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Old 2nd October 2003, 09:42 PM   #21
haldor is offline haldor  United States
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Quote:
Originally posted by 00940
hum, I just realized that I had a tube amp running without earth. And no way to get one since my old outlet has no ground.

I just went back to my old manuals and .... Could I use an isolation transformer instead of earth ? More specifically, I've still at home one old heavy EI core stepdown transformer. It has jumpers for 1:1 and is rated for 600W (the amp is rated at 2w).

If you have a tube amp then it already has a transformer in it. All you need to do is install a 3 prong line cord and connect the green wire to the amp chassis. When you do this, check to see if you have one of the 3 position ON-OFf-ON power switches. Is so replace it with a normal ON/OFF switch. The 3 position switches were to let you swap the AC hot and neutral wires (the old 2 prong plugs aren't polarized to make sure you plug them in right). You won't need this 3 position switch once you have a proper line cord.

About your outlet not having a ground. Unless your house is very old, you most likely have a ground present in the outlet box. Older house wiring (pre-Romex) mostly used a two conductor cable inside of a flexible metal jacket. The metal jacket of the cable is your earth ground and it is terminated to the outlet box.

The simplest solution to your problem in this case is to use one of the 3 to 2 prong adapters and install it according to directions (connect the adapter ground wire or tab to the outlet cover screw which is grounded). I would prefer to replace the 2 prong outlets with 3 prong outlets and connect the outlet ground to the outlet box, but do this only if you know what you are doing.

If your house is really ancient, then it may not have grounded outlet boxes, in which case hire an electrician to install a proper circuit and outlet for your amp.

You can test for a grounded outlet box with an AC voltmeter. Measure between the hot and neutral and you should see around 120 VAC (US). Meter between the cover screw and the hot and you should also see around 120 V. Between the neutral and the cover screw should be less than 1 volt. If your box is not grounded then you will probably measure around 60 volts between the Hot and cover screw and 60 volts between the neutral and cover screw.

Phil
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Old 2nd October 2003, 10:03 PM   #22
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Default Failure Mode

I have not seen a short that has happen before the line fuse. Maybe you've had rats in your equipment.
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Old 2nd October 2003, 10:37 PM   #23
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Jim,

Phil has explained the technical aspects of this issue very clearly. You may wire your systems how you see fit, but what you have suggested in your previous post is unsafe. This is against the rules of the forum (Rule #4). Please consider this an official warning and not pursue this debate any further.
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Old 2nd October 2003, 11:11 PM   #24
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I would like to clarify the above, just to make sure I'm not gambling with my measurement set-up. It is a LM3875 in 2 boxes. The PS is in a 6x4-ish Radio Shack plastic box. It is connected via 3 wire Neutrik to a smaller box, also plastic, containing the rest of the amp. The only exposed metal are the screws (non-issue) and the RCA and binding posts. Even though there is a primary fuse, is there a real chance that the binding post or RCA could go live? Of course, since I'm asking the question, the plug is indeed a 2 prong.

Smart and safe or ignorant and dangerous?

Sandy.
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Old 2nd October 2003, 11:57 PM   #25
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Sandy,

The fuse is a separate issue. It is there mostly to protect the equipment from damage (if sized correctly). Phil explains the typical fault modes for a short in a metal box. The plastic box will certainly eliminate most possible ways of getting a shock from your PSU. That said, equipment sold retail with two-prong-plugs will come with a warning that it is to be serviced only by a qualified person. Therefore when you open your plastic box to do your DIY thing you take on the responsibility of possible electrocution.

Can you get shocked from your input and output connectors with the plastic case closed? Yes, if the hot AC accidentally shorts to one of the connectors. Is it like to happen? Probably not, but then again, it probably couldn’t get UL approval either.
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Old 3rd October 2003, 01:17 AM   #26
haldor is offline haldor  United States
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Quote:
Originally posted by haldor
About your outlet not having a ground. Unless your house is very old, you most likely have a ground present in the outlet box. Older house wiring (pre-Romex) mostly used a two conductor cable inside of a flexible metal jacket. The metal jacket of the cable is your earth ground and it is terminated to the outlet box.

The simplest solution to your problem in this case is to use one of the 3 to 2 prong adapters and install it according to directions (connect the adapter ground wire or tab to the outlet cover screw which is grounded). I would prefer to replace the 2 prong outlets with 3 prong outlets and connect the outlet ground to the outlet box, but do this only if you know what you are doing.

If your house is really ancient, then it may not have grounded outlet boxes, in which case hire an electrician to install a proper circuit and outlet for your amp.

You can test for a grounded outlet box with an AC voltmeter. Measure between the hot and neutral and you should see around 120 VAC (US). Meter between the cover screw and the hot and you should also see around 120 V. Between the neutral and the cover screw should be less than 1 volt. If your box is not grounded then you will probably measure around 60 volts between the Hot and cover screw and 60 volts between the neutral and cover screw.

Phil
My humblest appologies. I just noticed you are located in Belgium. All of my comments about standard electical practices are based on my experiance in the USA. I have no idea things are done in Belgium. You need to talk to someone who is knowledgable about your country's electrical practices.

One comment, I have noticed that some European power plugs (French I believe) are round with two prongs and an recess on the edge of the plug. This recess is where the ground connection is made. It is possible your amp has a ground connection even though you only see 2 pins in the outlet. Again, don't take my word for it, check this with an electrician in your country.

Phil
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Old 3rd October 2003, 09:37 PM   #27
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Quote:
Originally posted by Fred Dieckmann

My father in law raises cows. Should I visit his farm and install ground straps around his cows in the middle, to be on the save side?

I wouldn't, because if lightning strikes the cow, then
it's dinner time. BBQ already done.
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Old 9th October 2003, 02:07 PM   #28
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Default [B]20 Pregnant Cows Killed by Lightning[/B]

Well Look what just rolled off the AP newswire!

20 Pregnant Cows Killed by Lightning

http://wireservice.wired.com/wired/s...storyId=789127

I think they shoulda been using that Corona Livestock shampo....

-Dave,
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Old 9th October 2003, 07:09 PM   #29
sam9 is offline sam9  United States
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An addendum regarding North American mains based on my own experience:

Don't assume that even a new house is wired correctly. This especially if it was built during a housing boom or part of a tract that went up fast. I have found the following in my place:

A- In a couple of wall sockets the ground was not wired correctly (i.e. an open ground).

B- Hot and neutral reversed.

One was sloppiness in construction the other was due to the previous owner doing his own "handyman" installations. Of course this is not supossed to happen if everybody follows the rules - but a fat lot of good that does me from the grave. So I check evey outlet in my house. Trust no one!

Also it is appearentlt legal to omit the ground if a GFI socket is used. Maybe this is OK but I still don't like depending soley on a circuit to detect a ground fault based on ME serving as the low impedance path to ground!

The moderator correct me if I'm wrong - one precaution against hot and neutral being reversed is to use a DPDT or DPST power switch when I buld something plus fusing both entry lines.
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Old 14th October 2003, 03:20 AM   #30
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Not true, the reason is about 9 posts back.
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