Amp blows mains fuse...maybe 1 out of every 10x?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by ChickenLover, Jan 16, 2008.


  1. ChickenLover

    ChickenLover Member

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    It does it immediately upon power up...standby switch is still in the standby position. This is a homebrew 50W NMV Marshall (built around 1999) and I did it a little differently. The first filter node (which is a 50x50 LCR can, both + lugs connected together) is straight off the diodes...then from there it goes to the HT fuse then to the SB switch. When I originally did it...it went from the diodes to the HT fuse and then to the filter then to the SB switch but it blew the HT fuse everytime. Now that the HT fuse is after the LCR can it doesn't blow the HT fuse. The mains fuse only does this maybe 1x out of every 10x power ups...if that.

    Also, I just happened to have these massive 25A FREDs around and that's what I used for the recifier in this amp. Do the smaller diodes we normally use 'limit' the inrush current at all?

    Thanks for any help/suggestions.
     
  2. John Phillips

    John Phillips Member

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    What fuse value are you using?
     
  3. ChickenLover

    ChickenLover Member

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    Hi John,
    The mains fuse is a 2A slo-blo and the HT fuse is a 1/2A fast-acting.
     
  4. John Phillips

    John Phillips Member

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    You probably just need a slightly larger fuse (2.5A slo-blo, if you can get one, or 3A if not). Marshall schematics show 2A for the original 50s, but 3A (at 110/120V) for the MkIIs, which means they must have found the 2A was not quite adequate.

    If it only blows on power-up and never when the amp is running normally, I think it's this and that it is safe to increase the fuse value just that little bit. Not the right thing to do normally but if it only blows rarely I don't think there is any other actual problem, it's just the initial surge which is doing it, and only by a fraction even then.

    I doubt the diodes have much or anything to do with it, since the forward resistance of even a standard 1N4007 type is very low anyway.
     
  5. ChickenLover

    ChickenLover Member

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    Thanks John. I'll up it to 2.5A and see how that goes.
     

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