Two Prong Power Cord Replacement

[Ptron]

For the umpteenth time I'm replacing a two prong power cord on a friend's amp and it's got me wondering how other folks handle some of the issues that come up with this most common of amp repairs. Feel free to offer any advice on the matter or ask your own questions but the things I'm wondering about specifically are:

The strain relief - Since this never works with the new three prong cord (without modification) what do you usually do? If I can, I cut or grind down the pincher part of the strain relief so the new cord fits. Labor intensive, but more satisifying to keep the old strain relief. I once drilled out the hole to use a larger strain relief. I don't know why but for some reason I felt bad doing this to an old amp. I've also used the rubber grommet with zip ties on either side method. It's the least work but it's not as satisfying and doesn't feel as secure since the cord isn't gripped tightly.

Neutral wire - Do you just splice and heatshrink the neutral wire to the transformer wire and let it float (mechanically speaking) if there's no convenient solder point or do you find a way to add a terminal?

The Death Cap - I have seen amps that either already had the cord replaced or always had a three prong cord that still have a "death cap" from neutral to chassis ground. Should this be removed?

Sorry if these have been covered to death but I didn't see anything.

[Gibsonman63]

I have done all of my vintage Fenders.

For the strain relief, I insert it back in the chassis without a cable and drill it out a little larger carefully with a cordless drill. I also try to pick a power cable that isn't overly thick.

For the nuetral connection, I use one of the lugs on the polarity switch as a connection point, since it is no longer in the circuit.

I always snip out the death cap.

[AtomicMassUnit]

I drill out the old strain relief to fit the larger cord, and solder a connector to the ground wire and bolt it to the nearest transformer mount. For the neutral wire I connect directly to the transformer lead and heat shrink it, and the hot side gets the fuse and on/off switch.

[R.G.]

It is really, really important not to do this wrong. Doing it wrong puts you and anyone who is using the amp or in the building with the amp plugged in at some risk of death by electrocution or fire. Maybe not a big risk, but it's there nevertheless.

Here are some important points if you want to do it right. There may be other things needed, but at least these need to get done.

1. The safety wire should be connected to a bolt on the chassis that is not used for anything else except safety grounding the chassis. One way I had explained to me to do this is to (1) drill the hole (2) sandpaper the inside of the chassis around it to bare, shiny metal (3) put down a toothed lock washer (4) put on the ring terminal (not space terminal or fork terminal) that has been properly crimped to the green wire (5) put on another toothed lock washer (6) put on a nut and tighten it down. The toothed washers ensure metallic contact to the chassis. Using another bolt that's already there used to be done, but would now fail a safety inspection because other parts held down with it may interrupt the connection to chassis or cause the bolt to loosen.

2. The cord has to be strain relieved in a way that will withstand a pull of X newtons. I don't remember X. But it should be enough that you worry that the cord will break first. The strain relief must also protect the cord from being cut into by the edges of the hole. I just buy the plastic pressure strain reliefs for about $0.50 from Mouser when I do these. They work. Yes, they're hard to do with pliers, but can be done, and the special tool is not all that expensive. Again, remember that you're talking about safety here. They're much better than dinking with grommets, clamps and such. You might even have to hand-file a proper sized flatted hole for them to go in. OK, takes 15 minutes of filing and tinkering. I want to live. If you want to do the double-D hole properly, The makers of these plastic widgies post the data sheets on line for them, including a drawing of the right sized and shaped hole. Draw the right shape and size on masking tape, then put the drawn-upon shape into place on the amp. Punch through the masking tape with a sharp pointed awl, filed-sharp nail point, whatever, then drill a hole in the center if you haven't put this over the old cord-hole, and file to the edges of the drawing on the masking tape with a round file and small flat file. The punching is for if you accidentally move the masking tape. You can file with the masking tape in place. The file doesn't much notice it's there.

Don't have those? Harbor Freight, about $10, and last a long time, and good for many other things, too.

Worried about metal shavings? Masking tape is a wonder material. You can use it and paper to mask off inside the chassis so it collects any filings that come through.

As for feeling bad about drilling out and making a bigger strain relief in an old amp, don't start something like this unless you can do the necessary stuff with some skill; and don't skimp on doing the right thing because you can't do the right thing skillfully and cosmetically well. Even a modest funeral costs more than buying another old amp even if it's a collectable one. Lest you think that's harsh, remember that you get good at playing guitar by practicing in places where it doesn't matter if you mess up. Eventually, you get good enough that you don't mind people hearing - or looking at - your work.

3. The inside wires must be of a length where if the strain relief does give up and let the cord pull out, the safety ground wire has more slack than the other two so it breaks last.

4. The safety wire should not be spliced and shrunk. Cut more slack out of the incoming cord.

5. Do not leave the power switch in one side of the incoming AC and the fuse in the other like many old amps have now, no matter how seductive it is to have a tie point for each side of the incoming AC line. If you have to, put in a new double pole power switch, or add a terminal strip so you have a point to attach neutral to. The fuse and (if you only use one switched side) power switch must break the hot side; otherwise it could kill YOU if there is a blown fuse in the neutral side but the power switch still puts AC power on the chassis through the power switch. It is safer is you can bring yourself to put in a new double pole switch to break both sides in case the AC power outlet in the wall was wired backwards (it happens).

[Jazz P Bass]

How do you make this excellent post a "stickY"?
Well phrased R.G.
Thanks.

[Jazz P Bass]

3 Wire AC Line Cord Installation

I made a PDF file of the post.
Copyright R.G.

[JoeM]

How do you make this excellant post a "stickY"?
Well phrased R.G.
Thanks.

Yes, that post should be a sticky.

[Enzo]

Since amp chassis are generally steel, a magnet is handy to collect stray filings too.

[Intergalactic Tourister]

I also use a magnet.
However, put a tissue around the magnet or you'll have he!! getting the shavings off your magnet.