3-Prong Power Cord

Lets replace a 2-prong power cord on an old tube amp, with a 3-prong cord.

Now we need to pause right here danger signand I just need to say that there are some dangers in working with tube amps. So if you don’t know what you are doing I recommend that you don’t poke around inside your amp and you take it to a professional. There are some pretty big capacitors in here so even with the amp off there is still potential to do some damage.

And I am just a guy on the Internet so I wouldn’t trust your life in my hands.

So now that you have been warned, lets get on with it.

I have an old tube amp that has a crusty ungrounded 2 prong power cord. Not only is it unsafe to have a two prong cord on an amp but this cord has also seen better days and especially has some wear around the grommet so it is definitely worth replacing.

backpanelNow most old two prong cords are reversible, which means you can plug the cable in either way around, so the two wires coming into the amp can be either hot or neutral. This means that typically you will see one wire come in and go through a fuse and off to the transformer. And the other wire will come in and go through the power switch, and then that often goes through a ground switch which has a capacitor attached to the chassis. Either of these two paths could be our hot or neutral.

We want the switch and the fuse to be both in line with our hot wire.

If you just solder your new cable where the old wires were than you either have the hot go through the switch, which means the neutral goes through the fuse, so if the fuse blows the hot wire will still send power to the amp, which we don’t want. Or if you hook up your hot wire to the fuse, then the power switch turns off the neutral, which we also don’t want.

schematicWhat I am going to do first is unsolder this red wire, which goes to the power switch, and also unsolder this jumper wire that connects power to this ground switch. This will disconnect our ground switch and death capacitor that we don’t need, and don’t want hooked up with a three-prong cord.

Next I am going to cut the black wire that goes to the fuse, but I am going to leave enough length so that I can attach it to the power switch later. Now I can remove the old power cord and throw it away.

Now I can install a new rubber grommet in the power cord hole and then feed in my new three-wire power cord.

Then I can remove the remaining wire on my power switch and solder the wire we left attached to the fuse to one side of the power switch, and my black wire from my new power cord to the other side. Now we can see that our hot wire comes into the amp, through the power switch, through the fuse, and off to the transformer.

Next I am going to attach a ring terminal to our green, ground wire and now I can simply remove one the screws that holds down this terminal strip and attach the ring terminal underneath. Usually a good place to attach a ground wire is the screw that holds the transformer but we can see that this screw is easier to get to, and is right next to our transformer screw, and it will work just fine.

Lastly we just need to attach our neutral wire and to do that I am going to follow this cable back until I find where it attaches into this switch here. Not every amp will have this but this one has a switch to change the amplifier from 110 to 220 volts. The terminal on this switch will be perfect to attach our neutral wire. So I removed the switch just to give me more room to work, and then soldered the white wire right where that old wire came from.

Then I could put the switch back in and lastly I attached a zip tie to the power cable to prevent it from pulling out of the amplifier chassis.



Before we plug in and power up the amplifier it is a good idea to double check your wiring and make sure it is ok, and you check for shorts with a multi-meter if you want. And it is also a good idea to check the fuse in the fuse holder, since these somehow always end up being bigger fuses than they should be.

And sure enough we can see here that this amp calls for a 2 amp fuse but actually has a 3 amp, not as bad as some I have seen, but I will go ahead and swap that out for the right fuse and then we can power up this amplifier.

Now that is really all there is to swapping out a power cable. If you are uncomfortable though working with electricity I recommend you take your amp to a professional.

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