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Vox AC-30/6 TB problem

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  • Vox AC-30/6 TB problem

    Can anyone tell me if they?ve experienced the same thing with their Vox AC-30. I?ve been trying to find a blog/thread of the same problem but have been unable to find anything similar to this specific problem. I just purchased this amp and have been using it non-stop for the last two months without any issues. It is the class-A power AC-30/6TB model made in (I believe 1997) with the blue Vox Alnico speakers.

    When I brought it out to my last gig I noticed that after about a half hour of playing a disturbing sound was intermittently starting to happen.

    It starts to produce a strange octave pedal type noise where when you play one note it sounds like it?s being multiplied (much like an octave pedal) in a slightly out of tune way and starts to sound very muddy. When this ?effect? is produced, it?s accompanied with a loud hum. The amp is usually pretty quiet hum-wise. It happens in degrees, but when it?s in full blown-out warble mode you can?t play an open chord with any consonance. I?m almost positive that it?s heat related as when I used it at our last practice it only started happening after a few hours of playing and at which point the amp was really hot. The funny thing is that it remained in this ?mode? without popping a fuse or dying? It kept on working but making this horrible sound.
    When it starts to happen, it?ll make the sound for maybe a few seconds and then come back to normal. It?s only as time goes on that it starts to become more frequent until it stays in this ?mode?.
    Both lights stay on all the while, with no flickering or anything.
    Can anyone help?
    Thank You!

  • #2
    Yes. Try a new set of power tubes. If it is not the problem, at least you will then have a spare set. But your symptoms sound to me like a failing output tube.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Thanks Enzo!

      I'll try that.
      Cheers!

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      • #4
        My first thought was the filter caps, one of the 1st 2. If they go bad, or there's a bad connection to them, the amp will turn into a 30 watt 60Hz modulator. But like Enzo says, the 1st step should always be subbing in known good tubes. Peter.
        My band:- http://www.youtube.com/user/RedwingBand

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        • #5
          Originally posted by pdf64 View Post
          My first thought was the filter caps, one of the 1st 2. If they go bad, or there's a bad connection to them, the amp will turn into a 30 watt 60Hz modulator. But like Enzo says, the 1st step should always be subbing in known good tubes. Peter.
          Thank guys i'll try that to.

          best regards.

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          • #6
            I've seen this or something very similar before on a 64 model. Is the annoyance aggravated by the tremolo control? Or any of the other pots?

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            • #7
              Just a quick thought I had one from this era with a faulty hum balance pot.
              It looked like it was made by Marshall as some of the switches etc looked like parts Marshall stocked.
              Could be complely wrong there though.
              Its a pcb mounting preset. From memory had a problem with the last coupling caps into the power amp.
              The amp had a rough life though sleeping on the streets.....no I jest... but it was a bit rough round the edges.
              The schematic was hard to find but they are about.
              Just thought might be worth checking.
              ahh just re-read your post '97 not 70's vox..so the above doesn't apply
              Last edited by oc disorder; 07-23-2008, 08:51 AM. Reason: ignorance

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              • #8
                "cold" solder joints? Thermal failure?

                Hi Vegetable Man,
                Have you thought about "cold" solder joints? If you have one around or some with cracks, the bad contact could act like a resistor of unpredictable ( and variable ) value and give rise to all sort of problems....This was not the case with older Voxes, which were hand-wired, but it could be true with the "newer" ones ( from 1970 onwards, with the exception of the hand-wired reissues )

                If the problem arises after playing for a while, as you stated, another reason could be some thermal failure, if so it could be good to have a "freezing gas" spray bottle at hand; these are helpful troubleshooting faulty components, if they fail heating up, usually cooling them down should bring the amp back in working order for some time, so spraying the components under suspicion one by one could reveal which the faulty one is.

                Hope this helps

                Best regards

                Bob
                Hoc unum scio: me nihil scire.

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                • #9
                  AC -30 hot warble solved!

                  Two rehearsals and one gig later it looks like the problem has finally been solved!!
                  Thank you to all of you who answered this thread with your suggestions it has been very helpfull! I forwarded all your responses to a tech in town.
                  So the culprit, sure enough was mainly a bad output tube. Given the nature of the problem I would've never thought it would've been that. And Enzo, pdf64 you're both the man! You guys called it!
                  Although a few things were done to the amp with varying degrees of success, (a few solder joints were re-soldered, a rectifier tube was replaced, some loose components re-tightened and a few other specifics that I'd have to ask my tech), but it was that 1st output tube which had a silver ring around it near the top which meant it (the tube) must have been running too hot.(?)
                  Anyway, the amp now sounds great and it's nice knowing that this forum exits and that there are people like yourselves out there answering them!
                  Thank You!
                  Sincerely,
                  Vegetable Man.

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