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60 hz hum

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  • #1

    60 hz hum

    Actually not sure if 60 or 120 but likely harmonics...

    You may recall the 5AR4/p-p5881/2-12AX7 HOME BUILT amp that had fried power tranny with the Zener diode in center tap.

    I sized-up an new Hammond pt with the help of you guys and we hit the nail on the head. B+ came out perfect for the power tubes and I can add the zener to my collection of stuff. The b + on v2 is fine appx 200 but v1 seems low at 110 vdc.

    Amp sounds good except there is hum with volume pot fully ccw but the tone control seems to have effect. Hum is gone if I pull the 2 preamp tubes. Hum also seems to disappear when I max out the volume pots fully cw.

    The electrolytics are good. The input jacks clean though shouldn't matter.

    The leads from the PI to the push pull are relatively long and unsheilded.

    Where would you guys look to eliminate the hum? Thanks.
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  • #2
    Layout and grounding. Start with the heater wiring, how is that referenced to ground?
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      From recollection, center tap for green winding is grounded.

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      • #4
        Please do determine if it is 60Hz or 120Hz hum. They sound similar, are the same note one octave apart. Plus not likely to be a sine wave, so there will be overtones. But the base freq tells us where it comes from - 120Hz is power supply ripple, and probably from ground path issues. 60Hz hum comes from shielding, heaters, radiated noise, etc.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          Its 60 Hz, and it is coming from the HV (red leads) coming from the transformer. If I move the leads, which i twisted, and left relatively long, it will disappear.

          So, assuming I should 1) shorten the leads and 2)untwist? 3)shield?

          Likely nothing can be done with bypassing since it is ac at that point

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          • #6
            You want them twisted, but you also want them as far from sensitive wires as possible - wires that go to grids for example. All the rules for heater wiring apply to any other transformer wiring. So if a transformer wire and a signal carrying wire must go similar places, do not bundle them, no matter how neat it might look. Things like grid wires ought to cross AC bearing wires at a right angle when possible.
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              That solved it. I shortened, twisted tighter and ran close to chassis. Thanks.

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