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Ampeg SVT-VR Ch 2 Buzz/Hum

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  • Ampeg SVT-VR Ch 2 Buzz/Hum

    While I had this same amp here searching for output level dropping, I found Ch 2 once again had very nasty loud 60Hz buzz/hum from the Standby Switch coupling into the Ch 2 input stage immediately adjacent to the switch. I've normally been successful in dressing those two yellow Mains wires to the S/B switch, tightly twisting and wrapping spirally with the AC mains wires, and folding them away from the input connector. My notes from May 2018 indicated I had done so and got it quiet. Yet now, it was louder than ever.

    On one of the SVT-VR's, I had gone as far as cutting up some 7mil Brass Shim Stock from a roll of it I've been using for the past 30 yrs, and folded up another shielded box to put the Standby Switch into.

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    After cutting the flat (4" x 2"), and punching a 1/2" hole a bit above center with a 1/2" dia Whitney XX Punch, I cut the flaps, folded the sides and back (2" deep, 1" wide, 1" side wall, 1/2" rear wall), along with the flaps, then soldered them together to hold it all together, and wrapped the outside walls and bottom with mylar insulation tape.

    It's a snug fit, but goes in, and after connecting the switch back up, and folding those two wires back away from the input cable (which has been re-routed along the front of the PCB under all the pots, instead of along the rear of the PCB in parallel with the heater traces), I mounted the chassis back in to have a listen.

    Buzz & Hum gone now. I can crank the Ch 2 Volume, Treble & Bass fully CW, Ultra-High & Ultra Low on, and just hear the normal tube noise without the annoying Buzz that comes with the stock amp. Sometimes on SVT-VR's, you have to go further than just lead dress, as was the case here. I don't know why it was so bad yesterday and not like that last year when I last serviced it.
    Logic is an organized way of going wrong with confidence

  • #2
    I would not have thought that a non-ferrous metal would work, unless it's an RF issue. ??

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    • #3
      Originally posted by TimmyP1955 View Post
      I would not have thought that a non-ferrous metal would work, unless it's an RF issue. ??
      Any conductive and grounded surface can shield against electric (interference) fields. The mains wires mainly radiate electric AC field. Ferromagnetic materials also shield against magnetic fields. Magnetic interference fields are mainly produced by transformers and other inductors.
      - Own Opinions Only -

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      • #4
        Originally posted by nevetslab View Post
        While I had this same amp here searching for output level dropping, I found Ch 2 once again had very nasty loud 60Hz buzz/hum from the Standby Switch coupling into the Ch 2 input stage immediately adjacent to the switch. I've normally been successful in dressing those two yellow Mains wires to the S/B switch, tightly twisting and wrapping spirally with the AC mains wires, and folding them away from the input connector. My notes from May 2018 indicated I had done so and got it quiet. Yet now, it was louder than ever.

        On one of the SVT-VR's, I had gone as far as cutting up some 7mil Brass Shim Stock from a roll of it I've been using for the past 30 yrs, and folded up another shielded box to put the Standby Switch into.

        Buzz & Hum gone now. I can crank the Ch 2 Volume, Treble & Bass fully CW, Ultra-High & Ultra Low on, and just hear the normal tube noise without the annoying Buzz that comes with the stock amp. Sometimes on SVT-VR's, you have to go further than just lead dress, as was the case here. I don't know why it was so bad yesterday and not like that last year when I last serviced it.
        I have the very same issue with a new Ampeg 50th Anniversary SVT, with its standby switch adjacent to the preamp circuit board. Would this sort of shielded box work in that application? I am in the Burbank/Los Angeles area.

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        • #5
          Originally posted by robal View Post

          I have the very same issue with a new Ampeg 50th Anniversary SVT, with its standby switch adjacent to the preamp circuit board. Would this sort of shielded box work in that application? I am in the Burbank/Los Angeles area.
          Is this 50th Anniversary SVT an SVT-VR, or an SVT-CL. I assume it's a VR, as this buzz doesn't happen on the SVT-CL family.

          My shop as at CenterStaging in Burbank....on Winona Ave, sling-shot distance from the runway at Burbank Airport (Hollywood Way & Winona. 3407 Winona Ave, back of the building...easy to find.
          Logic is an organized way of going wrong with confidence

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          • #6
            Just a quick 2 cents here: Get a magnet and put a piece of aluminum between it and something the magnet would normally stick to. Then do the dame with copper and other metals. Simple experiment with results that will amaze you. The magnet still works but aluminum seems to be quite the magnetic shield. Of course the original post isn't about magnetism but hey whatya get for 2 cents nowadays.

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            • #7
              Originally posted by SimTech View Post
              Just a quick 2 cents here: Get a magnet and put a piece of aluminum between it and something the magnet would normally stick to. Then do the dame with copper and other metals. Simple experiment with results that will amaze you. The magnet still works but aluminum seems to be quite the magnetic shield. Of course the original post isn't about magnetism but hey whatya get for 2 cents nowadays.
              Aluminum doesn't shield the field of a magnet any better than air.
              In your experiment a piece of wood of similar size would have the same effect, namely increasing the distance between the magnet and the attracted object.

              Magnetic field lines can't be stopped anyway. The can be redirected by ferromagnetic objects, though.
              Last edited by Helmholtz; 08-04-2023, 03:38 PM.
              - Own Opinions Only -

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