Putting capacitor in NFB?

Discussion in 'Amp Central Station' started by alexwilds, May 6, 2021.

  1. alexwilds

    alexwilds Tele-Meister

    Age:
    66
    Posts:
    181
    Joined:
    Jan 27, 2018
    Location:
    Japan
    I am trying to get some of the shrillness out of my 5F1 Champ, as well as the high pitch whine and buzz. Would putting a small cap, say, 150pF across the NFB resistor do the trick? Anybody ever done that?
     
  2. jimgchord

    jimgchord Tele-Afflicted

    Age:
    48
    Posts:
    1,600
    Joined:
    Apr 3, 2018
    Location:
    victor,ny
    There shouldnt be shrillness or whine or buzz. May want to address the actual problem instead of putting a band aid on it.
     
    corliss1, Fendereedo and oldfish like this.
  3. Lynxtrap

    Lynxtrap Tele-Holic

    Posts:
    959
    Joined:
    Feb 29, 2016
    Location:
    EU
    While I agree with the above, the answer is yes. You can put a capacitor across the NFB resistor to cancel high frequencies. To what extent it does the trick in your particular case is uncertain.
     
  4. Guitarteach

    Guitarteach Doctor of Teleocity

    Posts:
    10,936
    Joined:
    Aug 6, 2014
    Location:
    UK
    I made my NFB variable on a pot... acts like a presence control. I could add a cap too but the variable resistor lets me dial in more NFB which softens it anyway. Cranked it might be less shrill, so i can adjust too.

    As mentioned, be worth locating buzz/noise issue, if elsewhere.
     
  5. dan40

    dan40 Friend of Leo's

    Posts:
    2,198
    Joined:
    Aug 19, 2015
    Location:
    Richmond Va
    Is this a vintage amp, newer reissue or a kit that you built? The squeal could be caused by several issues that are easy to fix once we know what your amp is.
     
  6. alexwilds

    alexwilds Tele-Meister

    Age:
    66
    Posts:
    181
    Joined:
    Jan 27, 2018
    Location:
    Japan
    In a perfect world, you are right. In Japan there are no three wire outlets, so lousy grounding wherever you go. That and florescent lights are almost universal, as is the resulting high pitched whine they cause. Rather than rebuild the society and re-wire the nation, I'll go with the band aid - LOL!
     
    Big-ez likes this.
  7. alexwilds

    alexwilds Tele-Meister

    Age:
    66
    Posts:
    181
    Joined:
    Jan 27, 2018
    Location:
    Japan
    5F1 Champ, home brew, build from the scraps of other home brews. I have several very straight, wonderful performing amps, both factory and home built, that I do not mess with. And then I have several frankenstein/ test bench amps that spend a few months as 6V6 push/pull, then as twin 6L6 single-ended with a fixed bias, etc. I have decided to restore this one back to stock, and out it in the stop-fixing-it collection of working amps.

    And then, of course, I'll build a new frankenstein.

    My problem is not the amp, rather funky two-wire electrical grid, lots of florescent lights, etc., cause background hum. I need lots of shielding around here.
     
  8. jimgchord

    jimgchord Tele-Afflicted

    Age:
    48
    Posts:
    1,600
    Joined:
    Apr 3, 2018
    Location:
    victor,ny
    Does any type of line conditioner help?
     
  9. dan40

    dan40 Friend of Leo's

    Posts:
    2,198
    Joined:
    Aug 19, 2015
    Location:
    Richmond Va
    @alexwilds ...Has the squeal been occurring since the very first power up after the build? Just wondering if the NFB circuit is creating positive feedback which could be fixed with a quick swap of the OT's primary wires.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.