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HELP - Vox AC15C1 Loud Hum

Discussion in 'Amp Central Station' started by Scott57, Jan 15, 2021.

  1. Scott57

    Scott57 Tele-Meister

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    Location:
    Pennsylvania
    I recently plugged into my Vox AC15C1 and was greeted with a very loud hum. Trying a couple of things, I decided it must be a bad tube and ordered a complete new set of tubes. I replaced the tubes today but I'm still getting that loud hum. Here's what I tried:

    Different guitars with different pickups (SC and HB) - no difference;
    Different instrument cables - no difference;
    Different amp inputs (Normal and Top Boost) - no difference;
    Different amp power cables - no difference;
    Reverb/Trem footswitch connected and not connected - no difference;
    Different electrical outlets - no difference;
    A complete set of new tubes (all five were replaced) - no difference;

    Hum is not present until Stand-by switch goes to ON;
    Hum is not present until Master Volume is turned up;
    Once the tubes are warm the hum isn't audible with the Master Volume turned completely down;
    Hum increases and decreases in volume with only the Master Volume;
    Normal volume and Top Boost volume knobs seems to have little effect;
    Hum is present whether an instrument is plugged in or not;
    You can barely hear the guitar when it's plugged in.

    Tremolo will affect the hum;
    The amp is about ten years old with the original stock tubes (until I replaced them today)
    The amp worked fine the last time I used it approximately six months ago. Nothing changed since then except the introduction of the hum.
    It's a Vox AC15C1 but not hand-wired.
    The bright cap mod was never done on the amp.
    All stock with the exception of the reverb tank, which was replaced approximately nine years ago.

    It sounds like huge grounding issue or - from what I heard on a YouTube video - a bad filter cap.

    I guess the new tubes (or one of them) could be bad (purchased from the Tube Store) but what's the possibility?

    Any suggestions? Can filter caps be replaced on this amp model?

    Outside of changing tubes, anything else will need to go to an amp tech but I like to know what the possible issue might be and if it can be corrected/fixed.

    Thanks.
     
  2. Wally

    Wally Telefied Ad Free Member

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    Location:
    Lubbock, TX
    With the MV up and the input gain down, it hums???
    Warm the amp up.....in play mode. Turn the := down to a bare minim7m so the hum does not run you out of the room. After a minute or few, quickly touch each power tube. You should be able to make a very brief contact for part of a second without burning yourself. The power tubes should have some heat in them. Is one of them cool to the touch? I ask because I am suspecting a dead power tube. With one dead tube, you could have a hum coming from that imbalance in the output. If both EL84s are equal in temperature, they are both conducting current. Sub in a 12AX7 in the phase inverter position...one triode could be dead. If that is no cure, sub in a 12AX7 for the preamp tubes one at a time.
    if none of this points to a problem, you can pull the tubes in order fro: V1 through the power tubes. When the hum stops, you have found the problem area.....but it is tech time...inside the chassis work.
    A bad filter cap will cause a hum with all controls at zero. A bad cap somewhere circuit can cause a hum that is dependent upon a pot adjustment. A bad ground can give you a hum anytime depending where the bad ground is.
    if you don’t f8nd the problem with these simple tests, it is tech time.
     
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  3. gusfinley

    gusfinley Tele-Afflicted

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    SLC, UT
    This sounds like a bad filter cap prior to the output section. The master volume on this amp mixes the out-of-phase signals going to the power tubes. If the master volume kills the hum then it is originating before the phase inverter.
     
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  4. gusfinley

    gusfinley Tele-Afflicted

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    Joined:
    Jun 11, 2014
    Location:
    SLC, UT
    Also keep in mind that this amp uses operational amplifiers to mix the normal and top boost channels and to mix the dry and reverb signals. You could have a bad op-amp power supply cap, too.
     
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