Question about filtering.

Discussion in 'Amp Tech Center' started by Paul G., Aug 18, 2019.

  1. Paul G.

    Paul G. Friend of Leo's

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    I recapped my AB165 Bassman recently, and the more I play it, the more it seems like I'm not getting enough filtering. There's an ugly doubling, especially around A or notes that would have a strong A harmonic.

    The schematic calls for two 70uf/350V in series for 35uf/700V. 70uf/350V not available anymore so I put 2 80uf.

    Now, the interesting thing is the caps I removed from the first positions were 250uf, so I'm guessing this was a problem for the last guy? I'm going to try going up, but the amp is more or less stock, why would it need such a variation from the schematic. Just trying to improve my knowledge here.
     
  2. Wally

    Wally Telefied Ad Free Member

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    Paul, if that amp did have 2 x 250mfd caps in series in the first stage, then perhaps you got used to a strong, punchy low end and extreme articulation, eh? I have never installed that size, but I have used 2 x 200mfds in series. That amp would punch you in the chest. Fwiw, take a look at some high end boutique amps. You will see large capacitance I. The first stage of most of them...especially if they are other than small.
    Now, could the amp have other problems? Sure. Ime, with 2x80’s...which is as close as one can get to the original 2x70’s and might even be within the tolerance range, right?....one of these amps should have good sonic production. The attack will not be as punchy, and articulation is not as precise especially when the amp is pushed.
     
  3. Paul G.

    Paul G. Friend of Leo's

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    Yup, punchy as all get out! I got it with 7581s in there, and I assumed that was part of the big powerful sound. Thanks for your input. I'll experiment with some bigger caps in the first stage.
     
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  4. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Ghosting of notes relating to the mains frequency are a common sign of poor filtering, but one would expect the B at 61Hz to be more of the culprit than the A at 55Hz.
     
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  5. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    How would you solve that ^ B issue, Bendyha?
     
  6. Wally

    Wally Telefied Ad Free Member

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    Ime, Adequate filtering, which is the opposite of poor filtering, resolves that problem. Fender’s original 2x75mfd arrangement is adequate. I like more. More filtering and different approaches elsewhere in the circuit lead to very lively, articulate, hot amps.
     
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  7. Paul G.

    Paul G. Friend of Leo's

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    I was using a guitar as the source, and it was probably somewhat sharp with the AC off.
     
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  8. Commodore 64

    Commodore 64 Friend of Leo's

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    SS rectification? Go back to the 250uF in series.
     
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