Ground in a circuit

[SavannahLion]

This may sound like a stupid question, so I'll just come right out and ask.

On a properly grounded circuit, the ground prong is the reference point for ground in the circuit. Correct?

If that's the case, on circuits that simply have the older two prong socket, where's ground supposed to be? Is ground left to float or is it tied to the neutral side? How is ground resolved when polarity isn't established?

A more important question is, what happens if I tie a true ground from one machine to the "floating" ground of another?

[Ed_McCarron]

We're talking power plug, right?

Ground and neutral are tied common at your electrical panel.  If its a subpanel, they are not.

Ground is not supposed to carry current.

Ground is only there for safety reasons - a short inside the metal case will follow the ground conductor - if its a GFCI outlet, the difference in currents between hot and neutral is what trips it.

Now that that is out of the way...

In a 2 wire circuit, there is no ground.  Current comes in on hot, returns on neutral.  You'll see the term "double insulated" on many 2 wire items now... It means theres no way for a hot wire to electrify the case, internally.

If you have a cabinet with no ground pin on its plug, theres a good chance that metal parts inside may be at line potential - think the chassis of a CRT that still needs its isolation transformer.  Grounding it by tying it to another machines ground is a bad idea.

Did that answer anything?  I'm not 100% sure that its what you asked.

[SavannahLion]

Oops, Yes and no.

By circuit, I meant a piece of equipment. Sorry for the confusion there.

I'm asking because I have an antique player with a two prong cord. There isn't any modern PCB. It's all wires and an enclosed box with a bunch of vacuum tubes sticking out of it. The speaker has a wire tracing back to the vacuum tubes. The other speaker wire is attached to the enclosure of the player. I can touch the enclosure and not get shocked, so the only thing I can assume is that wire is tied to the neutral line.

The last patent date stamped on the player is 1947.

I was scratching my head for a bit until it dawned on me that the vacuum tube(s) have a rectifier somewhere in there.

My goal is to get the audio feed from it to a PC without popping a vacuum tube. Don't care about the PC popping.

[2600]

If you are that worried, use a transformer that way the 2 will be isolated.

[SavannahLion]

If you are that worried, use a transformer that way the 2 will be isolated.

On a speaker line? Hhmm... I've never really thought of that before. I'll look into it.

[Ed_McCarron]

Ah, OK.

Is the output of the player line level or speaker level?

If line level, just use a ground loop isolator -

http://www.radioshack.com/product/index.jsp?productId=2062214&cp=&parentPage=search

If speaker level, try one of these -

http://www.crutchfield.com/App/Product/Item/Main.aspx?i=142SLC4&search=line+level+converter&tp=2001&tab=review

[SavannahLion]

If speaker level, try one of these -

http://www.crutchfield.com/App/Product/Item/Main.aspx?i=142SLC4&search=line+level+converter&tp=2001&tab=review

Neat! Didn't know those existed. Thanks.