Mains fuse in the neutral wire

[wpaulvogel]

Why do some amps have the neutral wire fused? Isn’t the purpose of the fuse to protect the “house “ from an over current situation. If a short to ground occurs before the fuse, there’s no protection inside the chassis.
If the secondary becomes over current, the fuse will open and protect the internals, hopefully. Some opinions would be helpful.

[R.G.]

Got examples?

Your concept is correct - neutral should not be fused in any case that comes to mind.

But I have seen neutral fused in some poorly thought out three wire conversions. The old two wire amps often put one AC line wire to the fuse, one to the power switch. I have never figured out why, other than it's convenient to hand wire this, as the fuse and switch each provided a convenient terminal lug to wire to. But if you do the simple three-wire conversion, safety grounding the chassis and removing the death cap, if you don't also rewire the fuse AND switch into the hot wire and run neutral directly, you might come up with a fused neutral.

I can't remember seeing fused neutral in other situations. But - maybe? Got examples?

[martin manning]

The old two wire amps often put one AC line wire to the fuse, one to the power switch. I have never figured out why, other than it's convenient to hand wire this, as the fuse and switch each provided a convenient terminal lug to wire to.

This is where I'd put my money ;^)

[wpaulvogel]

The example I’m going to use is Marshall. Plexi’s are wired this way.

[R.G.]

The old two wire amps often put one AC line wire to the fuse, one to the power switch. I have never figured out why, other than it's convenient to hand wire this, as the fuse and switch each provided a convenient terminal lug to wire to.

This is where I'd put my money ;^)

Me too, but I have a new resolution to be kinder, gentler, and less dogmatic.

[R.G.]

The example I’m going to use is Marshall. Plexi’s are wired this way.

Can you give me a schemo link? I don't keep Marshalls in my schemo folders.

[gktamps]

I have seen this many times. I correct it any time I see it.
If I recall, the JTM45, and possibly the 18 watt, but definitely with many old audio amps like Dynaco, even the VTA modern versions of this amps.

[R.G.]

Thanks for the links.

Yes, those are all done "wrong" by modern standards. They come from a time when early all outlets had only two terminals. Even though these were (almost always) line and neutral in the USA, there was no way to know which way the plug would be turned in the wall socket, so an argument could be made that a fuse in either line was just as good.

Not so, as we know from the deaths of some folks. The issues raised by having only two AC wires were improved by going three-wire.

You're doing the right thing gtk - eliminate that danger any time you find it.

[wpaulvogel]

I’ve seen it in 6 Marshall amps that had been converted or they had three wire plugs anyway. I changed them and said nothing to the owners. I just posted this because I just built a MetroAmp 1969 100 watt and the instructions specified to also wire it neutral fused. I’ve only seen these 6 plexi amps in my time, keep a lookout for this in the future. I also see it in Marshall schematics with 3 wire connections. Look at the 2203 from 1970 and the 1959T from 1970. Very odd.

[gktamps]

For the past few years I've taken a break from guitar amps to work on tube and solid state audio gear. I've seen many, many pieces of two-wire equipment that compound the problem by using non-polarized jacks in preamps for other gear, non-polarized plugs for their own supply, among other things we now see as safety concerns. Wiring convenience didn't seem to be a factor, because they could have easily been wired as we now see as properly without any real modification. Another risk is that many owners of equipment clipped off the tangs on the polarized plug spade to remedy ground loops.

I am working on a preamp that cost $3,500 back in the early 1980's that has all of these "features". I could have replaced the non-polarized jacks with polarized, but instead bypassed them, and replaced the nonpolarized power supply cord. I did this to a Fender Bandmaster I owned 40 years ago because I kept getting shocks when I'd plug in a cooling fan to that jack.

[Leo_Gnardo]

The old two wire amps often put one AC line wire to the fuse, one to the power switch. I have never figured out why, other than it's convenient to hand wire this, as the fuse and switch each provided a convenient terminal lug to wire to.

This is where I'd put my money ;^)

Me too, but I have a new resolution to be kinder, gentler, and less dogmatic.

I'm with Martin on this one. Dogmatic? Well that is a dog in his avatar. Bow-wow-wow!

[pdf64]

Even in 1960, Vox were doing it right https://el34world.com/charts/Schematics ... 301960.pdf
Selmer too https://el34world.com/charts/Schematics ... ss_mki.pdf
If Marshall had used a competent engineer to help put their prototyped 5F6A copy into production then it would have complied with UK good practice, rather than being a hodge podge of North American and UK wiring arrangements.

[wpaulvogel]

My sing sentiments exactly pdf64!

[Tony Bones]

"Neutral" wire? What's that?

I am aware that in some countries one of the two power wires happens to be connected to ground back at the breaker box, but really that's not a good reason to treat it differently, at least not as far as I'm concerned. Start out by thinking of both wires as equally hot, and then go from there...

[pompeiisneaks]

"Neutral" wire? What's that?

I am aware that in some countries one of the two power wires happens to be connected to ground back at the breaker box, but really that's not a good reason to treat it differently, at least not as far as I'm concerned. Start out by thinking of both wires as equally hot, and then go from there...

That's a very true point. here in the US TONS of houses have them wired wrong/backwards, and if I find them in mine I fix them, BUT any well designed electronics device should expect this type of wiring error.

~Phil