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Fuses blowing repeatedly on a Sunn 300T

Discussion in 'Amps and Cabs [BG]' started by Bad Gas, Dec 21, 2004.

  1. Bad Gas

    Bad Gas Guest

    Sep 28, 2004
    Okay, so there are three fuses that protect the six 6550 tubes. The furthest left of these fuses blew twice yesterday. The head is set to 4 ohms, which my 8-10 cabinet is rated, so what would cause this problem?
  2. Eric Moesle

    Eric Moesle

    Sep 21, 2001
    Columbus OH
    An electrical short. Have it evaluated by a qualified tech.
  3. Sparkie


    Nov 15, 2004
    If you are blowing fuses then you probably have a short of some kind. There are VERY DEADLY amounts of voltage hiding in your electrolitic caps even when the amp is unplugged. Based on your question I highly recommend you take your amp to the nearest reputable amp tech.

    If you want to know more about tube amps and want to service your own amp I suggest you visit Ampage over at www.firebottle.com and pick up an amp book or two. Dave Funk's book (of Thunderfunk fame) is very good for starters.

    Good Luck
  4. If the tubes are biased too "hot" that can lead to blown fuses. Be sure the ones you replace them with are the same rating. It could be a simple matter of a bias job or possibly bad tubes. Ignore the stupid bias LED's; I had a Sunn 300T and the bias lights always indicated a problem, and I know for a fact that amp was biased fine because I'm the one who set it. Find a good tech.
  5. Thunderfunk


    Mar 27, 2004
    McHenry, IL
    I'm not familar with that head and not sure what fuse you mean, but it sounds like a shorted 6550. Could be as simple as changing the tubes and replacing the fuse if it's the B+ fuse that's blowing. Sometimes (especially with 6550's because they are high gain tubes the grid is close to the cathode) the grid shorts to the cathode (especially under vibration) at that means the bias voltage goes away, the tube runs away, and that blows the B+ fuse.

    Dave "Thunder" Funk
  6. dwylie75


    Aug 23, 2006
    i have the same issue with mine. I guess i will be taking it in!!

  7. i had similar issues with mine as well. the tube on the farthest to the left malfunctioned and fried a wire on the inside. it cost $170 to fix. better nip that problem in the butt soon before it gets expensive.
  8. Bassmec


    May 9, 2008
    Ipswich UK
    Proprietor Springvale Studios
    Thats why I like KT88's
  9. dwylie75


    Aug 23, 2006
    i might have to put a set of KT88s in there