Trimmer capacitors - anyone use them?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Normster, Aug 17, 2005.


  1. Normster

    Normster Member

    Messages:
    921
    Joined:
    Oct 14, 2004
    Location:
    Sacramento, CA
    Mouser has Xicon ceramic trimmers (225VDC) with a range of 12-100pF. I'm just wondering if anyone has tried using these in parallel with bright caps, treble caps, or snubbers to help dial in tone.
     
  2. ekp

    ekp Member

    Messages:
    402
    Joined:
    Sep 2, 2003
    Location:
    Berkeley Springs, WV
    The max value is generally much lower than the cab value so it would not do much. Tacking in caps with a soldering iron is your best bet.

    Have a great day, Eric
     
  3. Normster

    Normster Member

    Messages:
    921
    Joined:
    Oct 14, 2004
    Location:
    Sacramento, CA
    Yeah, I kinda figured as much, but I was just curious. Sure would be nice though, to have more "adjustable" stuff. ;)
     
  4. ekp

    ekp Member

    Messages:
    402
    Joined:
    Sep 2, 2003
    Location:
    Berkeley Springs, WV
    There is a device that does have some sort of "adjustment". It is the capacitor substitution box. You connect it with a couple of leads to the amp and then switch in caps with a switch. The values are in decades: 1,2,3,.....,9,10 for one switch and then 10,20,30,....for the next, etc.

    The problem is that they aren't cheap and they introduce lead dress issues that can be a real problem at high gains. Hence the tack them in suggestion.

    I have been a circuit engineer for 40 years and I don't have a cap substitution box. Now, I use a circuit analysis program to get me close and then tack as needed. Certainly, Phil Zuckerman has watched enough of this operation as we tailored my amps.

    Have a great day, Eric
     
  5. scottl

    scottl Member

    Messages:
    17,042
    Joined:
    Apr 11, 2003
    Location:
    Cherry Hill, NJ
    Actually if 100pf was the high end value, it would be very effective. If you put it in parallel with the bright cap or treble cap, you could have a nice range. You could also use it as a master volume bright cap.

    Scott
     
  6. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Indianapolis, IN
    I'm with Eric -- when prototyping I model in software first then tack something in.

    Then again, I (and I'm guessing Eric) am after a circuit that sounds right with standard component values. So, even if 103.256pF is the spot-on value for ultimate tone, I'm looking to see if a 100pF+-20% or 120pF+-20% cap is better.

    I'm also guessing that, in general, the difference in tone from slight tweakage isn't going to produce significant enough results to justify the hassle. Of course all those folks who pay for the Dumble-esque trimmers all over the place would disagree with me :)
     
  7. scottl

    scottl Member

    Messages:
    17,042
    Joined:
    Apr 11, 2003
    Location:
    Cherry Hill, NJ
    I hear small differences and I disagree with you ;)

    LOL

    Actually, I tack in caps as well. I added a few on/off/on switches to switch between several values as well. I like Dumblesque trimmers. Does it for me. :D
     
  8. ekp

    ekp Member

    Messages:
    402
    Joined:
    Sep 2, 2003
    Location:
    Berkeley Springs, WV
    OK, I can see that on certain caps in certain amps that you can use a trimmer of 100 picofarads and notice a difference. However, if you move from a Fender to a Marshall, you will find that difference smaller. Since Marshalls use bright and treble caps that are larger than the corresponding ones in Fenders.

    The Fender bright capacitor can be tweeked from 100+% to about 200%, but the treble cap only goes from 100+% to about 145%. The corresponding ranges in a Marshall are from 100% to 102% and about 120%.

    And then I will also admit to thinking of my own amps first and they use capacitors that 10 times larger because all of the impedances with exception of the input circuit to the input stage are all a tenth as much as corresponding impedances in vintage amps.

    Before someone jumps on this lowering of impedances as some sort of huge error, since ALL of the impedances are lower, ALL of the time constants and frequency responses remain the same. The advantage is that the lower impedances make leakage currents and stray capacitances less effective and gives the amps greater tonal reliability.

    Have a great day, Eric
     
  9. scottl

    scottl Member

    Messages:
    17,042
    Joined:
    Apr 11, 2003
    Location:
    Cherry Hill, NJ
    Your amp does rock Eric!
     
  10. ekp

    ekp Member

    Messages:
    402
    Joined:
    Sep 2, 2003
    Location:
    Berkeley Springs, WV
    Thanks Scott, wait until you get a load of the new 4-12 combo.....

    Nice to hear from you, have a great day, Eric
     

Share This Page