position of fuse in Champ circuit

[d. spree]

I am about to help a friend and his son build a Champ. I have found a couple of wiring diagrams and each has a different position for the fuse. One has the hot wire from the mains go first to the fuse, and then to the on off switch. That's the way I've done it in the three 5e3s I have built. A drawing from Weber has the fuse come after the on off switch. Why the difference?

[soundguruman]

I am about to help a friend and his son build a Champ. I have found a couple of wiring diagrams and each has a different position for the fuse. One has the hot wire from the mains go first to the fuse, and then to the on off switch. That's the way I've done it in the three 5e3s I have built. A drawing from Weber has the fuse come after the on off switch. Why the difference?

Both ways protect the amp, but it's better before the switch.
If the switch (wires or other) shorts to the chassis, the fuse will blow.
Also, we like double pole switch, that shuts off the neutral also, besides shutting off the hot wire.
Nowadays, just about all amps have double pole shut off switch...it's safer.

[Jazz P Bass]

Power source-fuse-switch.
It is good practice, also, to specifically fuse the hot leg of the mains (120V service).

[Jazz P Bass]

Just thought I would add that the reason for the fuse is NOT to protect the amp.
It is there to isolate the equipment from the mains.
So with that in mind I think you can see WHY I said Power Source- Fuse- Switch.

[Mick Bailey]

A caveat about fusing - one terminal of the fuseholder is still at mains potential when the fuse blows, so make certain that the fuseholder is the right type and is wired up correctly. With many fuseholders there's a right and wrong way to wire them (is nothing straightforward?). Not all fuseholders are safe when opened and sometimes at gigs people forget to unplug the amp.

You'll see schematics for older amps where the neutral was fused, possibly to make the fuseholder 'safe'. No longer a good idea - it doesn't offer full protection for some fault conditions.

I can see the arguement for fusing after the switch - the fuse blows and the switch can be turned off to isolate the fuseholder. The downside is if there was a neutral-live short within the switch (if it's double pole), or a live-ground fault somewhere between the switch and the fuseholder, the equipment fuse wouldn't 'see' the short and it would rely on a fuse or circuit breaker upstream to let go.

[Enzo]

When the fuse was in the neutral line, I think it was more a matter of layout than a electrical rationale. If we wire the switch and fuse all in the hot lead, then we have to join the neutral and the transformer primary wire "in midair." But when wiring an old Fender chassis, they could run the hot wire to the switch, and the neutral wire to the fuse holder, and so no mid-air splices are needed.

Old Fenders - and other amps - had fuse holders with caps that did not extend far into the holder. The cap came off, and the end of a potentially hot fuse was right there accessible to touch. They can't make those any more. Modern fuse holders typically have a cap that reaches far enough into the body of the holder, you cannot touch a fuse end. The contact is thus also too far down to touch.

[loudthud]

Remember, the fuse doesn't protect your amp, it protects the world from your amp.

[Leo_Gnardo]

And if that all wasn't enough... to the best of my knowledge UL-approved fuse wiring has the AC hot lead coming to the END tab on the fuseholder, not the one on the side. (If your fuse holder is one of those that has 2 tabs sticking out the end, then I don't know what...)

Feel free experts, to sound off if I'm mistaken.

Practically every amp I see is wired wrong. That's no reason we all can't do it right.

[Mick Bailey]

Modern fuse holders typically have a cap that reaches far enough into the body of the holder, you cannot touch a fuse end. The contact is thus also too far down to touch.

Here's a modern one where the fuse is live if the side terminal is wired 'hot';

20mm Finger Rls Fuseholder 10a 250v

There are quite a few others on the market.

[Enzo]

You are correct. But that part does not meet UL specs in the USA, which is what I am used to.