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Mains fuse location

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  • #1

    Mains fuse location

    Apologies in advance for the 101 questions but quick search didn't turn up quite the answers I was looking for.

    I'm switching out the two-prong cord in a friend’s Silvertone. It has the switch on one side of the primary and the fuse on the other. This seems to be fairly common in old amps (perhaps mostly in ones that originally used two-prong cords?).

    The current standard of the hot going to the fuse first, then the switch, then the PT and the neutral going straight to the PT makes sense in that the fuse protects everything it can from the mains voltage....but... I always think "what if somebody who didn't know any better stuck a fuse directly in the socket without using the cap/holder, while the amp was still plugged in?" To me, this seems like the greater risk. This doesn’t appear to be the consensus. Why?

    Assuming the current standard is truly the right way to do it, should old amps be converted to this setup when the cords are swapped out?
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  • #2
    Originally posted by Ptron View Post
    The current standard of the hot going to the fuse first, then the switch, then the PT and the neutral going straight to the PT makes sense in that the fuse protects everything it can from the mains voltage....but... I always think "what if somebody who didn't know any better stuck a fuse directly in the socket without using the cap/holder, while the amp was still plugged in?" To me, this seems like the greater risk. This doesn’t appear to be the consensus. Why?
    You should use a 'touchproof' fuseholder which doesn't make contact to the fuse until it is screwed fully home. Before there were touchproof fuseholders the hot wire was connected to the end contact of the fuseholder not the ring. The end connector was too deep inside for the fuse to touch it when you were holding the fuse. Even if the fuse is on the neutral side you could still get shocked through the transformer primary.

    Dave H.

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    • #3
      Originally posted by Dave H View Post
      Even if the fuse is on the neutral side you could still get shocked through the transformer primary.
      Point taken. I guess I was thinking of the power cord lead (whichever one) going to the tip of the fuseholder and the ring being more difficult to accidently touch. So I guess the obvious answer would be to have to the power cord hot lead go to the ring. Thing is, the first couple tutorials I find tell you to hook it to the tip and IIRC this is what you see must often (maybe I'm mis-remembering that?) In the two fuseholders I checked, one new and one from the sixties, the fuse makes electrical contact with the tip when it's just sitting in there without the cap.

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      • #4
        The bayonette cap fuse holder has been around for ages. But look at the ones they sell now and the ones on a Fender amp of 30 years ago. They are not the same. The fuse is recessed. You cannot touch it while it is in contact with the mains current.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          So what's the right thing to do on an old amp? I'm guessing folks don't usually replace the fuse holder.

          Originally posted by Enzo View Post
          The bayonette cap fuse holder has been around for ages. But look at the ones they sell now and the ones on a Fender amp of 30 years ago. They are not the same. The fuse is recessed. You cannot touch it while it is in contact with the mains current.
          You can on the "new" ones I have. They came from Hoffman or some other dedicated guitar amp part supplier so maybe they're overzealous recreations or something.

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          • #6
            Possibly new old stock. No one would "recreate" an decades old design on a $1 part that violates current safety practices.
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              Wire it up properly, to interrupt the hot, and don;t worry about the odd circumstance where if, if, if, if then someone gets a shock.
              Education is what you're left with after you have forgotten what you have learned.

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              • #8
                I must have been 10 at the time and trying to fix an old Zenith TV.

                none of the tubes wer glowing so I pulled the fuse while it was plugged in. Well no problem, the fuse was blown. BUT when I put the new fuse in ZZZZZZZTTTT!

                you never forget lessons like that

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                • #9
                  Originally posted by stingray_65 View Post
                  I must have been 10 at the time and trying to fix an old Zenith TV.

                  none of the tubes wer glowing so I pulled the fuse while it was plugged in. Well no problem, the fuse was blown. BUT when I put the new fuse in ZZZZZZZTTTT!

                  you never forget lessons like that
                  See! It happens.

                  Just ribbin' but seriously, I do think this is a more likely scenario than a lot of the other safety issues we worry about.

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                  • #10
                    Just keep on foiling Darwin with your safety fuseholders and you'll have no one but yourself to blame when the human race eventually becomes too stupid to sucessfully tie non-velcro shoes!

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