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Old 15th January 2009, 08:42 AM   #1
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Default earthing advice on a US amp

Hi all,


I reckon I know the answer to this, but need some input to confirm....

A friend has bought a pretty high dollar commercial tube amp from the USA. He is using it in a 220V area (europe). He has had a large step down transformer made to match up the voltage (with adjustable output so that the output is correct and not eg 140V on a high input line). The amp has a captive power lead which only has 2 wires ie live and neutral (black and white). There is no earth or mains safety ground.


So should he:

a) connect it up as is, just use away and ignore the lack of earth?
b) connect a fly wire to somewhere on the chassis and connect back the earthing point on the step down?
c) something else?


First thing here is to do no harm to him or the amp! followed closely by getting the thing running!


I'll be calling to him later to give a hand setting it up so would love to hear what you guys think.

Fran
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Old 15th January 2009, 01:30 PM   #2
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As long as the step-down transformer is correctly grounded,I don't see why you would need to ground the amp.
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Old 15th January 2009, 02:03 PM   #3
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The step down is indeed grounded properly. Its just normal practise here that all equipment is either double insulated or else is grounded.

In amps I would have built myself I always add a safety earth and then circuit ground is brought to that point.


For those who have US amps - do all yours only have a 2 core power lead?

And double check me on this: white is neutral and black is live?

Fran
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Old 15th January 2009, 02:07 PM   #4
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As far as safety, a step-down isolation transformer SHOULD be adequate, but if it's a custom made one-off part, of course it won't have any safety tests or approvals. The amp chassis SHOULD be grounded for noise reasons, though since it's totally isolated, the ground through the preamp may be enough. I myself would add an IEC connector and 3-wire cord if that's practical, a 3-wire permanent cord if it's not.
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Old 15th January 2009, 02:31 PM   #5
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The step down is a one-off job alright but it is professionally made by a company who specialises in power supplies, usually for hospitals, sensitive installations etc. Basically they know what they're at. I know they included taps in the transformer to allow for adjustment of the output voltage from 110 to 125V (tolerance on the input voltage here is 220-240V so they did that to give the closest output to 120)

I think I would be happier to run an extra wire from safety earth to somewhere on the chassis. Hopefully it won't add any noise to the amp. That way at least if something does occur across the chassis it will be carried right back to the fuseboard and trip whatever is relevant.

I guess I'm trying to figure out do I need to do this!


Fran

ps thanks to those who have commented so far......
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Old 15th January 2009, 07:35 PM   #6
m6tt is offline m6tt  United States
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http://en.wikipedia.org/wiki/Three-p...er#Color_codes

Is the amp too valuable to change power transformer for UK mains? In the long run that may be the best solution, if you can find very similar secondary Vs with a 240v primary. Surely the US amp was distributed in the UK? Can the UK PT be sourced?
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Old 15th January 2009, 07:48 PM   #7
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Quote:
Originally posted by woodturner-fran



For those who have US amps - do all yours only have a 2 core power lead?

And double check me on this: white is neutral and black is live?

Fran

In the US really old equipment (50's/60's/70's) can have 2 wire power cords, usually not keyed for hot/neutral, some 80's equipment has 2 wire cords with keyed plugs, the rest use 3 conductor cords with keyed hot/neutral and earth ground. The advent of universal input power supplies and IEC power inlets allows manufacturers to make one product and just ship it with different power cords for different world markets.

Modern equipment with 2 conductor power cords have to meet UL/CSA/CE etc. double insulation stds.

US color code is black is hot, white is neutral, and green (or green/yellow stripe) is earth ground.
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Old 16th January 2009, 01:25 AM   #8
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ok,

thanks to everyone for the advice. I went around to my friends place adn gave him a hand hooking it all up. As it turned out it has a 2 lead captive cord as I said above, but there is also a grounding post on the back panel so I ran an extra wire from there to the step down which then runs back through the normal house wiring.

There is a very slight rise in the hum from the amp with the earth connected but its very slight and so he will leave it in place and be safe!

Thanks again for all the help - great to get a good, safe and complete solution.


Fran
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