bypass cap on silverface twin reverb

Discussion in 'Shock Brother's DIY Amps' started by duffer426, May 26, 2019.

  1. duffer426

    duffer426 TDPRI Member

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    This bypass cap was in v5 circuit of a silverface twin reverb, can I use a 25 mfd 25 volt instead?
    what size cap is that anyway? IMG_0802.JPG IMG_0779.JPG IMG_0778.JPG IMG_0802.JPG IMG_0779.JPG IMG_0778.JPG IMG_0802.JPG IMG_0779.JPG IMG_0778.JPG
     
    Last edited: May 26, 2019
  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I believe that's a 5uF 50v. The silverface AA1069 Twin Reverb layout shows 5uF 25v for V5B and going with higher voltage rating is not a problem. I'd stick with a 5uF.
     
  3. Andy B

    Andy B Tele-Afflicted Gold Supporter

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    The 5uf cap replaced the 25uf cap of the Blackface amps. as I recall the smaller cap is supposed to speed up the response of the speed control. On some BF amps you can hear a lag if you sweep the speed control from one end to another.
     
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  4. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    You'll be much more likely to find 4.7uF than 5, as that is the modern day "standard" value.
     
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  5. duffer426

    duffer426 TDPRI Member

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    Thanks Rob,Andy B, and Corliss1 for the info. I`ll order a 5uF or 4.7 don`t have any of those on hand. Hey Corliss, I`am from Charlotte,Mi. moved to southern IN. in 1973
     
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  6. Ten Over

    Ten Over Tele-Meister

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    The DC voltage on the cathode of V5B is higher at mid speed than it is at high speed. The bypass capacitor has to discharge when the speed control is changed from mid to high speed and a 5uF capacitor will discharge faster than a 25uF capacitor. While the cathode voltage is still higher than usual for high speed the magnitude of the amplified voltage is diminished. The speed changes instantly when you turn the knob, but the intensity takes a little time to reach its full potential. Fender probably couldn't have cared less about this phenomenon.

    With the 25uF/100K cathode on V5B, the lower vibrato speeds have a higher intensity than the higher speeds. If you reduce the value of the bypass capacitor, the cut-off frequency increases and the lower frequencies get amplified less. This evens out the intensity across the vibrato speed range.

    The DC voltage on the cathode of V5B can approach 25V, so a capacitor rated at 50V would be prudent.
     
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