I recently bought a guitar tube amp in Germany, with european 220V configuration, now I live in Mexico, where 110 or 127V is the standard. In order to use my amp here I considered three options:

1) Take my amp to hughes&kettnerUS to change the transformer to a 110V one (which cost me about $300USD or 30% of the amp price)

2) Buy a 110V to 220V converter (which I didn't found in Mexico) not for the 200W rating of my amp

3) Make a 127V to 220 converter.

I went for the third option, and actually I went to a local transformer builder (I don't have any experience in the subject) and asked him to make a transformer that steps up the 127V to 220V, with a power handling of 300W, 100W spare.

It cost me about 20bucks (250 pesos) and I have been using it so far like that. It hasn't give any trouble, but my concern is that the amp (as many high quality audio equipment) has a IEC C14 JACK with neutral, line and ground pins.

What I did is just connect the neutral and the hot lines from my 127V AC line to the transformers input, at the two output pair of cables I read 220V which is good, and I connect there the power inputs of the amp, leaving the ground connection unconnected. I don't know if this ground connection is really important for audio performance, or it is a safety concern, but until now I haven't receive any shocks when singing and playing the guitar.

My question is,should I ground my amplifier? In order to do so, should I just connect the AMPs IEC Ground Pin to my physical ground, which is normally connected to the neutral line at the main breaker panel (Ive read a little on internet ), should I jump the primary and the secondary windings of the transformer (neutral from the 127V to the one in phase of the secondary winding)? Or should I leave it like that, and say "got sei dank" I haven't had any electrical shock.

PD. I know that I could install a 220V socket in my house, but the movability is important to me, as I play regularly in bars and parties.

Thanks a lot in advance, for your help and advice.

Ramon Grande, Mechatronics Engineer ITESM

Images are attached

 

If your house mains wiring system has a ground connection provided then connect your amp ground pin to that. Other possible connections you mention could be dangerous.

 

For added safety, you can re-reference the 220vac secondary to earth ground, this entails connecting one of the 220v secondary connections to your earth ground conductor, the conductor that then connects to your amp from this point would be the 220v neutral.
If this transformer if always going to be fed from an earth grounded supply, then there is an earth ground available on the primary side.
The metallic core of the transformer you had built should also be earth grounded.
Max.

 

The transformer you added isolates your amp from the power line neutral which is fine. For added protection you could connect the amp ground to the power line protective ground (not neutral).

 

The method I outlined restores it to the same condition it would, be had you obtained a 120v version of the amp.
Max.

 

The transformer output is a separately derived system (under US codes). The neutral-to-ground connection is at the secondary (and only at the secondary) of the transformer like MHR outlined.

http://www.mikeholt.com/videodisplaynew.php?pageid=4395

 

Like this:
.....

 

Like this:
.....

Maybe a dumb question, but doesn't wiring like that violate the "ground and neutral at one point only" rule? Shouldn't the 'plug' ground go directly to the chassis of the amp/machine and not the 'neutral' of the secondary?

 

the transformer provides isolation from the primary to the secondary, so no rules are violated.
if you wanted a 120 to 240 volt transformer, look for "control transformer" in industrial catalogs, lots of different standard voltages and current ratings. most are for continous use also.

 

To the best of my knowledge, that is one of the odd things about a locally derived supply. All the grounding rules transfer to the secondary winding as if it was on a telephone pole. Ground and neutral are born together, created, at the transformer and that's your star ground point. Farther distant from there and the neutral and ground shall be carried on separate wires....AFIK

 

if they werent seperated, gfci would not work.

 

Maybe a dumb question, but doesn't wiring like that violate the "ground and neutral at one point only" rule?

But you have lost the grounded neutral at that point if steps are not taken.

The NEC states that in the case of using a local isolated control transformer where you have lost the neutral ground reference, it is optional to either provide a re-referenced grounded neutral immediately at one terminal of the secondary.
This has been done now for some decades, in the case of non referencing a grounded neutral then both conductors will be fused and treated as two live conductors, otherwise a fuse in the live conductor only, and all other conditions apply to the new neutral.
Max.

 

MaxHeadroom, correct! As long as the earth wire is not used as the return wire; i.e. as long as it does not carry current.

 

So with a secondary wired like this, you can also chose which side of the secondary winding to be 'neutral'? Since the primary and secondary are galvantically isolated?

 

Right. (It would be even more obvious if the secondary had a center tap which was connected to Earth.)

 

So with a secondary wired like this, you can also chose which side of the secondary winding to be 'neutral'? Since the primary and secondary are galvantically isolated?

See X2 of 1T transformer.
NFPA79.
Max.

 

For added safety, you can re-reference the 220vac secondary to earth ground, this entails connecting one of the 220v secondary connections to your earth ground conductor, the conductor that then connects to your amp from this point would be the 220v neutral.
If this transformer if always going to be fed from an earth grounded supply, then there is an earth ground available on the primary side.
The metallic core of the transformer you had built should also be earth grounded.
Max.

I disagree. Although I can't vouch for Mexico, neither side of a 220 feed is grounded in the US. the 220 is a centertapped secondary with a split bus to provide balanced load 110/110 and neutral is grounded. It's possible that the 220 elsewhere is wye/delta 440 w grounded CT but it had better be known before you arbitrarily ground one side of a 220 line.

 

#1 we are not talking of the US/Canada.
Besides the load is not a '110/110 balanced load', it is simply a 240v CT transformer.
If I ground one side of a 240v supply it has nothing to do with the type of source, 1ph secondary or star point of a three phase transformer.
If the practice in the country of use is to support a earth ground referenced neutral.
Max.

 

While I wouldn't expect IEC to be compliant with NFPA sec 70 I would wager there aren't too many differences.

The only thing that lets you ground the secondary is that it is isolated but you may get ground loop issues if your primary is connected to a grounded CT. #12's drawing of carrying the ground through the core to the socket ground is the best given the requirement of portability.

As has been previously mentioned, in an actual install of a step down transformer such as a 240v 5kva to splitbus subpanel, the secondary is grounded with new grounding not bonded back to source.

Also if you have an autoformer(do we really know) you can have issues.

Grounding is by far THE thorniest issue. IMHO because NFPA is primarily interested in life safety not preventing damage to equipment.

 

So far, 12# Thanks for your schematic. Another doubt is if I should care about the "polarity" of the output cables in the transformer, I mean..

On the 110V side neutral is connected to earth ground at the main breaker...

Now if we put an oscilloscope at the 110V outlet and at the transformers output, I think we would see two synodal signals, the 220V twice the amplitude of the 110V, and depending on how we connect the probe's ground and input to the output of the transformer, it could be in phase or with an offset of PI...

Is this important?

Thanks for your reply...