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Thread: Ampeg SVT-VR Ch 2 Buzz/Hum

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    Senior Member nevetslab's Avatar
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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.

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    Senior Member TimmyP1955's Avatar
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    I would not have thought that a non-ferrous metal would work, unless it's an RF issue. ??

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    Quote 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.

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