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AC30 continous buzz

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  • #1

    AC30 continous buzz

    Hello,

    I wonder if anyone could help me diagnose this problem with my late-80's anniversary reissue AC30.

    After i switch the amp on, after about 15 seconds a continous buzz starts which sounds a little like earth hum.

    It is independant of volume, and independant of input. ie even if the amp is on with all volumes down and no guitar plugged in, the buzz will still be there.

    It is loud enough to be noticable even when playing, and certainly too loud for recording.

    The buzz is the same under all conditions. I have tried removing the earth on the plug just to test and no change.

    Does anyone think they may have any idea what the problem is?

    Thank you

    S Price
    Tags: None
  • #2
    The fact that it is the same with the volume turned down,and I assume you mean completely turned down,I would suspect the output stage,possibly mismatched or worn power tubes or it could be time for new filter caps.

    Comment

    • #3
      Hi Stokes

      Thanks for your post.

      That's correct it is the same with the volume shut off. On further investigation there is one factor which is affecting it - the cut eq. the other eq's do not but the cut does.

      I'm not too familiar with the workings of the ac30, but does this give any more clues or support your theory?

      Many thanks

      S Price

      Comment

      • #4
        The cut pot I see on a 1989 version is wired as a variable resistor in series with a cap between the two sides of the PI feeding the output tubes,so this leads me to believe the hum is coming from the PI,could still be the filters tho.Try pulling the PI and see if the hum is still there.Are you capable of opening the amp and probing with a multimeter?

        Comment

        • #5
          Does the hum disappear when you turn the īNormalīvolume up halfway?

          Comment

          • #6
            Hi Stokes

            Sorry for the late reply.
            I should say that my electronics skill is on the amateur side but not hopeless!
            I tried removing the inverter but still the buzz was there. Just to clarify, the inverter is in the preamp section of tubes correct?
            Also this model is actually a 1990 (sorry). i have attached a couple of pics of the amp out. i know there is not a lot to see, but it just may help with visual identification.

            http://www.tracques.com/vox1.jpg

            http://www.tracques.com/vox2.jpg

            I could probe around with a meter.

            Two Tone, the buzz is constant regardless of volume level of normal and brilliant channels. However your question has found another piece of info out. The buzz dimishes by about 90% when i put the trem channel to around 60%. This sounds like a phase inverter issue to me but do you agree?

            Many Thanks

            Stuart

            Comment

            • #7
              The fact that the noise is still there with the PI removed only leaves the power section.Although the PI is a preamp type tube (12AX7) it is actually part of the output section.Removing this tube and still getting the noise removes suspicion from the preamp section and PI.So I am going back to my initial thought,mismatched or bad power tubes or time for a re-cap,or both.Your caps are 16 years old,6 years older than what I consider their useful life.From the pictures they look like cheapo Jap caps,I would say you will surely notice an improvement in the overall response and especially the bass response with fresh filter caps.I would also suggest using good quality Sprague brand,much better than those Xicon or Nichicon caps.

              Comment

              • #8
                Pull the power tubes one at a time and listen with only one in there each time. Do both sides make the hum? DO both pass signal?

                Verify that the screen grids on each power tube have B+ voltage. I am looking for one side of the power amp not to be running and the hum is from misbalance. Check at each tube, not just one.
                Education is what you're left with after you have forgotten what you have learned.

                Comment

                • #9
                  Originally posted by S Price View Post
                  Hi Stokes

                  Sorry for the late reply.
                  I should say that my electronics skill is on the amateur side but not hopeless!
                  I tried removing the inverter but still the buzz was there. Just to clarify, the inverter is in the preamp section of tubes correct?
                  Also this model is actually a 1990 (sorry). i have attached a couple of pics of the amp out. i know there is not a lot to see, but it just may help with visual identification.

                  http://www.tracques.com/vox1.jpg

                  http://www.tracques.com/vox2.jpg

                  I could probe around with a meter.

                  Two Tone, the buzz is constant regardless of volume level of normal and brilliant channels. However your question has found another piece of info out. The buzz dimishes by about 90% when i put the trem channel to around 60%. This sounds like a phase inverter issue to me but do you agree?

                  Many Thanks

                  Stuart
                  Stuart,this is an issue with most 90īs Korg models,
                  poor heater wiring,
                  though I believe your amp uses DC heaters?

                  Comment

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