how to install grounded 3-prong power cord?

[onusx]

i know this should be a basic thing, but i haven't been able to find clear instructions online, and want to make sure i do this properly. i mentioned in another post that i have a couple of effects that i hope to quiet down with a grounded plug. i also have a vintage magnatone amp with a two prong plug that's been shocking me, and i'd like to fix that; hopefully after i've practiced on my stuff i can replace the cord on my friend's marshall jmp, which i'm worried will kill him eventually since it's a lot higher powered than my amps, and he plays out pretty often with it.

sorry if this is the dumbest question ever, but if someone could just point me toward some good directions on this i'd appreciate it. do i just buy a length of cord with the plug attached, and solder up to wherever the two prong connections are inside the units? how do i know how to connect the positive/negative sides of the power supply, and where to connect the ground?

thanks, brian

[elberto]

for the amps:  do as you say with a length of cord.  when you cut it open, you'll see three conductors:  two wires to where the old two-prong wires went (probably to the power switch), third conductor (for third prong) attaches to the chassis--I use a bolt + star nut.  You don't need to worry about polarity for the two prongs.

[onusx]

thanks, that makes it pretty simple. i thought it would be, but have read many times the recommendation that people add a three prong mod without ever describing how. i guess it should be self-explanatory, but i didn't want to get it wrong. thanks, b

[R.G.]

The tricky part is what to do with the "line reverse" switch, AKA the "death switch".

The answer is - disconnect everything from the back of the line reverse switch. Leave it totally disconnected.

Here are some other things to consider.
- Where's the fuse? Two wire amps usually put the fuse in one of the incoming power lines, the power switch in the other. Bad, bad, bad with a three wire. For three wire, the green wire is tied hard to chassis, with more slack in th wire than either the white or black wires.  The white or neutral wire is tied to one side of the power transformer. The black wire goes first to the fuse, then to the switch, then to the other side of the power transformer. An ohmmeter should show an open circuit between the chassis and either power wire, performed on the end of the power cord (pulled out of the wall, of course).
- it's not a bad idea to use a double pole power switch and disconnect *both* hot and neutral power wires.

[Paul Marossy]

it's not a bad idea to use a double pole power switch and disconnect *both* hot and neutral power wires.

This is how I have all three of my tube amp builds wired. It makes the most sense to me to do it that way.

[PaulC]

I'll add this...  I've changed a gaggle of ac cords, and about 20% were wired up backwards inside the molded end.  Also I've done alot of gigs with lame house or generator pwr  which had reversed lines.  So if you do go to the trouble of using a DPDT mains switch fuse both sides so you'll always be fused on the hot side.

PaulC

[R.G.]

Yep. I'd also fuse each transformer secondary. The power transformer is usually the  single most expensive part in the amp. It's worth protecting.

[Paul Marossy]

if you do go to the trouble of using a DPDT mains switch fuse both sides so you'll always be fused on the hot side.

Not a bad idea. I guess it wouldn't hurt to check those IEC power cords with a continuity tester.

Yep. I'd also fuse each transformer secondary. The power transformer is usually the single most expensive part in the amp. It's worth protecting.

Hmm... I never heard anyone suggest that, but it's a darn good idea!