Help with Vox AC15 problem -rectifer sparking + blowing fuse

Discussion in 'Amps and Cabs' started by smogfalls, Apr 6, 2011.

  1. smogfalls

    smogfalls Member

    Apr 8, 2007
    Nottingham, England
    So I'm having a problem with my recently acquired Vox AC15HTV1 amp. It's on of the heritage handwired series from a couple a' years back. I've had it for a month now and it's been working well, and sounding great, until a few days ago. I turned it on as normal, and a few seconds later there was a light crackle, and the whole thing turned off.
    The mains fuse had blown, so I replaced it with the correct one, started it up and the same thing happened again. I did some research and saw that the rectifier or power tubes could be the cause so I took the rectifier out, put a brand new fuse in, started the amp and it was fine... no fuse was blown. So, happily, I assumed it must've been the EZ81 rectifier, so I went and ordered a replacement. Recieved that yesterday, (brand new JJ EZ81), popped it in, started the amp, everything was fine for maybe 30 seconds, the tubes were all glowing, and then a slight crackle along with blue sparks inside the recifier tube happened for maybe 3 seconds and the power died again! Again, it had also blown the mains fuse.

    Any idea in what could be up with this? I'm out of my depth with this, and am on the verge on taking it to a tech, but just wanted to check in with you knowledgable folks first incase I've missed something obvious, or something cheap to repair.

    Thanks in advance,
    Xan :)

    P.S: I should also point out that although the amp was running well before this, it did have some moments of odd behaviour. ie, the power light would go off randomly even though the amp was still running and performing perfectly. And, in pentode mode there was always a loud hum, loud enough that it seemed odd to me. But switching to triode mode made this disappear completely and the amp was as quiet as a mouse!
  2. MichaelK

    MichaelK Member

    Jan 7, 2002
    Fort Mudge
    Sounds like you need a tech.

    You might ask him to start by testing the rectifier. When I got a new AC30CC years ago I was warned by a tech that new rectifier tubes coming out of Asia are unreliable and often mislabeled, and he suggested that I have it tested at some point. Sure enough, the stock Sovtek 5AR4 tested as something other than a 5AR4 – never found out what exactly, but definitely not a 5AR4. I replaced it with NOS.

    Obviously I can't say if that's the cause of your problem, but it's a piece of information that might be of help. Best of luck!
  3. JMItones

    JMItones Member

    Nov 7, 2008
    Nashville, TN
    It sounds to me like the rectifier tube you received was damaged. Often the fuse blowing means the rectifier is bad.
  4. Griz

    Griz Member

    Sep 23, 2006
    Toronto, Canada
    If you're getting sparks, don't forget to check the socket for signs of arcing. If this happens, a carbon "path" will sometimes develop between pin sockets, making future arcing more likely.

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