Posted by Caucasian Blackplate (P) on June 28, 2009 at 12:27:17

I would like to do some parts swapping on an amp I have that needs a fairly large parafeed cap. Plate voltage is only about 190v, so I was considering moving the parafeed cap to between the ground and output transformer (it is now connected at the plate). How much AC can I expect across the cap at this loaction? I would presume very, very little, but I figured I should check with the experts before I perform this modification. The output section swings about 100v max pk-pk, and I am considering using a film cap rated for 250V-DC.

-Paul

• Nevermind, found my answer. - Caucasian Blackplate 13:46:35 06/28/09 (2)
  In Reply to: RE: AC on a parafeed cap posted by Caucasian Blackplate on June 28, 2009 at 12:27:17

  In case this gets hit on a future search, the article that follows has the answer built into it. B+ slightly under 400v, parafeed cap specified at 400v.

  • Single Amp   (Open in New Window)

  
  

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  • Attach the bottom of C6 to the top C5 - VoltSecond 20:05:25 06/29/09 (1)
    In Reply to: RE: Nevermind, found my answer. posted by Caucasian Blackplate on June 28, 2009 at 13:46:35

    Connect the parafeed cap to the cathode of the output tube. It sounds much better that way.
    
    The parafeed cap can ring higher than the plate. I would rate the coupling cap for at least 1.4 times the plate voltage. With the cap or parafeed transformer going to ground (instead of the cathode), I've seen the voltage rating of the cap recommended to be 2 * Vplate - 1 * Vcathode .
    
    Play safe and play longer! Don't be an "OUCH!" casualty.
    Unplug it, discharge it and measure it (twice) before you touch it.

    . . .Oh!. . .Remember: Modifying things voids their warranty.
    
    

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    • RE: Attach the bottom of C6 to the top C5 - Caucasian Blackplate 20:23:02 06/29/09 (0)
      In Reply to: RE: Attach the bottom of C6 to the top C5 posted by VoltSecond on June 29, 2009 at 20:05:25

      This is on a very different circuit (6C33 in fixed bias). I have 180 volts of B+ and I have a 250v cap from ground to OT. So far so good!
      

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