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Discussion in 'Amps and Cabs [BG]' started by BuckoSama, Apr 13, 2010.

  1. BuckoSama


    Jun 30, 2008
    Plattsburgh, NY
    So, to start with, I'm cheating, because I'm actually asking this on behalf of my brother/gui****. But, you know, they're a necessary evil, and I know you guys are always helpful, so I wanted to ask for your help.

    He's got a Fender Super Twin Reverb. I think it was discontinued in 1983 or so. Anyway, he blew the fuse, which was not the original, but had been in there for years, and predated his ownership. The back of the amp and the schematic call for a 6 1/4A 125V fuse. What had been in it was a 6.5A.

    Anyway, after lots of running around in real life and on the internet, I can't seem to find that particular rating available anywhere.

    We've used three 6A 250V (bussmann AGC fast-action) fuses in the last two weeks, but we can't afford to get stranded at a gig, and I worry about the effects of repeatedly blowing the fuse on all the tubes. They get us up and running for a while, but we can't rely on them.

    Bussmann doesn't even make any 6 1/4 A fuses except in Slo-Blo. The amp doesn't SAY slo-blo on it, so should I assume it would be a dangerous mistake to use one? Could be what's intended, I don't really know how to know that. Also, those 6 1/4A slow-blo bussmanns are 250V, not 125V. Does that make a difference?

    Obviously, I don't know the ins and outs of the safety of different amps, and I was hoping that someone out there could help me decide on what would be a good choice to take the place of this seemingly-mythical fuse. It's also a possibility that I just haven't located the correct place to order such esoteric things on the web yet, so any guidance in that area would be helpful.

    I appreciate the help even though this is a guitar amp. It is quite a nice one, however, and I want to treat it properly.
  2. Axtman

    Axtman Supporting Member

    Mar 3, 2008
    Seattle, Washington
    Here is my $.02.

    I can't imagine that fuses are super accurate, therefore a 6 amp fuse should work fine in lieu of a 6.25 amp fuse. Underfusing is better than overfusing.

    I agree that if it does not say "slow blow" on the amp that "fast blow" should be the default.

    Typically shorted output tubes (or transformers) cause fuses to blow on tube amps. Cranking the amp should not blow a fuse. If it does, you have more serious problems.

    Always bring a few extra fuses in your gig bag or taped to the amp.
  3. BuckoSama


    Jun 30, 2008
    Plattsburgh, NY
    The 6A 250V fuses are blowing consistently (3 over 2 weeks including 3 shows and 5-8 practices). The blow when the amp is first turned on, not when we're actually playing. All the tubes were replaced in late 2007.

    Does any of that help? Maybe we should replace a power tube? We've never had to do that (i've only got experience with the little 12AX7 tubes in my compressor) - how do we diagnose a power tube?
  4. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Fast-blow fuses are just that, they blow fast on an overload. Turning on an amp typically IS an overload, as the caps charge up (solid state rectifiers) and tube filaments warm up and increase in resistance, etc.

    Fast-blow fuses also are much more susceptible to blowing early just because the particular manufacturer's fuses of a particular amp rating don't take a surge well...... even though they will reliably blow on correct current per the general fuse spec.

    I don't see any particular reason why a fast blow fuse has to be used......... And I do see a couple reasons why not to use one.

    Pretty much all my Fender schematics show a slow-blow... go for it.
  5. toomanybasses


    Feb 20, 2009
    One of the companies I use sells what your looking for;

    I do remember one band I was in someone had a Fender Bandmaster that had the same problem, we did fix the problem but I don't remember at the moment exactly what we did. It was back it the late 70's so you will have to forgive my memory but if I do recall how we fixed it, I will post the answer.

    Good Luck
  6. Axtman

    Axtman Supporting Member

    Mar 3, 2008
    Seattle, Washington
    Here is another place to buy fuses. http://www.tubesandmore.com/

    Try the 6A slow blow fuse. If you still have a problem then go to an amp tech.

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