What could be the reason for too high voltage from filter caps?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by O RUde, Jun 21, 2019.

  1. O RUde

    O RUde Member

    Messages:
    22
    Joined:
    Mar 31, 2015
    Location:
    Norway
    Hi, I wonder what could cause too high voltage from filter caps? Might be a difficult question, but if there are some general rules?
     
  2. ironman28

    ironman28 Member

    Messages:
    553
    Joined:
    Jun 24, 2014
    How much is too high?
     
  3. Vanyu

    Vanyu Member

    Messages:
    654
    Joined:
    Oct 11, 2015
    You'll have to elaborate a little bit.

    First, what amp is in question here?

    Second, where are you measuring your voltages at, and which filter caps are in question? And what makes you think your voltages are too high? 400v-500v is nothing out of the ordinary at the first node. Typically these caps see whatever voltage the rectifier is putting out (which is usually really high!).

    Is your amp working in its current state? Blowing fuses? Blowing tubes?
     
  4. Tron Pesto

    Tron Pesto Member

    Messages:
    763
    Joined:
    Feb 18, 2016
    Location:
    Mahwah, NJ
    And just to help you zero in on the details, capacitors themselves do not supply voltage. No voltage comes "from caps." Capacitors are passive components that affect a circuit's performance to a desired result.

    In the case of filter caps, voltage/current is typically supplied through a rectifier powered by a transformer (secondary). In your search for why your voltages are too high, you have to look up the chain at what the transformer and rectifier are doing. Those two things will have the greatest effect on the voltage. Filter capacitors simply smooth the AC pulses coming from the rectifier.
     
    Last edited: Jun 21, 2019
  5. Blue Strat

    Blue Strat Member

    Messages:
    30,002
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Yes, what amp? Are the power tubes installed? If not, the plate voltage will be high. What voltage are you getting and what's the voltage rating of the amp? I should have mentioned this first, but change your meter's battery first.
     
    zenas likes this.
  6. wall_of_sleep

    wall_of_sleep Member

    Messages:
    752
    Joined:
    Jul 31, 2011
    Location:
    child of the corn
    You could rewire them to form a voltage doubler.


    Don't mind me, carry on.
     
  7. tarweed

    tarweed Member

    Messages:
    681
    Joined:
    Jan 29, 2019
    Location:
    Orbis Tertius
    if one or more caps are blown (i.e. show as open circuit to AC), then your meter may be showing something closer to the peak (unfiltered) voltage.
    Also, what else is in the filter circuit? Choke, or resistor - to make a LC or RC filter? If shorted, then the output voltage would be higher
     
  8. PushedGlass

    PushedGlass Member

    Messages:
    498
    Joined:
    Sep 26, 2014
    Yeah - there's simply nothing to go on here. I'll just say that these power supplies are most thoroughly unregulated; they arrive at the voltages they produce as a result of how much they're loaded by the circuit in whatever state it happens to be.
     
    pdf64 likes this.
  9. O RUde

    O RUde Member

    Messages:
    22
    Joined:
    Mar 31, 2015
    Location:
    Norway
    Sorry about the vague kind of question. I am building a Dumble clone, and when I finished it there was sound going through, although not the right sound. Then at some point it all went dead. I have changed all the tubes and they seem to be OK, glowing as usual. Normally you can get some "scratch" sounds when you touch certain pins on the tubes, but nothing. When I measure the B+'s I seem to get a bit high voltage, about a 100 more voltage on b5 and b4 than the schematics states. It seems like the tubes doesn't get any working feed, there is only a very low hum in the speaker. This amp is difficult, bit too much I must admit.
     
  10. ironman28

    ironman28 Member

    Messages:
    553
    Joined:
    Jun 24, 2014
    Post your schematic or a link to it on the internet.
     
  11. Vanyu

    Vanyu Member

    Messages:
    654
    Joined:
    Oct 11, 2015
    I hope you didn’t build the amp solely off of that layout... Do you have an actual schematic to your amp or just that layout?
     
  12. O RUde

    O RUde Member

    Messages:
    22
    Joined:
    Mar 31, 2015
    Location:
    Norway
  13. Vanyu

    Vanyu Member

    Messages:
    654
    Joined:
    Oct 11, 2015
    That’s still only the layout you posted.

    The reason I make mention of the layout is because this is a quite complex build, and not referencing the actual schematic is a recipe for gremlins inside the amp. You really need to find an actual schematic for your amp so you (along with all of us) truly understand what’s happening in your amplifier. A layout should only be used as a cross reference for component placement.
     
    HotBluePlates and pdf64 like this.
  14. HotBluePlates

    HotBluePlates Member

    Messages:
    6,681
    Joined:
    Mar 27, 2017
    Now that we have a schematic, it's very easy to devise a troubleshooting strategy!

    Measure the voltages at pins 3 & 8 of V1 and V2. The schematic shows you should be seeing something between 1.5 to 2v at each cathode. What do you have?
     
  15. O RUde

    O RUde Member

    Messages:
    22
    Joined:
    Mar 31, 2015
    Location:
    Norway
    I have 21.75v there measured to ground, same on all pins...
     
  16. pdf64

    pdf64 Member

    Messages:
    6,021
    Joined:
    Aug 9, 2008
    Location:
    Staffordshire, UK.
    Perhaps the 0V return for the cathodes from the board to the chassis tab has been omitted (black wire, shown as dashed when under the board)?
     
    HotBluePlates and O RUde like this.
  17. O RUde

    O RUde Member

    Messages:
    22
    Joined:
    Mar 31, 2015
    Location:
    Norway
    I just checked that yesterday, and it seemed fine.
     
  18. killer blues

    killer blues Member

    Messages:
    2,775
    Joined:
    Mar 10, 2006
    Location:
    East of Philly
    Check the screen grid resistors they may be open
     

Share This Page

  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.
    Dismiss Notice