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B15 Preamp issue

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  • B15 Preamp issue

    Hiya folks, just completed a single channel B15 pre w/ 12au7 CF stage, all crammed into a cute little enclosure.
    Having an issue with the Treble control, when it is zeroed the amp breaks into what seems like a low Hz oscillation. Not a squealing or wailing mind you, more like an angry, prehistoric hornet.

    I've triple checked my wiring to this schematic:
    http://unofficialampeg.com/schematics/b-15-n68.gif

    ...and tried replacing the pot, no dice. Anything suspicious in that schem?
    Thanks--
    Last edited by hylaphone; 12-30-2013, 09:08 PM.

  • #2
    Since you only have 1/2 of the schematic built try 500K treble.
    What amp are you using, and how is it coupled?
    As usual, the layout of the circuit can make quite a bit of difference.
    So show us the layout, and how you crammed it into a tiny box.

    I suspect you are amplifying heater buzz from AC filaments.
    So, try it again with DC to power the filaments. That will confirm the problem.

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    • #3
      I should add, the onset of this noise is very sudden at the last 1/10 or so of CCW rotation.

      Wouldn't heater buzz enter gradually?

      I will try a different pot value.. and snap some pictures later.

      Testing this thing through a SS power amp/PA speaker.

      It plays beautifully otherwise, very quiet and convincing

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