Changing to grounded cord

12bobDecember 31, 2012

Hi, I have an old organ. (Tube model) We haven't used it in a number of years because the power cord is falling apart, and I didn't think it was safe since tubes use so much power.

I'd like to install a new cord on it, and I'd like to use a 3 pronged grounded cord.

Can I just attach the green lead to one of the screws in the giant aluminum chassis? Since its a tube model, everything inside is metal, and I'm guessing that its all grounded or groundable.

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Ron Natalie

The first thing you need to do is find out if the chassis inside is connected to one side of the power cord lines. Inspect or get a meter out. If the chassis is connected to one DO NOT CONNECT IT TO THE GROUND PIN ON THE NEW CORD. Connect it to the NEUTRAL (i.e. grounded) side of the plug (the larger of the two blades (or in the case of a three pin plug, with the center (ground) pin down, the on en the right looking end on at the blades).

If the chassis appears to not be connected to either of the supply lines (but to some part of the transformer secondary), then go ahead and connect it to ground. However, be prepared based on other quirks of the circuitry that you may induce a hum doing this, in which case, just unhook it.

    Bookmark   December 31, 2012 at 11:59AM
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Ok. The white lead is connected to a soldering block, and the black lead is connected to this bias test plug, which seems odd to me.

I checked for continuity between the metal chassis and both leads and got infinite impedance, so I don't think anything is connected unless there is some kind of switched connection.

    Bookmark   December 31, 2012 at 12:57PM
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I guess you can only post one picture at a time, so here it the other one.

    Bookmark   December 31, 2012 at 12:58PM
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Ron Natalie

I presume the cord goes to a power switch somewhere before it gets to the chassis.

It looks to me (though it's hard to see) that the white wire on that terminal strip then feeds the white wire in the transformer (big black thing on the bottom right of the lower picture, top right of the upper one).

The black wire also appears to depart from the bias test plug and head to the expansion cabinet octal socket and then to the transformer black wire.

So, yes it appears your chassis is electrically isolated.

    Bookmark   December 31, 2012 at 1:05PM
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No, there isn't a switch before the cord gets there. The cord you see in the picture entering the outside of the chassis comes straight from the wall plug, so the black wire must route through the on/off switch somehow and then eventually complete the circuit back to the transformer.

Would it be better to just connect the black and green leads both to the bias test plug terminal where the black one is now, and the white one to the lug where the old white one is now?

    Bookmark   December 31, 2012 at 1:14PM
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Ron Natalie

DO not connect he black and green wires together. That is going to be quite spectactular when you plug the thing in. Sure, I would take the black and white from the new cord and connect it to the exact same place that they are in the old cord. If you're bent on connecting the ground to the chassis, find a terminal that has that connection. Often one of the pins on a tub socket is also connected to that. Again be warned is that if the thing doesn't have a hum in it now, this may introduce one.

    Bookmark   December 31, 2012 at 1:17PM
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I guess I can just leave it floating. I just thought it would be safer to have something connected to the ground pin on the plug.

    Bookmark   December 31, 2012 at 3:16PM
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I think if you look again, you'll find the black wire from the cord goes to that 5 amp fuse holder - as it should. The bias test socket connects to the power tube cathodes for setting bias after a tube replacement.

However, as Ron already mentioned, I think you may end up with 60 cycle hum issues if you ground the chassis. I really wouldn't worry about grounding the chassis for additional safety.

    Bookmark   December 31, 2012 at 5:34PM
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Just use a polarized plug.

    Bookmark   January 1, 2013 at 7:27PM
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