Adding 3-prong cord to hifi amp.

[beedoola]

How would I go about adding a three prong cord to this?

The picture attached is not of my unit but of someone else's here on the forum.

You can see the 2 brown wires on the back of the chassis - both are the original power cord wires.

One goes to the volume pot which is also the On/Off switch. From there a white wire goes to the accessory power outlets. There is a .01 cap to ground and a black wire from the Power transformer - they both join with the white wire.

The other poles of the outlets join with the other brown power cord wire, and also meets with another black wire from the power transformer?

how would I go about adding the three prong cord? I already have the new cord's ground tired in to a bolt from the PT.

What about the other two wires?

[Tom Phillips]

Yours is a common question. Check out the discussion at " http://music-electronics-forum.com/t35564/ " which includes the basic information. Your amp doesn't have a "ground" switch but everything else discussed applies.

Another tip is to look inside modern equipment that was shipped with a 3 wire line cord and see first hand how it was wired.

Cheers,
Tom

[R.G.]

There are many descriptions of this, but I guess typing another won't hurt.

1. Before you go messing with this any further, ask yourself if you really already know how to do the wiring so that (a) you don't electrocute yourself or start fires now and (b) when you've passed this modded unit along, people with less skills and experience than you won't electrocute themselves or others, or start fires with it. If the answer to any of these is "no", do not proceed, and get an experienced tech to take these risks for you. If you are SURE the answer to all of them is "yes" you can proceed.

2. The ground wire connection to a mounting bolt of the power transformer used to be OK, but modern safety practice says that it's prone to coming loose and letting shock hazards exist. The right way to do this is to drill a hole in the chassis for a bolt that will hold this, and be used for no other purpose. Drill the hole; sandpaper to bare, shiny metal on the inside where the ground wire will go; insert the bolt; put on a toothed washer to bite into the chassis; put on the ground wire ring terminal. Do not use the bare copper wire or solder fill it to an ersatz ring terminal. Crimp on a ring terminal. With the ring terminal on, put on a second star washer and a nut. Tighten the nut firmly. Check for continuity to the ground pin of the power cord.

3. Determine which of the incoming wires is "hot" or line, and which is "cold" or neutral. Do not mix these up.

4. Connect the incoming line wire to the end of the fuse holder furthest from the panel. The fuse must be the first thing the incoming line hits.

5. From the terminal of the fuse holder nearest the panel, run a suitably rated wire to the AC power switch, doing the wire in a manner so that the 8mm creepage and clearance distances for safety are observed.

6. For extra credit, run the cold/neutral wire to the other pole of the AC power switch so both sides are broken by the switch in case some yokel of an electrician has wired the plug wrong.

7. From the switch, run suitably rated wire to the AC powered terminals inside the unit, again using wire practices as in 5.

8. Wire the AC power indicator, if any on the AC power lines, so that they're past the fuse and switch.

9. Remove all vestiges of wiring to any line reverse switches; they're not needed, and a failure in these wires or the death cap could cause damage to the unit - or you.

10. Remove all wiring to accessory AC plugs. Don't daisy chain AC, and don't enable others to do so.

[beedoola]

There are many descriptions of this, but I guess typing another won't hurt.

1. Before you go messing with this any further, ask yourself if you really already know how to do the wiring so that (a) you don't electrocute yourself or start fires now and (b) when you've passed this modded unit along, people with less skills and experience than you won't electrocute themselves or others, or start fires with it. If the answer to any of these is "no", do not proceed, and get an experienced tech to take these risks for you. If you are SURE the answer to all of them is "yes" you can proceed.

2. The ground wire connection to a mounting bolt of the power transformer used to be OK, but modern safety practice says that it's prone to coming loose and letting shock hazards exist. The right way to do this is to drill a hole in the chassis for a bolt that will hold this, and be used for no other purpose. Drill the hole; sandpaper to bare, shiny metal on the inside where the ground wire will go; insert the bolt; put on a toothed washer to bite into the chassis; put on the ground wire ring terminal. Do not use the bare copper wire or solder fill it to an ersatz ring terminal. Crimp on a ring terminal. With the ring terminal on, put on a second star washer and a nut. Tighten the nut firmly. Check for continuity to the ground pin of the power cord.

3. Determine which of the incoming wires is "hot" or line, and which is "cold" or neutral. Do not mix these up.

4. Connect the incoming line wire to the end of the fuse holder furthest from the panel. The fuse must be the first thing the incoming line hits.

5. From the terminal of the fuse holder nearest the panel, run a suitably rated wire to the AC power switch, doing the wire in a manner so that the 8mm creepage and clearance distances for safety are observed.

6. For extra credit, run the cold/neutral wire to the other pole of the AC power switch so both sides are broken by the switch in case some yokel of an electrician has wired the plug wrong.

7. From the switch, run suitably rated wire to the AC powered terminals inside the unit, again using wire practices as in 5.

8. Wire the AC power indicator, if any on the AC power lines, so that they're past the fuse and switch.

9. Remove all vestiges of wiring to any line reverse switches; they're not needed, and a failure in these wires or the death cap could cause damage to the unit - or you.

10. Remove all wiring to accessory AC plugs. Don't daisy chain AC, and don't enable others to do so.

The amp didn't come stock with a fuse. What rating should be used?

Can I use the number 2 method below, but with no ground switch?

[loudthud]

RG already posted before I could complete my post so I just deleted what I wrote.

May I assume you are in the USA? That looks like about a 5 Watt amp. A 1/2 Amp fuse will be good for 120V, 1/4 Amps in countries with 220-250V mains.

[beedoola]

RG already posted before I could complete my post so I just deleted what I wrote.

May I assume you are in the USA? That looks like about a 5 Watt amp. A 1/2 Amp fuse will be good for 120V, 1/4 Amps in countries with 220-250V mains.

Yes. I'm in the USA. the amp is a Teeco HIFi amp - 2 EL34s, 2 12AX7s and a 5Y3 rectifier.

[loudthud]

Yes. I'm in the USA. the amp is a Teeco HIFi amp - 2 EL34s, 2 12AX7s and a 5Y3 rectifier.

I didn't see the 5Y3. I assumed it was one EL84, one preamp tube and some kind of 9 pin rectifier. 1/2 Amp might be too small. If it blows, try something slightly larger, 3/4 or 1 Amp.

[R.G.]

The amp didn't come stock with a fuse. What rating should be used?

Wow. That's an old one if no fuse was used. This is going to sound facetious, but it's not intended that way. You want to use the smallest standard fuse rating that doesn't either pop or have the filament sag or otherwise visibly heat up at power on. You want this to be a time delay /slow blow type. As noted, 1/2A, 3/4A, or 1A would be great starting points.

Can I use the number 2 method below, but with no ground switch?

The number 2 schematic is correct, but the wiring makes me nervous. It's OK to remove the ground cap and use the switch lugs as terminals if you have to. But I personally would very much prefer that you put in a small tag strip for wiring the AC power terminals to that doesn't involve the ground switch or accessory AC outlet.

On the accessory outlet: The outlet is not grounded, and therefore represents a hazard to equipment and people for things plugged into it. If it simply must be used for an accessory outlet, replace it with a grounded outlet, and fuse the AC mains current before it gets to the outlet. You can possibly start fires with uncontrolled currents through that outlet that are not limited. I personally would NOT use the accessory outlet as a place to terminate the incoming AC mains wires, which is what I think is being done here. The incoming AC mains line wire should go directly to the fuse as the first thing it touches other than isolated terminals for mechanical positioning.

The ground switch can be left there, even used for isolated terminals as long as you use the throws, not the pole. However, this is confusing for anyone coming after you to work on this amp, and the pole terminal appears to be unconnected, but really is connected either to the neutral or line wires depending on the switch position. It's a camouflaged shock hazard for the next tech who opens it up, even YOU in a year or two when you've forgotten what you did. You can minimize this by putting heat shrink over the pole, etc. but that's a lot of work to avoid putting in a terminal strip.

I would prefer that you bring in the AC power line (and use a strain relief grommet, not a knot in the cord) and terminate the line wire at the fuse holder, the neutral at an isolated terminal; I would advise you to leave the AC accessory outlet and ground switch disconnected, no wires at all as far as AC mains go.

An unrelated bit of wiring advice I realize I left off from the earlier post. Modern wiring practice says to make the safety ground wire to the chassis long enough that if the line cord is pulled out of the chassis, the line and neutral conductors tear apart before the slack is taken up in the ground wire. This is one example of why learning to wire on the internet is a bad idea - people forget things, even if they know them. I certainly do at times. That's why I warn people when I give wiring advice that the advice may be incomplete or even incorrect in some ways, although I try to tell the straight stuff.

... and you HAVE been warned. I'm not an expert on AC wiring, just have some background in doing it. I may miss stuff.

[beedoola]

Wow. That's an old one if no fuse was used. This is going to sound facetious, but it's not intended that way. You want to use the smallest standard fuse rating that doesn't either pop or have the filament sag or otherwise visibly heat up at power on. You want this to be a time delay /slow blow type. As noted, 1/2A, 3/4A, or 1A would be great starting points.


The number 2 schematic is correct, but the wiring makes me nervous. It's OK to remove the ground cap and use the switch lugs as terminals if you have to. But I personally would very much prefer that you put in a small tag strip for wiring the AC power terminals to that doesn't involve the ground switch or accessory AC outlet.

On the accessory outlet: The outlet is not grounded, and therefore represents a hazard to equipment and people for things plugged into it. If it simply must be used for an accessory outlet, replace it with a grounded outlet, and fuse the AC mains current before it gets to the outlet. You can possibly start fires with uncontrolled currents through that outlet that are not limited. I personally would NOT use the accessory outlet as a place to terminate the incoming AC mains wires, which is what I think is being done here. The incoming AC mains line wire should go directly to the fuse as the first thing it touches other than isolated terminals for mechanical positioning.

The ground switch can be left there, even used for isolated terminals as long as you use the throws, not the pole. However, this is confusing for anyone coming after you to work on this amp, and the pole terminal appears to be unconnected, but really is connected either to the neutral or line wires depending on the switch position. It's a camouflaged shock hazard for the next tech who opens it up, even YOU in a year or two when you've forgotten what you did. You can minimize this by putting heat shrink over the pole, etc. but that's a lot of work to avoid putting in a terminal strip.

I would prefer that you bring in the AC power line (and use a strain relief grommet, not a knot in the cord) and terminate the line wire at the fuse holder, the neutral at an isolated terminal; I would advise you to leave the AC accessory outlet and ground switch disconnected, no wires at all as far as AC mains go.

An unrelated bit of wiring advice I realize I left off from the earlier post. Modern wiring practice says to make the safety ground wire to the chassis long enough that if the line cord is pulled out of the chassis, the line and neutral conductors tear apart before the slack is taken up in the ground wire. This is one example of why learning to wire on the internet is a bad idea - people forget things, even if they know them. I certainly do at times. That's why I warn people when I give wiring advice that the advice may be incomplete or even incorrect in some ways, although I try to tell the straight stuff.

... and you HAVE been warned. I'm not an expert on AC wiring, just have some background in doing it. I may miss stuff.

The amp in question doesn't have a ground switch... I was just going to use the wiring two method, minus the ground switch.

I disconnected the AC outlets but I may just drill the rivets out and remove the whole assembly. In case I do sell it at some point, as you said, it will prevent anyone from trying to use the outlets.

Having said that, about the AC outlets and lack of ground switch, do I really need tag terminals?

I didn't see the 5Y3. I assumed it was one EL84, one preamp tube and some kind of 9 pin rectifier. 1/2 Amp might be too small. If it blows, try something slightly larger, 3/4 or 1 Amp.

ya the 5Y3 is in between the PT and OT, and the other 12AZ7 is behind the skinny can cap.

[R.G.]

The amp in question doesn't have a ground switch... I was just going to use the wiring two method, minus the ground switch.

OK. I think I misunderstood. I took the wiring diagram to mean the incoming AC was from the accessory outlet(s).

I disconnected the AC outlets but I may just drill the rivets out and remove the whole assembly. In case I do sell it at some point, as you said, it will prevent anyone from trying to use the outlets.

Having said that, about the AC outlets and lack of ground switch, do I really need tag terminals?

From your question, I realized I did one of those leaps over the steps in how I got to what I said. There's an issue with wiring the AC power cord into the chassis in all the older amps which don't have an IEC power entry jack. That issue is "where do I solder the ends of these wires?" For a non-grounded AC line, this is easy. One goes to the AC power switch, and one goes to the fuse. That's really convenient, and doesn't need any more tag terminals. And there is no way to say which of the incoming AC power wires is "neutral" and which is "line". It will change when the plug is reinserted into the wall. In fact, this is part of what the ground switch was intended to do - connect the chassis to the neutral wire for RF frequencies, when it changed. Even earlier gear told people to turn the plug around.

When you go to ground, line and neutral wires, the objectives shift. When you break the incoming AC line, you really, really want to break the line wire, not the neutral. Breaking neutral leaves the thing hot if there's any connection between neutral and safety ground. A fuse opening the neutral leaves this condition, as does a switch in the neutral only. So the fuse and the AC power switch need to be only in the hot/line wire. It's a nice bit of extra safety to use a DPST or DPDT power switch and break both line and neutral, not just line; but if only one is broken, make it be line.

That leaves the question of where to solder the neutral wire from the power cord. There's no solder location for it like there was when it was the fuse or the power switch. The DP power switch solves this, as does a tag strip. You could use a wire nut from the neutral wire to one power transformer primary, but I don't know if that is any better. You could solder the neutral to the power tranny primary and heat shrink it, etc.

My unconscious choice was to advise you to put in a tag point to solder it to.

See how this simple process gets complicated in the real world??

[beedoola]

OK. I think I misunderstood. I took the wiring diagram to mean the incoming AC was from the accessory outlet(s).



From your question, I realized I did one of those leaps over the steps in how I got to what I said. There's an issue with wiring the AC power cord into the chassis in all the older amps which don't have an IEC power entry jack. That issue is "where do I solder the ends of these wires?" For a non-grounded AC line, this is easy. One goes to the AC power switch, and one goes to the fuse. That's really convenient, and doesn't need any more tag terminals. And there is no way to say which of the incoming AC power wires is "neutral" and which is "line". It will change when the plug is reinserted into the wall. In fact, this is part of what the ground switch was intended to do - connect the chassis to the neutral wire for RF frequencies, when it changed. Even earlier gear told people to turn the plug around.

When you go to ground, line and neutral wires, the objectives shift. When you break the incoming AC line, you really, really want to break the line wire, not the neutral. Breaking neutral leaves the thing hot if there's any connection between neutral and safety ground. A fuse opening the neutral leaves this condition, as does a switch in the neutral only. So the fuse and the AC power switch need to be only in the hot/line wire. It's a nice bit of extra safety to use a DPST or DPDT power switch and break both line and neutral, not just line; but if only one is broken, make it be line.

That leaves the question of where to solder the neutral wire from the power cord. There's no solder location for it like there was when it was the fuse or the power switch. The DP power switch solves this, as does a tag strip. You could use a wire nut from the neutral wire to one power transformer primary, but I don't know if that is any better. You could solder the neutral to the power tranny primary and heat shrink it, etc.

My unconscious choice was to advise you to put in a tag point to solder it to.

See how this simple process gets complicated in the real world??

So could I wire it like this below? Omitting the ground switch? I added the colors for each wire to similar that wire being solid, and not breaking at any switch lugs.

So the "white" wire would connect straight to one of the black PT wires (insulated with heat shrink.

the remaining wire from the power cable would run to the fuse holder.

Next question. The power cable I have has a green wire (ground lug), brown and blue.

If you were to look at the 3 prong cord, as if you were holding it in front of you (ground on the bottom), the brown wire would be the upper left contact, and the blue the upper left. Is the blue then the "white" wire connection?

[R.G.]

So could I wire it like this below? Omitting the ground switch? I added the colors for each wire to similar that wire being solid, and not breaking at any switch lugs.

So the "white" wire would connect straight to one of the black PT wires (insulated with heat shrink.

the remaining wire from the power cable would run to the fuse holder.

That looks right, assuming that the "accessory outlet" in the drawing is really the end of the AC cord. Be sure that your joint on the "neutral" to PT wire is solidly made mechanically before being soldered and coated with heat shrink. You don't want that wiggling loose over time.

Next question. The power cable I have has a green wire (ground lug), brown and blue.

If you were to look at the 3 prong cord, as if you were holding it in front of you (ground on the bottom), the brown wire would be the upper left contact, and the blue the upper left. Is the blue then the "white" wire connection?

I went to the Knower of All Things, wikipedia for the current color code for AC flexible cables. Wikipedia says that the green/yellow is safety ground, brown is line or hot, and blue is neutral or cold. That appears to match your cord.

I don't do this often enough to remember from time to time, so I look it up. I found this: Fileomestic AC Type B USA.jpg - Wikipedia, the free encyclopedia looking for "NEMA 5 plug polarity"; many things said that the taller prong is neutral if there is a taller prong.

[beedoola]

That looks right, assuming that the "accessory outlet" in the drawing is really the end of the AC cord. Be sure that your joint on the "neutral" to PT wire is solidly made mechanically before being soldered and coated with heat shrink. You don't want that wiggling loose over time.


I went to the Knower of All Things, wikipedia for the current color code for AC flexible cables. Wikipedia says that the green/yellow is safety ground, brown is line or hot, and blue is neutral or cold. That appears to match your cord.

I don't do this often enough to remember from time to time, so I look it up. I found this: Fileomestic AC Type B USA.jpg - Wikipedia, the free encyclopedia looking for "NEMA 5 plug polarity"; many things said that the taller prong is neutral if there is a taller prong.

Gah! I feel like a fool for having not seen the labeled "AC outlet" on that diagram. No wonder you were going on about it. Yes - there is no outlet, just the AC cord.

Thanks for the help.