Fast blow or slow blow fuse?

Discussion in 'McIntosh Audio' started by Borat, Feb 13, 2009.

  1. Borat

    Borat New Member

    Messages:
    25
    First I have to thank you guys for sharing your knowledge.

    I initially came to this forum to ask about the possibility of converting my two MC275's voltage from 120V to 240V. Lots of members chipped in and told me that it's an easy thing to do - all I need to do is to switch a few tabs on the primary wirings of the transformer.

    So I paid an electrician with over 20 years of experience of hi-fi repair to do the switch for me. He told me that he switched a few tabs of the primary wirings, and then tested his work by putting 220V through the primary wiring and then measure the output of the secondary wiring. Everything was looking good.

    After I took the amp home, I went to Radioshack and bought some 2A fast-blow fuses. I switched out the original fuse which has a 4A rating (I don't know whether it's fast-blow or slow-blow)

    But when I tried to start the amp by giving it 220V, the 2A fast-blow fuse blew immediately. A friend of mine told me that I shouldn't use a fast-blow fuse because the initial current surge would definitely blow it right away.

    Does anyone know which fuse type is the right one to use? I must admit that the original 4A fuse supplied by Mac looks just like a fast-blow fuse. The slow-blow fuses I saw at Radioshack all have slightly thicker filaments. But my friends say some slow-blow fuse filaments are just as thin and it is difficult to tell the difference.
     
  2. jdandy

    jdandy Super Member

    Messages:
    4,198
    Borat.......I looked at a schematic I have on the original MC275. It shows F1 as a Slow-Blow fuse. The original MC275 calls for a 5A fuse at 105-130 volts, but that amp has a receptacle mounted on the amp that is powered through that fuse, as well as the amp. If your amp is labeled for a 4A fuse at 120 volts, the 2A slow-blow fuse at 240 volts should do the job.
     
  3. Borat

    Borat New Member

    Messages:
    25
    Thank jdandy..you're my hero..
     
  4. 'cisco

    'cisco Well-Known Member

    Messages:
    894
    Location:
    808/415
    Borat....he has that effect on people :D
     
  5. jdandy

    jdandy Super Member

    Messages:
    4,198
    Enough........Please, I'm blushing. :eek:

    Thank you.
     
  6. drbryant

    drbryant Member

    Messages:
    53
    Location:
    Los Angeles and Tokyo
    Could I revive this thread? I also have an MC275, and have used fast-blo fuses because someone told me that I am less likely to damage the amp with a power surge, as the fuse will blow easily, and that there was no downside to a fast-blow fuse, other than the cost of replacing them from time to time. True?
     
  7. wolfcub

    wolfcub Active Member

    Messages:
    146
    Location:
    UK
    The slow blow fuse is designed not to blow during the initial high current switch-on surge so I am very surprised if your fast blow fuses survive if they are the same rating as the slow blow - can I ask if you are using the same amp rating fast blow as you should slow blow ?
     
  8. drbryant

    drbryant Member

    Messages:
    53
    Location:
    Los Angeles and Tokyo
    Yes. The sticker above the fuse on the base of the MC275 says Slo-blo T5A 250V. So I have purchased regular 5A 250V fuses, and they seem to work ok, although I had a blow recently (nothing too violent - about 1/8" of filament was gone).
     
  9. BwanaJim

    BwanaJim Well-Known Member

    Messages:
    917
    Location:
    Puget Sound
    Normally I would advise you to replace your Quick-Blo fuses with Remington 22LR cartridges but with your switch to 240V it could get messy....
     
    John James likes this.

Share This Page