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Need a suitable sub for a 500mA slo-blow fuse

Discussion in 'Amps [BG]' started by webmonster, Dec 26, 2013.

  1. webmonster

    webmonster

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    ...a slap for anyone who suggests tin foil or a nail ;)

    I went to get some spare fuses for my amps the other day and I couldn't get a particular one: a slo-blow 500mA fuse for the H.T. on my early 1970's Selmer Treble'n'Bass 50.

    Can I use a non slo-blow fuse instead? If so what value would be OK?

    Out of interest, why would this one need to be slo-blow anyway? If something nasty happens in the H.T. wouldn't it be better for the fuse to go 'pop' straight away?
  2. two fingers

    two fingers Loud Mouth Know It All Blowhard Gold Supporting Member

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    I can't speak to your amp. However many times when slow blow fuses are used it is because the equipment they protect may start up with a quick burst of in - rush current that is higher than the running current. If you have a regular fuse in it there is a possibility it could pop every time you turn it on. Again this is having zero experience with your amp. Rather just offering up a possible reason for the fuse being slow blow to begin with.
  3. beate_r

    beate_r

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    A 500 mA slow-blow fuse can be replaced by a 500 mA slow-blow fuse.

    BTW: the fuse is on the secondary, isn't it? Not all amplifiers have fuses on the HT secondary.

    With some lack of security (!!!!) slightly larger values can also be used, and if larger, the fuses might also react a bit quicker. A 600 mA slow-blow might still be pretty safe. You might also try if 400 mA slow works sufficiently reliable (It should fail during initial heating when the current drawn is largest).
  4. beans-on-toast

    beans-on-toast

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    You can use a ½ amp slo-blo fuse if that helps. :p

    If the fuse is inside the amp and it doesn't have a fuse holder, you might need one with pigtail leads (wires attached to each end of the fuse). Try an electronics supplier such as mouser.com, digikey.com, or newark.com.

    There is a surge in the HT when you first turn on the amp as the capacitors charge. This exceeds the normal operating voltage and current. This is why you need a slo-blo fuse. It is designed to take the initial surge without blowing.
  5. 1958Bassman

    1958Bassman

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    You could wrap the old one in aluminum foil (aluminium, for those of you who prefer the correct name).:bag:

    Do I get points for acknowledging the correct name? :D
  6. WingKL

    WingKL

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    A good sub for a 500mA slo blow fuse is a 500mA slo blo fuse. If these aren't the ones you are looking for, you can try different parameters and or other sites like newark.com or mouser.com.
  7. nashvillebill

    nashvillebill

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  8. beans-on-toast

    beans-on-toast

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    If the HT fuse inside the amp has blown, it could be that your GZ34 rectifier tube has an issue. If you change the fuse and it blows again, change the tube as well.
  9. Cirk

    Cirk

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    Heard about a guy who used a .22 LR cartridge as a fuse in his truck. Won an honorable mention for a Darwin award due to the injuries sustained. Yeah, he got hit in an unfortunate place.
  10. webmonster

    webmonster

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    A curse of 1000 plagues for those who suggested solutions other than fuses ;)

    Thanks for the explanations for why it needs to be a slo-blow fuse.
    No tube rectifier in this amp.

    Both the mains and HT fuses are easy to access on the back panel and neither have blown - I just want spare fuses for the gig bag.
    I'll try a different supplier.
  11. Cirk

    Cirk

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    Glad to give you something to laugh about. Possible sources for your fuses are parts express, mouser electronics and antique electronic supply. Best of luck in your search.
  12. beans-on-toast

    beans-on-toast

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    I was looking at the Selmer Treble N' Bass 50 MKII schematic that had the tube rectifier. It must be a different version of your amp.
  13. Mr. Foxen

    Mr. Foxen

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    Disclosures:
    Amp tinkerer at Ampstack
    Taping the spare inside the amp is a good plan.
  14. webmonster

    webmonster

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    Sorry, I should have been more specific given there were several versions made over the years. It is one of these:
    Treble 'n' Bass 50 S.V. from the early 1970's
    DSCF8808.JPG
    DSCF8809.JPG

    On another note - the light inside the original orange standby switch gave up so my tech replaced it with the green switch.

    I still have the orange one and it appears that it is not a sealed unit and that there is a neon inside. Is it feasible to replace this? I have no idea of its value/spec at this point.
  15. webmonster

    webmonster

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    That's clever - I'd never thought of this!
  16. beans-on-toast

    beans-on-toast

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    I've got the right schematic now. See below. There were a number of versions with this name.

    If you can open the switch, you can replace the neon lamp. The neon lamp needs to work with your AC power line voltage. As you can see in the bottom right hand corner, there isn't a part number for the lamp. It works with a range of AC line voltages from 110VAC to 245VAC. Sometimes there is a resistor soldered onto one leg of the lamp. The problem is finding a neon lamp that fits inside the switch. Best to find one locally if you can. Try http://nz.element14.com/, they are a big electronic supply company (called Newark in North America). They would have your fuses as well. You can buy the neon lamp just for your local voltage since the line setting on your amp doesn't change.


    [​IMG]

  17. hsech

    hsech Your opinion doesn't trump mine. Gold Supporting Member

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    In a previous life when I played guitar, I did this on a Fender Super Reverb more times than I care to remember. A chewing gum wrapper with a foil backing would work in a pinch also.
  18. BassmanPaul

    BassmanPaul Gold Supporting Member

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    Folk who do tech work just love players who do this. It's a constant source of incoming work!

    I had a very nice Marshall brought to me. The HT/B+ fuse was a nail and the mains fuse was wrapped in cigarette foil. Both transformers needed to be replaced.
  19. beans-on-toast

    beans-on-toast

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    What, nobody's used a paper clip in place of a fuse? :p
  20. Mr. Foxen

    Mr. Foxen

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    Disclosures:
    Amp tinkerer at Ampstack
    I hate changing transformers.

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