On/Off via the Neutral

[C Moore]

I have noticed several Fenders, including the one I am working on now, use the Neutral to turn the amp On and Off. Is there anything fundamentally wrong with that?
Thank You

[paulster]

It'll work just fine but it wouldn't get past any European safety laws and I'd doubt if it would pass current US ones either. It's not as bad as the 2-core cable and 'death' cap, but still far from ideal.

Switching the live wire means that in the event of a fault there's significantly less of the amp energised and able to deliver 120V from a very low impedance and high current source.

It should be wired live -> mains fuse (with the mains in on the bottom terminal and the mains out on the side terminal since the side terminal can be reached by a child's finger with the fuse out) -> switch. Ideally you'd use a double-pole switch too, to switch the neutral as well in case of a wiring issue at the outlet.

[C Moore]

Thanks Paulster, that all makes sense. My next question is how much noise is generated by the hot lead ? Is it really necessary for the Hot and Neutral to be twisted?
On a Fender Twin, the On/Off switch is quite a distance the transformer. But the neutral could be terminated to the transformer almost right where it enters the chassis. So what I am asking.... is there any noise issues with running a single 120 VAC line from the tranny to the fuse, then to the On/Off switch and back to the transformer? It would be traveling across the top of the power tubes to the fuse, and then from the O/F switch back to the transformer.
Thank You

[Phil_S]

Paul is really on target here with the safety issues. I have one minor consideration on which terminal to wire on the fuse holder. If you've got the kind of fuse holder that grips the fuse in the cap, do what Paul said. If you've got the kind that doesn't pull the fuse, you might want to consider wiring the line feed to the side terminal, as that connection is broken when you pull the cap. Little pinky's aside, for us grown-ups, we want the circuit to be cold as soon as we pull the cap. I realize there may be a mix of opinions on this, but that's my 2 cents (pence).

The real bottom line is that you shouldn't be using the cheap fuse holders, but the reality is, they are in use.

[paulster]

So what I am asking.... is there any noise issues with running a single 120 VAC line from the tranny to the fuse, then to the On/Off switch and back to the transformer? It would be traveling across the top of the power tubes to the fuse, and then from the O/F switch back to the transformer.

I usually route the wires around the side of the chassis since I also use voltage selectors on all my amps, so I have a number of wires that need to go from front to back to transformer. So long as they are as far away from anything carrying audio as you can get they will be fine even without being twisted.

I have one minor consideration on which terminal to wire on the fuse holder. If you've got the kind of fuse holder that grips the fuse in the cap, do what Paul said. If you've got the kind that doesn't pull the fuse, you might want to consider wiring the line feed to the side terminal, as that connection is broken when you pull the cap. Little pinky's aside, for us grown-ups, we want the circuit to be cold as soon as we pull the cap. I realize there may be a mix of opinions on this, but that's my 2 cents (pence).

I absolutely agree with you. I was assuming that all of the barrel-type fuseholders these days grip the fuse in the cap, as those ones that leave it floating inside the fuse have the potential to be fatal, and your method would be preferable where replacing the fuseholder isn't an option in these circumstances.

[C Moore]

All my questions and concerns have been well addressed, and I really appreciate it.
Thanks Again

[Structo]

I agree that ideally the power switch should be double pole so both hot and neutral are switched.

I've seen some amps with the fuse on the neutral side.
The argument I have read is that if the fuse is on the hot side before the switch and the fuse holder is the type that holds the fuse and you screw it in, that your fingers can touch part of the fuse and you can get shocked while screwing it in.

Of course you should unplug an amp before replacing the fuse but when these kinds of devices are designed they have to make them idiot proof.

If the fuse was after the switch, then it would not be hot unless the switch was on.

Not sure what UL calls for these days and there seems to be more and more restrictions that could be detrimental to tube amps.

[paulster]

A fuseholder designed to be mounted on the outside of the chassis has to be touchproof these days, so there is no excuse not to fuse the hot/live side in any new build. Ideally it should be integrated into the IEC inlet if you are using one to further limit the extent of the chassis that can remain live in a fault condition.

The point of the fuse is to interrupt the supply in the event of a fault, so if your fault involves putting 120V or 240V on the chassis or something like that then a fused cold/neutral would be about as useful as a chocolate teapot!

It's about time some of these old conventions and ideas were properly laid to rest, as there's a genuine risk that one of us could be responsible for the death of someone else otherwise.

[jaysg]

examples

[Structo]

A fuseholder designed to be mounted on the outside of the chassis has to be touchproof these days, so there is no excuse not to fuse the hot/live side in any new build. Ideally it should be integrated into the IEC inlet if you are using one to further limit the extent of the chassis that can remain live in a fault condition.

The point of the fuse is to interrupt the supply in the event of a fault, so if your fault involves putting 120V or 240V on the chassis or something like that then a fused cold/neutral would be about as useful as a chocolate teapot!

It's about time some of these old conventions and ideas were properly laid to rest, as there's a genuine risk that one of us could be responsible for the death of someone else otherwise.

I have seen those IEC inlets that have the small 5mm fuse holder built in but haven't tried one.
What you say makes sense about the possibility of having the hot side going to the chassis during a catastrophic failure of a switch or transformer.

I also like the idea of those dual gang switches for the AC supply.

Does anybody know if the companies supplying amp kits have to have the layout certified or not? I would guess not.
Or does that come under a different type of regulation since it is a hobby built amp?

What kind of hoops does a small amp company have to jump through in order to market an amplifier for retail sale to the general public?

Underwriters Laboratory and what else, CE or Conformité Européenne rating?
So you only need the CE mark if you plan on selling to European consumers?

I was reading a little on the CE rating and apparently you don't need to have your device tested, you can simply mark it CE and you're good to go.
Not sure about the UL rating.
The way I understand it, anything powered by house current or above has to be rated by UL. Battery powered devices are exempt.

[Phil_S]

Gosh. I didn't expect so much commentary on fuse/line safety. Good stuff. You can't be too safe.

['67_Plexi]

Thanks Paulster, that all makes sense. My next question is how much noise is generated by the hot lead ? Is it really necessary for the Hot and Neutral to be twisted?
On a Fender Twin, the On/Off switch is quite a distance the transformer. But the neutral could be terminated to the transformer almost right where it enters the chassis. So what I am asking.... is there any noise issues with running a single 120 VAC line from the tranny to the fuse, then to the On/Off switch and back to the transformer? It would be traveling across the top of the power tubes to the fuse, and then from the O/F switch back to the transformer.
Thank You

No need at all. Neutral is at ground potential, twisting it with a live wire has no use except to save on a few cable ties. The only time twisting AC wires has a noise use is when they come from a centre tapped transformer and each wire is above ground potential.....ala filament wires.

Keeping mains cabling short and especially the live cable from the IEC to the fuse is just sensible.

In some Countries such as England there is fuse in the actual mains plug which protects the cable and the apparatus. In other Countries such as the US....the mains fuse is in the apparatus itself, so keeping any wiring short is just sensible.

Switching and/or fusing the neutral is a bad practice....beause the neutral is at ground potential. Yes it will blow if there is a fault but the live is still live in the amp and the chassis is a ground in itself. You still have a potentially dangerous stuation.

Running mains cables near any of the power amp circuit causes little to no noise. It takes a lot more than a mains cable to induce noise in the power stage. The closer you get to the preamp the more noise you will get though.

Alan.

['67_Plexi]

A fuseholder designed to be mounted on the outside of the chassis has to be touchproof these days, so there is no excuse not to fuse the hot/live side in any new build. Ideally it should be integrated into the IEC inlet if you are using one to further limit the extent of the chassis that can remain live in a fault condition.

The point of the fuse is to interrupt the supply in the event of a fault, so if your fault involves putting 120V or 240V on the chassis or something like that then a fused cold/neutral would be about as useful as a chocolate teapot!

It's about time some of these old conventions and ideas were properly laid to rest, as there's a genuine risk that one of us could be responsible for the death of someone else otherwise.

I have seen those IEC inlets that have the small 5mm fuse holder built in but haven't tried one.
What you say makes sense about the possibility of having the hot side going to the chassis during a catastrophic failure of a switch or transformer.

I also like the idea of those dual gang switches for the AC supply.

Does anybody know if the companies supplying amp kits have to have the layout certified or not? I would guess not.
Or does that come under a different type of regulation since it is a hobby built amp?

What kind of hoops does a small amp company have to jump through in order to market an amplifier for retail sale to the general public?

Underwriters Laboratory and what else, CE or Conformité Européenne rating?
So you only need the CE mark if you plan on selling to European consumers?

I was reading a little on the CE rating and apparently you don't need to have your device tested, you can simply mark it CE and you're good to go.
Not sure about the UL rating.
The way I understand it, anything powered by house current or above has to be rated by UL. Battery powered devices are exempt.

Hobby amps fall through the hoops....but if you build one and sell it and it hurts someone..........you're all done.

All these regulations are to protect people and the environment.
Demonstration of Compliance is different depending on what Regulations you are following.
RoHS and CE you have to be able to prove that your products are compliant. Having an independant Agency test is one way.
Documentation is the key to all these. If it's not documented, it hasn't happened.
For the European RoHS amps, we keep a database file per model of every part and sundry item used and a copy of their RoHS compliance documentation.

UL testing is brutally expensive and only the biggest players in the game can afford to do this. It isn't law but having certification would most certainly play on your side in a liability case that went to court.
Falsely putting a UL label on your product could end your business, yet I know some Companies do it.

To build and sell to the public even at small part time levels should only be done by people that can prove they are qualified to carry out the work. "I've been tweaking amps since 1978 and I was a Member of 18Watt" doesn't count as qualified if something goes wrong and you are facing a judge.

You also have to be fully insured and you have to be fully aware of the laws governing the intended final destinations for your products and make steps to show that you are following them.

Alan.

[Structo]

Thanks Alan, that clears that up.