Technics. I know they are reasonably safe because I have been inside the units. I do not 100% trust them, as they are aged units. Therefore, they are not ever left on while I am away. They are never worked on when they are plugged in. They are not ever electrically moddified to suit a trend or flavor of the month notion.
 
WaynerN said:
Put an ohm meter on the neutral of the gear and the metal cabinet. The neutral should not be bonded to the metal cabinet, as it is a current carrying conductor. The cabinet should (if properly designed) be isolated from the primary power. The line (120v) should go directly to a fuse, then on to the power on button.

.
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what you say is true but is not contrary to what I have said. the earth bond, ground, would be at the cabinet - if equipped. the topic was stereo equipment, which as most here know, the older vintage silver stuff had 2 conductors and one was attached to the chassis and frame (and in may cases the silver face)

have things changed 1970s vs 2018's yes. but that does not alter what exists.
 
Well. I know a whole lot more than I did yesterday. AND.....I don't know any more than I did yesterday! LOL

Whew! What I DO know is....I will either shorten these big cords (most likely), or find some thinner, more flexible cords with the three prong configuration, and solder/connect them up inside the cases just like the original!

Thanks for all your answers.
 
quaddriver said:
what you say is true but is not contrary to what I have said. the earth bond, ground, would be at the cabinet - if equipped. the topic was stereo equipment, which as most here know, the older vintage silver stuff had 2 conductors and one was attached to the chassis and frame (and in may cases the silver face)

have things changed 1970s vs 2018's yes. but that does not alter what exists.
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I was referring to older 2 prong, non-polarized equipment, as I refereed you (old Dynaco, Marantz, Technics). However, no piece of equipment should have any leg of the power bonded to the chassis. The is reserved for the safety ground (earth ground).

As I said before, the neutral should not be bonded to the chassis in older equipment (Dynaco, Marantz, Technics, etc.), the cabinet should be isolated.
 
Zygmo said:
Well. I know a whole lot more than I did yesterday. AND.....I don't know any more than I did yesterday! LOL

Whew! What I DO know is....I will either shorten these big cords (most likely), or find some thinner, more flexible cords with the three prong configuration, and solder/connect them up inside the cases just like the original!

Thanks for all your answers.
Click to expand...

I would keep the equipment as it was in its original state. If it was designed as a 2 prong piece, I would leave it that way. If it came with a grounded 3-prong cord, I would leave it that way.
 
blhagstrom said:
Where ARE you?

There was a pissing contest on another thread about adding a third wire to ground the vintage system and increase the wire size. Total BS.

IF you have US 110-120v gear on a US 120v power grid. The third wire is BS and you can lose that dumb f**k monster power cords.

70s vintage gear used unpolarized 18ga “zip cord” AC cords except on the monster gear.

Open your gear. Look at the white and black AC connections. That’s where you tap a 2 wire AC cord on. The green is probably tapped with the white. If the green goes to chassis, I’m surprised you don’t have a hum.

I chop off polarized 2 wire replacement all the time and put the unpolarized 2 wire back all the time. The accessory sockets on the back are unpolarized and the wide blade on polarized won’t fit.

The AC line goes into the unit through a strain relief. Hopefully they didn’t f**k up the original hole so you can replace the strain relief in the normal cord. Once inside, one AC line taps directly to the transformer. The other goes through the main fuse, through the power switch to the transformer. Many times all of that is very secure with straps and or held in looms.

That’s it. The rest of the system is secondary winding off the power transistor and that only goes to the bridge rectifier then to the filter caps.

Everything after the filter caps is DC powered. The audio signal is a low voltage AC that rides in top of the DC.

So, the only thing the AC main does is energize the transformer to create the required AC secondary voltage to power the DC system.
There may be some low voltage taps running a light or two.

There is nothing about powering the transformer that requires polarity.

Lose those stupid monster power cords.

All the talk about bigger power cords working better or sounding better is pure BS. All the AC goes through the main fuse. A tiny little thin wire.
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Where am I? In Arkansas, US.

I did not add a third wire. Nor change the size of the wire. I just replaced the original cords, which had three ROUND prongs, for cords of the same size and number of prongs but the right type to use here in the US. I know virtually all stereo equipment does not need huge power cords, nor do speakers need huge cables.
I used cords of the same size so I could use the original strain relief grommet.
 
WaynerN said:
I was referring to older 2 prong, non-polarized equipment, as I refereed you (old Dynaco, Marantz, Technics). However, no piece of equipment should have any leg of the power bonded to the chassis. The is reserved for the safety ground (earth ground).

As I said before, the neutral should not be bonded to the chassis in older equipment (Dynaco, Marantz, Technics, etc.), the cabinet should be isolated.
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open her up! read *any* post when someone is asking for voltages on legs of transistors. where are they in most cases told to put the black probe..... ;-)
 
Zygmo said:
Well. I know a whole lot more than I did yesterday. AND.....I don't know any more than I did yesterday! LOL

Whew! What I DO know is....I will either shorten these big cords (most likely), or find some thinner, more flexible cords with the three prong configuration, and solder/connect them up inside the cases just like the original!

Thanks for all your answers.
Click to expand...
The amount of current your equipment uses is used to properly size your cord. Under 10 amp fuse use 18 gauge wire. 10~13 amp, use 16 gauge wire. 13~15 amp, use 14 gauge. Never use 3 lead if it wasn't originally equipped. Line goes to the switch, neutral to transformer. If on 220~240 volt, you need to understand what you are doing inside. Using a grounded lead can and will lead to ground loops, and run the risk of equipment failure.
 
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