Draining Caps: Resistor Question

Discussion in 'Shock Brother's DIY Amps' started by BradKM, Sep 23, 2012.

  1. BradKM

    BradKM Tele-Holic

    Posts:
    927
    Joined:
    Apr 27, 2003
    Location:
    Charlotte, NC
    I'm preparing to tackle my first amp repair after watching a few online videos and reading a few tutorials.

    When draining the caps will a 10 watt - 100 ohm resistor work? The ones I've seen used in my source material were of lower wattage, but higher resistance (at least 480 ohms).
     
  2. hackworth1

    hackworth1 Friend of Leo's Vendor Member

    Posts:
    3,578
    Joined:
    Oct 11, 2009
    Location:
    Volusia County, Florida
    The bigger the resistor, the longer it takes to drain the caps. You can drain the caps very quickly using no resistor. There might be a spark. The answer to your question is yes.
     
  3. BradKM

    BradKM Tele-Holic

    Posts:
    927
    Joined:
    Apr 27, 2003
    Location:
    Charlotte, NC
    Thanks!
     
  4. rz350

    rz350 Friend of Leo's Silver Supporter

    Age:
    55
    Posts:
    2,617
    Joined:
    Nov 15, 2009
    Location:
    Waubeka Wisconsin
    I would use the resistor, shorting certain electrolytics can damage them internally, plus it's hard on the terminals (and screwdriver)!!!
     
  5. Mat

    Mat Tele-Meister

    Posts:
    298
    Joined:
    Oct 14, 2003
    Location:
    Australia
    If you have a 12AX7 preamp then clip a simple jumper lead to earth and then on to pin 1 (not the other way around as you become earth if you touch the alligator clip unprotected).

    This will drain all the filter caps through the plate load resistor - leave it clipped on whilst working to ensure no drain back of current into the caps.
     
  6. tubeswell

    tubeswell Friend of Leo's

    Posts:
    2,365
    Joined:
    Jul 1, 2008
    Location:
    NZ
    A simple way is to clip one end of your (insulated) gator clip DCVmeter lead to the chassis and the other end to the plate resistor of V1. The filter caps will then safely discharge through the amp's power supply rail in series with the meter, and you get to watch the voltage go down :)

    When you've discharged it, it a good idea to add a 220k 2W (bleeder) resistor permanently in parallel with the first (reservoir) filter cap, so that the caps always discharge when the amp is switched off. JM2CW
     
  7. Knight_Hawkish

    Knight_Hawkish TDPRI Member

    Posts:
    23
    Joined:
    Sep 16, 2012
    Location:
    Millboro, VA
    Any ideas as to whether this might affect the character of the circuit/amp/tone? At that value (220k) you don't think there's any audible AC ripple being introduced to the amp via the power supply?
     
  8. printer2

    printer2 Poster Extraordinaire

    Posts:
    6,279
    Joined:
    May 24, 2010
    Location:
    Canada
    A drop in the bucket. Say you have one 6V6 in your amp running into a 5k transformer (impedance not resistance but the power supply does not know the difference) you could have 60mA running through the tube and 1.4mA through the resistor.
     
  9. KenH

    KenH Tele-Holic

    Posts:
    604
    Joined:
    Jun 30, 2010
    Location:
    Pennsylvania
  10. printer2

    printer2 Poster Extraordinaire

    Posts:
    6,279
    Joined:
    May 24, 2010
    Location:
    Canada
    Which I agree with but that is assuming you have a properly working amplifier. If you do as I do and test parts of you build as you go along you could put power on the power supply to check it out and still have voltage there when you turn it off. Or if you are having a problem with your heater circuit and the tubes do not get hot but the SS rectifier charges up the capacitors. I rarely discharge capacitors but I always check to see if they are holding a charge. And if they are I discharge them.
     
  11. KenH

    KenH Tele-Holic

    Posts:
    604
    Joined:
    Jun 30, 2010
    Location:
    Pennsylvania
    Makes sense. I never thought of it that way.
     
  12. DavidP

    DavidP Friend of Leo's

    Posts:
    2,420
    Joined:
    Mar 16, 2003
    Location:
    Vancouver, BC
    +1 on the 220K 2W bleeder...

    I do this on all my builds & rebuilds; its good insurance for that one time you mess up and think the caps are drained!!
     
  13. tubeswell

    tubeswell Friend of Leo's

    Posts:
    2,365
    Joined:
    Jul 1, 2008
    Location:
    NZ
    Doesn't affect the power supply at all. The filter caps are there to keep the power supply constant. The resistor just helps drain them quickly when the amp is off.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.