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I may have connected the a/c plug the wrong way around!

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roadtrip

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I had to replace the a/c plug on my guitar tube amp, and I may have connected the plugs the wrong way around. The a/c is 120 volts, and the amp works still, but I am concerned about the fuse. If I have connected the polarity the wrong way around, will the fuse still blow if the amp has a problem? Thanks.
 
The fuse would still blow, but then you could have a problem because components in the amp would still be at mains voltage and hence DANGEROUS. The fuse must be in the Line (Live) conductor, not the Neutral one.
Wiring guidance for US mains plugs can be found e.g. here.
 
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ok that makes sense. I didn't realise that the amp would still be live! I'm not sure how to test if the fuse is on the positive side as it slides into an opening, and I don't know if it is the outer connection that is connected to the mains, or the inner one, which will be hard to get to.
 
It really doesn't matter much at all
From the point of view of protecting the equipment from damage I agree; but it does matter for personal safety.
Can you give an example of intentional fusing of the neutral conductor, Nigel? I thought that was contrary to UK wiring code (don't know about elsewhere).
 
From the point of view of protecting the equipment from damage I agree; but it does matter for personal safety.
Can you give an example of intentional fusing of the neutral conductor, Nigel? I thought that was contrary to UK wiring code (don't know about elsewhere).

A good many UK TV's have fused, and actually even switched (with a single pole mains switch), the neutral mains lead. Not that anything modern even has a mains switch :D

There's not really any 'personal safety' issue, as un-qualified people shouldn't be messing about inside live equipment anyway, and there's very little extra risk (if any?) with the fuse in the neutral.

Bear in mind, most equipment these days is class II, and don't even have polarised mains connections.

Personally, when I'm building something, I would ALWAYS put the fuse in the live side of the mains - but it's more a 'convention' than anything else.
 
Thank you all for the replies. I don't know a lot about electronics, but I'm assuming that the mains goes through a transformer, so I'm wondering if it matters if the fuse blows.

The amp is 25 years old, and since it is a tube amp, the voltage to run the power tubes is 400 (440?) volts, and to reduce hum all guitars have the ground from the amp wired to the bridge of the guitar. (so that by playing, the amp is grounded through the hands.)

If the amp shorts out to ground and the ground goes hot, I will get quite a shock! (will it kill me? will 400 volts stop my heart? yikes!)

Fortunately I have found a circuit that can be wired between the ground and the guitar bridge, that will still allow for grounding, but will reduce the amps so that I will feel a small electric shock so that I will stop playing. ( for anyone who wants to know, wire a 220 k ohm resistor and a .001uf (1000pf) capacitor, both rated for a minimum of 500 volts, in between the bridge and the circuitry. wire them in parallel, not in series! solder them together and insulate the leads).

I'm wondering if I can test the amp by removing the fuse, and testing the resistance between the a/c lugs and inside the fuse opening. by putting the test lead just inside I should get a reading close to ) ohms. (I think).
 
If the chassis is connected to the guitar, as it is usually done, then the amp should have a three prong power lead and a solid connection between PE and chassis and chassis to the outer connector. That should protect you in any case of failure in the amp. However it also creates a more danger to the player since he is firmly grounded and should he touch some other high voltage he will be in danger.
 
Yikes! The ground circuit I mentioned might really come in handy then!

So I used an ohmmeter and tested both prongs on the plug. The live prong (the thinner one) connects to the back of the fuse holder, and gave me a reading of zero ohms. So I guess I go t lucky. Fifty - fifty, my dad always says!

Thanks for all the help, I'm really enjoying messing around with electronics! Now I'm going to try to build a bypass foot switch with red and blue LEDs. Wish me luck!
 
If the amp shorts out to ground and the ground goes hot, I will get quite a shock! (will it kill me? will 400 volts stop my heart? yikes!)

For such a thing to happen you need would multiple unlikely faults to occur - and it would be even more unlikely for 400V to be the result (that would require a further series of more unlikely failures), just mains voltage would be the most likely possible outcome.

So basically you would almost certainly need someone to do it deliberately, and know what they are doing as well.
 
Ok, thanks for the reassurance Nigel. I'm still going to install that circuit in my guitars. I bought the resistors (500 volt metal film 5%) and capacitors (1000 volt!).
 
Ok, thanks for the reassurance Nigel. I'm still going to install that circuit in my guitars. I bought the resistors (500 volt metal film 5%) and capacitors (1000 volt!).

Are you sure your bridges are grounded anyway?, in my experience they aren't, which is why you get hum from them. Just check with your meter.
 
In N.A. and the NEC, the neutral should never be fused.
A two pin plug is normally polarized in order to eliminate the mistake when wired correctly.
Max.
 
Nigel: All electric guitars and basses are grounded, usually wired to the bridge or tailpiece, or the springs in the back of the guitar on a vibrato (tremolo) system.
 
Seymour Duncan 60 Watt Power Supply Schematic.jpg
Okay so I purchased the schematic for my amp from musicparts.com. I can't tell from the schematic if the fuse goes inline with the hot or the neutral. Can anyone tell by looking at this?

I have done some household wiring and I know that black is hot and white is neutral. I wonder if they have followed it here. It is interesting that the fuse and the switch are on separate wires. If the switch switches the hot off, then the fuse should have gone in the neutral wire in the plug I replaced. Any ideas? Thanks!
 
Well for US NEC wiring color code is that Black is ~HoT~ and white is neutral. So it appears the fuse is on the white wire.
 
Since line power feeds only the transforner, the position of the fuse and switch don't make much difference. The transformer would need to short interally to create a hazard.
 
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