Crackling noise in my Hybrid build

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by bob-i, Nov 4, 2005.

  1. bob-i

    bob-i Member

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    I've had this amp for about a year and continually modding it. Right now it's pretty muc a Dumble Hybrid-A with some 97 type ckts.

    I'm getting a terrible crackling noise from either the PI or output sections. I've connected the preamp to another power amp and no problem, plus it doesn't start until I hit about 11:00 on the preamp out volume.

    http://www.patmedia.net/bob-ingram/Dumble/crackling.mp3

    My PI voltages are B+ 386
    plate 1 222
    plate 2 232
    cathodes 72

    The cathode seems high to me, I was expecting about 50-60. I'm wondering if this might have any bearing on the noise.

    Another thoughts?

    Thx.
    Bob
     
  2. scottl

    scottl Member

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    Maybe that is why the plates are really really low??

    386 B+ should yield 280 or so on the PI plates......
     
  3. bob-i

    bob-i Member

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    I replaced the tail resistor with a 15K which brought the cathode down to 50V, but it didn't change the crackling so I put the 22K back. I forgot to check the plates.

    I'm at a loss to why these voltages are off, and what's causing this noise.
     
  4. bob-i

    bob-i Member

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    So I swapped PI ground resistor from 330 to 820. The crackling stopped but the voltages are still off. :confused:
     
  5. TubeAmpNut

    TubeAmpNut Member

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    That sounds like distortion from parasitic oscillations. Use your scope and watch your waveform off the speaker jack. Try a small cap from the PI input to ground (like 10pF - 47pF). If it's PO's and lead dress doesn't clear it, that should do. Shielded cable to your power tube grids helps as well. -You might notice a little cut in highs with the cap fix...

    BK
     
  6. bob-i

    bob-i Member

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    Thx... you nailed it.

    I put the shielded wire on the output tube grids and maybe 80% of it stopped. I thin put a 47pF cap on the PI input and the rest cleaned up, with no real loss of highs, this amp has plenty of highs.

    I wish my scope would pick these up but all I have is a 40 year old tune type scope and I don't think it'll pick up these VHF oscillations.
     
  7. pfrischmann

    pfrischmann Member

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    Is there anything worse than PO?
     
  8. TubeAmpNut

    TubeAmpNut Member

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    I'd rather troubleshoot PO than stand next to someone with BO! -But PO sure sucks.

    :D

    BK
     

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