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Speaker Output Current Draw??? PBG?

Discussion in 'Amps and Cabs [BG]' started by VtypeV4, May 28, 2004.

  1. My old 50 watt Valve amp occasionally blows its output fuse which can be a little annoying but it doesnt matter because it doesnt really gig anymore. My point being, the fuse is rated at 2amps, is this not enough, should it be rplaced with a Time delay type? The Amp itself was made by my stepdad about 8 years ago so there are no official ratings etc. Its built from a LINEAR L50, based on a Marshall design incorporating a solid state rectifier. The mains Transformer is in SVT size league and puts about 600v on the plates! Is this likely to be the cause of these fuse failings? The other point worth noting is that the original circuit design did not have this fuse in it, my stepdad put it in to protect the output transformer just in case something goes wrong.

    Thanks, Matt
  2. I think I would keep the same fuse....why risk valuable components for the sake of a cheap fuse?
  3. fair enough i guess, would be very lucky to find another linear50 output transformer...
  4. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Output fuse on a valve amp? That doesn't make any sense. If the fuse blows, it's like running the amp without a load which will fry your output transformer.

    Are you sure that the 2 A fuse in is the output? Or is it the mains fuse?

    How old is the amp? Maybe it needs a cap job or something.
  5. An underbiased amp will blow its mains fuse from excessive current draw. What kind of tubes are you running?
  6. The output fuse is b4 the transformer (I think), The tubes are JJ EL34s
  7. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA

    Exactly. Output fuses make no sense in a tube amp, whether they're before or after the transformer. More likely it's a B+ fuse, or what Marshall calls an "HT" fuse. Definitely check into the cap job. Electrolytic filter caps have a useful lifespan of ten or fifteen years. When they begin to go bad, they can easily cause fuses to start blowing.
  8. notanaggie

    notanaggie Guest

    Sep 30, 2003
    600 volt plate supply and 50 watts is kinda wierd, but Ok I guess, if it uses 6550 or El-34 type tubes. If it uses 6L6, that's way high, in my opinion, and could be a problem.

    Dunno where the fuse is, but if it is used at high voltage, it shouldn't be a standard fuse, they are only rated 250V. There is a fatter style fuse for high voltage.

    Fuses ratings depend on temperature. if the fuse gets hot, the rting goes down somewhat.

    if the 2A blows on a 50W amp from plate current, there is a problem. The 50W shouldn't draw much plate current to get 50W at a 600V plate voltage.

    Might be from arcing in the tubes, if they are 6L6, or if they are bad.

    Might be from filter capacitor problems.

    I don't know your stepdad, could there be a design issue? Might be some parts over-rated a bit, that are now failing.