fender cathode bypass cap question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers T' started by texwest, Dec 15, 2009.

  1. texwest

    texwest Member

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    I replaced the cathode bypass caps in my blackface princeton reverb and don't like the sound quite as much. I replaced them with the standard sprague atom 25 uf caps. I measured the capacitance on the original caps and they had all drifted up to about 35 uf. what sound difference would raising this value from 25 to 35 make? I guess, in other words, what does higher capacitance do on the bypass caps?
     
  2. TweeDLX

    TweeDLX Member

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    Higher values allow more low frequencies through, lower values restrict low frequencies.

    Mike
     
  3. texwest

    texwest Member

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    Would 25 to 35 make that much difference in the cathode bypass caps?

    Thanks
     
  4. Axekisser

    Axekisser Supporting Member

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    I don't think so. The 25ufs let most everything through anyway. You're probably hearing a new/different cap material. I always tell my customers to let things break-in a bit too.
     
  5. schmidlin

    schmidlin Member

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    Found this a while back, and it helps in understanding the hi-pass roll-off of cathode caps. Draw a straight line accross it intersecting the C & R values and it will show the cut off frequency. Add a chart that shows guitar string frequencies and you have yourself a good start.
    [​IMG]
     
  6. zzmoore

    zzmoore Supporting Member

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    That is an awesome chart. I am going to ask an honest, sincere, question, not being a Smart Ass... Is it accurate/legitimate?
    Thank You
     
  7. texwest

    texwest Member

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    Thanks, I'm wondering if they need to be broken in, so I'm running a loop thru the amp right now. The amp sounds a little brittle and harsh with the new caps. I'm hoping a break in will resolve this. If that chart is correct there shouldn't be that much difference, especially since the frequencies seem to be lower than any note on the guitar.
     
  8. schmidlin

    schmidlin Member

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    Accurate? I'm sure there are math formulas that back it up, somewhere. It's a tool, not the answer.

    But more importantly, you need to understand that it is a frequency roll-off, not a hard cut. Not sure how sharp the roll-off is, or if this shows where it begins, peaks or ends. You EEs can chime in at any time. ;)
     
  9. schmidlin

    schmidlin Member

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  10. zzmoore

    zzmoore Supporting Member

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    Thanks -
    Those are good observations. I assume the indicated frequency is the cut line (if the number is 100Hz then no lows pass below that). But I never considered the taper or start of the roll.
    Just to reiterate, I am wondering if that chart is "generally" accurate. Apparently it is. I have printed it and will keep it for easy, visual reference.
    Thank You
     
  11. Patrick620

    Patrick620 Member

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    This thread is becoming very helpful for me. Thank you, to all who have contributed.
     

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