Bright Cap Question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by tubesR4tone, Mar 16, 2008.

  1. tubesR4tone

    tubesR4tone Member

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    Hello:

    I have a question for anyone who has any practical knowledge of electronics as I have next to none!

    I put a brightness cap in an amplifier. It had a value of .022pf. It did not add enough brightness to suit me, so I put in a .044pf. That wasn't too bright but seemed to let some mid-range seep past. So I figured that perhaps .033pf would be just about right. The problem is no-one around here has any .033pf capacitors and I don't feel like ordering.

    So, I was wondering: Would a .022pf and a .011pf wired together in parallel give me the same results as a .33pf?

    Thanks for your help!
     
  2. slider313

    slider313 Silver Supporting Member

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    First, did you put it across the volume pot? Second, the caps you listed are not really bright caps. 47pf or 120pf would work.
     
  3. tubesR4tone

    tubesR4tone Member

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    I wired the caps between the center and far right legs of the pot (looking at the pot from the rear with the legs up)

    As for those not being 'bright' caps...I certainly can't argue with you as I am admittedly ignorant...but it does just go to show how much bad info is out there...as I got those values from reading things others had written about 'bright' caps.

    I must mention that the caps did indeed brighten the sound though...whether they are the 'proper' values for the purpose or not.

    I find it somewhat confusing that some people use 0s preceding the numbers and some do not when referring to the same capacitor in an amp. I remember that from viewing a thread on the brightness cap in a Fender Deluxe Reverb.

    Any more info would be of interest.
     
  4. vibroverbus

    vibroverbus Member

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    TR4T - double check exactly what you have and get back to us... I think you might be confused by the units you see - a picofarad is 1 millionth of a uf microfarad (and a nf nanofarad is in between - 1 thousandth of a uf)

    the decimal point and zeros and units are important. amp caps cover a broad range of values - commonly including:
    large-value filter caps generally 10-100uf (microfarads)
    mid-value coupling, bypass and tone-network caps that are generally .001-.22uf (or more)
    tiny value caps used mostly for tone control - 47-500pf (picofarads)

    the caps themselves can be coded, and often could be marked in nf - so if you're talking .044nf that would be 44pf, which would be a more sensible value although not necessarily common.
     
  5. tubesR4tone

    tubesR4tone Member

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    Thanks both of you for your replies so far.

    If I understand you correctly; most brightness capacitors would be 47pf or larger...is that correct?

    Here's my problem. I had and tried a capacitor marked 47pf (it had no zeros or decimals) but it was way too bright. I am only trying to add a touch of brightness. The 47pf gave so much brightness that I had to drop the treble control down to about 2-3; much like some people complain they have to do with a Deluxe Reverb.

    So assuming that 47pf is way too bright(for my tastes): then are .47pf and .047pf viable alternatives to try?

    May I ask a few questions about capacitors in general? Does the number (example 47) determine the frequencies that can pass through (or are blocked from passing)? and

    Do the number of decimal places (or absence of one) determine the amount (or strength) of the signal within that frequency range that passes through?

    Thanks
     
  6. vibroverbus

    vibroverbus Member

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    It'd be nice to ref you to a basic RC filter page on say, wikipedia, but unfortunately the people who edit electronic pages for wikipedia seem to be, well, friggin d-bags who miss one point of a Wiki is generally educate the uneducated, so the page that should be perfect, is heavily loaded with the math and theory, and virtually unreadable for a basic pragmatic explanation of an RC circuit.
    http://en.wikipedia.org/wiki/RC_circuit
    (note the disclaimer at the top: This article relies on knowledge of the complex impedance representation of capacitors and on knowledge of the frequency domain representation of signals. - now if anybody understands those things, aren't they likely to understand RC filters? the title of this article should be "advanced formulas and theory of RC filters"... stupid scientists and their big brains)

    anyway, some of these be better.
    http://www.play-hookey.com/ac_theory/filter_basics.html
    http://www.angelfire.com/electronic/funwithtubes/Basics_02_AC_Circs.html#02-AC-08-back

    but to the pragmatic answer you want - in a bright switch, you are creating a high-pass filter that shunts high frequencies around the volume controls resistance. the lower the capacitance value, the higher the frequencies. so your basic premise is correct, except a .47pf cap would be way to small a value - likely would have no audible effect...

    also - are you sure it was 47pf cap you tried? often times the number on tiny caps is coded to make up for the tiny space...
    http://staff.bcc.edu/eet/Capacitor_Coding.html
     
  7. tubesR4tone

    tubesR4tone Member

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    I'm going to try and absorb some of the info on those sites...and also see if I can't find those old packs of capacitors and double check them.

    I'll be back if and when I know enough to continue.

    Thanks again!
     
  8. Rick1114

    Rick1114 Member

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    If you are just trying to add a little bit a brightness, you can order some small value ceramic caps from Mouser.com. I ordered 1pf, 5pf and 10pf to try in my DRRI, and I've got the 10pf in there right now.
     
  9. tubesR4tone

    tubesR4tone Member

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    OK... my question was 'off' from the very beginning...because...

    I found the packaging and the caps I have (APPEAR TO BE) .01UF, .022UF, and .044UF...not PF.

    So are any of these suitable?

    Thanks
     
  10. Dave C

    Dave C Gold Supporting Member

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    Those caps are too large for use as a brite cap . The .01uf=10000pf. As was mentioned most brite caps fall into the 47-120pf range.
     
  11. vibroverbus

    vibroverbus Member

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    aha! makes a lot of sense now - I thought that might have been the case. go getcherself some pico farad values and try those...
     
  12. Nolatone Ampworks

    Nolatone Ampworks Silver Supporting Member

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    I've wondered about the coding on smaller caps (like ceramic disc and silver mica caps).

    It seems the numbers always assume picofarads. I presume that's because caps of that size/type can only be in that value range. Am I right?
     
  13. vibroverbus

    vibroverbus Member

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    yeah I think it's all about space available to print on and be legible, although sometimes they're in nanofarads (aka 4n7 = 4.7nf) or often 'coded' so not exactly in pf (aka 223 = 22 + 3 zeros = 22000pf)
     

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