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Original Vox AC50 MKII sounds distorted

Discussion in 'Amps and Cabs [BG]' started by purplehearted, Apr 4, 2020.

  1. purplehearted


    Mar 27, 2020
    Recently, I picked up a Vox AC50 MKII head. I have connected it to a Vox Foundation cabinet (18inch speaker). Unfortunately, I can't seem to get a decent tone out of it. The normal channel sounds muddy, and I can't get a good clean tone out of it. When I put the volume at around 11 o'clock or higher, the tone is overdriven / distorted. I suppose this is not supposed to happen. Also the brilliant channel has the same problem (except for the muddiness). I tried it with several bass guitars (EB-3, Precision bass, Jazz Bass, Framus 50/150)
    Does anyone know what the problem could be? If it's helpful, I can make some pictures of the electronics.

    I am thinking about bringing it to a tech, but perhaps it's just a simple thing.
  2. abarson


    Nov 6, 2003
    Santa Cruz
    Have you tried it with a different cabinet?
    I say take it to a tech.
    Omega Monkey and RSBBass like this.
  3. purplehearted


    Mar 27, 2020
    Haven't yet tried it with a different cab (don't have one). But I can connect another head to the cab to check perhaps.
  4. abarson


    Nov 6, 2003
    Santa Cruz
    Yes, you want to discern if the problem is the cabinet or the head.
    Omega Monkey likes this.
  5. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    That's a pretty simple amp, the first step is to be sure the problem is with the amp itself. Check with a different bass too, a dying battery in an active bass can cause this as can a defective or turned down on board tone control.
    Omega Monkey likes this.
  6. Roland GR 88

    Roland GR 88

    Sep 16, 2013
    It's approaching 60yrs old ...... take it to the doctor.
    AstroSonic likes this.
  7. Wasnex


    Dec 25, 2011
    There is not much you can do without electronics knowledge. If both channels are distorted it probably rules out the preamps. Is the line voltage set to the proper range? Is the speaker good and is the amp configured for the impedance of the speaker? The schematic shows a link that can be configured for either 8 or 15 ohms. Is the speaker cable good? Is the instrument cable good? Is the bass good?

    It's a fairly simple amp. The first common tube is the phase inverter (PI) and it may be bad. You could try reseating all of the tubes. You could also try swapping a fresh 12AX7 into the PI position. If you don't have a fresh tube, you can try one of the preamp tubes in the PI. They should all be 12AX7s. Other names for 12AX7 include 7025, ECC83, ECC803, etc.

    It could also be the bias is way out of range. You should not mess with the bias unless you have the proper tools and know how. If the plates of the output tubes are glowing cherry red they are pulling too much current, you should turn the amp off immediately. It's possible one of the El34 output tubes or the GZ34 rectifier is bad. The problem could also be a wide variety of faulty components at various locations in the amp. I would suggest taking it to a technician for any of the problems in this paragraph.
    purplehearted likes this.
  8. purplehearted


    Mar 27, 2020
    Thank you. The head states 16 ohm on the back (written on steel plate with pen/marker). Perhaps the impedence has been changed over time by a previous owner. But it should still work fine on a 15ohm foundation cab right?

    Besides that, the output tubes seemed to get quite hot and light bright orange, but not cherry red. The output tubes also lightly touched the grill/vent above it.
  9. Wasnex


    Dec 25, 2011
    15 ohms is essentially the 16 ohm setting. The images I found had a panel on the back with a removable link, so the user could switch the settings. Sounds like your's is hardwired to 15 ohms. Should be fine with a 15 ohm speaker as long as it has not be changed over the years.

    I don't know how sensitive this amp is to impedance mismatches, but I doubt this is the source of your problem.

    The heaters should glow orange. Here is a video that shows what red plating tubes look like:

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