Need AC30 TBX technical help!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Mike M, Nov 17, 2005.


  1. Mike M

    Mike M Supporting Member

    Messages:
    431
    Joined:
    Jan 9, 2004
    I have a mid 90's Vox AC30 that blew a fuse the other day. I replaced it with one that radio shack said would work, but after installing it, it happend again and again. Does anyone know what the exact fuse replacement number should be? Also, could there be other issues with the amp that is causing it to blow fuses?

    Thanks
     
  2. VintageJon

    VintageJon Member

    Messages:
    155
    Joined:
    Nov 3, 2005
    Location:
    Austin TX
    Lots of fuses in these things. Is it the Mains fuse?

    Shorted/Leaky output tube is likely suspect.
    Fuses from Rat Shack are Import-Unknown-Cheap-As-They-Can-Find.

    Remove power tubes and rectifier tube.

    Get 2 Little Fuse or Buss fuses of the correct rating. (Made in the USA)

    Install fuse.

    Power-up. If fuse blows it's something else. Not tube...

    If fuse lives then install rectifier tube.
    (If fuse blos then it's the rectifier...)

    No blow?
    Install power tubes.
    Fuse blows- it's a power tube.
    Doesn't blow- it's a mystery or the wrong Rat Shack fuse

    Hope this blow-by-blow helps...

    -Jon
     
  3. PaulC

    PaulC Member

    Messages:
    1,819
    Joined:
    Jan 11, 2002
    Location:
    Murfreesboro,TN
    I used to be a service guy for korg, but I don't have the factory schematics on that amp anymore. That said the number one problem I had with those amps was rect tubes going south, and popping fuses.

    One of the things I did to the amps that made the rect tubes much happier was to rewire them. For whatever reason the rect tubes are wired wrong in the amps. They take the output off of pin 2 when the cathode connection inside the tube goes to pin 8. There is a difference, and the tubes held up much better after fixing it.

    Later, PaulC
     
  4. Enzo

    Enzo Member

    Messages:
    15
    Joined:
    Oct 3, 2005
    Location:
    Lansing, Michigan, USA
    When fuses blow repeatedly, the problem is not the fuses. Radio SHack parts might be cheap, but they are not so cheap that they don't work at all.

    As with any amp, it is most likely a bad output device, followed on the list by a power supply component.
     
  5. Mike M

    Mike M Supporting Member

    Messages:
    431
    Joined:
    Jan 9, 2004
    After tinkering last night, here is waht happend. I reinstalled another fuse and replaced the rectifier tube. The amp played for about 2 minutes then the amp started squeeling pretty bad. I looked inside of the amp and one of the power tubes was glowing red hot and smoke began to come out of the amp. I turned it off and replaced the two power tube with a JJ. Turned the amp back on and everything seemed to work fiune. I am a little skittish to push it too hard and damage something. The rectifier that I replaced was a brand new GT Gz34. So based on that should I just replace the one bad power tube and re install the older recifier or do you think the rectifier was the culprit all along? Or buy a new rectifier and replace all the power tubes. They power tubes were EI El84's.

    Thanks
     
  6. TheAmpNerd

    TheAmpNerd Member

    Messages:
    1,062
    Joined:
    Sep 23, 2004
    Location:
    Tejas
    Do what PaulC said first.
    Followed by vintageJon.

    Check condition and values of
    resistors and caps connected to
    the output tubes/sockets.

    If all those are okay, then you
    are probably good to go.

    Don't forget, SMOKE came out
    of something and there goes
    your tone.

    Once you let the smoke out of
    the components, it/they should
    be replaced.

    If there are any unusual readings
    post them and the good folks
    here will help.
     
  7. AC30TBX

    AC30TBX Member

    Messages:
    5
    Joined:
    Dec 17, 2010
    On my '03 Vox Ac30tbx I have a black and red wire on Pin 2 of the Rectifier and none going to Pin 8. Does one or both wires on Pin 2 go to Pin 8??
    Thanks!!
     

Share This Page