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Mechanical Buzzing in Peavey Classic 30 (Capacitor?)

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  • Mechanical Buzzing in Peavey Classic 30 (Capacitor?)

    Hey there,

    I have a Peavey Classic 30 that is emitting a faint but noticeable mechanical buzz. It appears it's coming from the filter caps, which in this amp are 6 10UF@450 caps.

    Video here:

    Should I suspect a bad cap?

    All voltages are spot on. Amp sounds really good. Operating well with no strange noises or buzzing in the output.

    The amp came to me with a broken power transformer (FYI, it wasn't the thermal fuse). I've just installed a new PT.

    Classic 30 schematic attached here is not an exact match to this amp, but should be close enough. This one is one of the newer models with Standby switch. If anyone has the correct schematic, I would love a copy.
    Attached Files

  • #2
    Actually I think the buzzing is coming from the .01 ceramic that is across the HV AC before it hits the rectifier (C38 in schematic). It all but disappears when I touch the cap with a chop stick.

    Updated video (may need headphones to properly hear):

    I was reading a bit about singing ceramic caps. Seems normal, but I still find it unpleasant. Think I should replace the cap? Would replacement with non ceramic type be ok here?


    • #3
      Why do you think it is the filter caps? I heard no change as you probed them. I'd suspect mechanical noise from a transformer, especially the one you just put in. Grab it with your hand, can you mute the buzz?
      It's weird, because it WAS working fine.....


      • #4
        Sorry, difficult to capture the audio on a phone recording.

        The buzz is definitely coming from right around that area on the PCB, seems like it is the ceramic cap.

        No mechanical noise coming from either transformer.

        Edit: confirmed 100% it's the ceramic cap. When grabbed from both sides the buzz is gone.


        • #5
          Q: clearly you have the chassis out on the bench since you are grabbing parts. But can you hear this when the amp is assembled into the cab? In which case why fix it. If the cap is really noisy, swap it out, it is just a cap. Or squirt some silicone around it. Or hot melt.
          Education is what you're left with after you have forgotten what you have learned.


          • #6
            I guess I was just wondering if it is a sign the cap is on it's way to a failure.

            I'm wondering why they chose ceramic for this application? And would this not need to be an 'across the line' X2 cap?

            Click image for larger version

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            • #7
              It's not across the mains line though, just across the HT which is isolated from the mains by virtue of the transformer. Ceramic caps are cheap and readily available in high voltage ratings.


              • #8
                It isn't across the line, it is across a transformer secondary. No matter what the part may do in a failure mode, it cannot cause any connection to the mains and the danger there.

                As to why ceramic, well, why not? it isn't in the signal path. It is also a 1000v part. In ceramic those are dirt cheap. In film caps, less cheap. it is also smaller. Look at C38 nestled in there by the fuse. A film cap would be somewhat larger.

                Ceramic caps are by their nature somewhat piezoelectric, so they can buzz... or be microphonic. That is part of why I suggested a glob of glue.
                Education is what you're left with after you have forgotten what you have learned.


                • #9
                  Copy. Appreciate all the info, I have learned a few things today, did a bit of reading on snubber caps and singing ceramic caps.

                  Replaced with a fresh cap and buzz is completely gone. I understand it would have been fine to leave it in, but feel better about getting rid of the buzz.

                  These amps actually sound pretty good


                  • #10
                    As soon as I read the title of the post, a buzzing ceramic cap across the rectifier diodes was the first thing that popped in my head. When I saw your photo, I was nearly positive that was the culprit. But, it looks like you tracked it down as well.
                    If you've never heard this happen before, it can be a little disconcerting. But this is extremely common when ceramic caps are used as snubbers. As Enzo mentioned, ceramic caps by nature are piezoelectric, and they buzz at the frequency the diodes switch on and off. Totally normal, and nothing to be concerned about.
                    Fender Hot Rod/Blues amps are notorious for this. At least a few times a year, someone writes a post asking the same question you asked.
                    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.


                    • #11
                      I am glad you found it. I must admit my first reaction wsa the famous EL84 tubes rattling. The Classic 30 is so prone to thins, I used to market a tube retainer to clamp them in firmly. Mine competed with "Tom's Tube Tamer".
                      Education is what you're left with after you have forgotten what you have learned.


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