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where do these begin to roll off?

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  • where do these begin to roll off?

    I know i'll get a lecture for not knowing the math, but trust me....numbers are to me what nuclear physics are to a 6 month old chimp. I have a 12AX7 gain stage with a 220k plate R and a 1.9k cathode. Where would the cutoff points be with the following caps...

    .68uf
    .47uf
    .33uf

    I have a .68 there now and i think it's cutting off right after a particular mid frequency that gives the gain a sorta metalish tone. i want to get rid of that and leave just high end enough to keep the lows from getting muddy as it does if i lift the bypass cap. the tone i'm getting makes me think the .68uf must be rolling off somewhere around the 700Hz area. Am i close?

  • #2
    I don't know the math either. But why should it matter? If you think this is the area you want to adjust, and you have experimented and know what the adjustment will do, Then just find a value you like for this amp and be done. Why should it matter what the exact knee frequency is.

    It's noble enough if you want to know the math. But if your just looking for a spoon fed frequency spec, I really don't see how it matters.

    I'm not trying to be antagonistic (though I often am). Maybe I don't understand the problem you're trying to solve.

    Chuck
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

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    • #3
      I just like to see the roll off points because i like to know in theory what frequencies a given value will give me. especially when i hear something where i think i know the range thats being peaked. I find it better to know what value gives what so i can start off with a close if not perfect value so i don't have to keep trying different ones and listening over and over. I can do it that way and in fact do most all of the time. But i find that listening to what seem like subtle differences can be hard to assess even when the results are not as subtle as they first seem later on, if that makes any sense. I have a chart that show the points for all combinations of cap and R values, but it's based on having 100k plate R when i'm using 220k.

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      • #4
        Hi Daz

        The cathode bypass freq roll-off (is that what you were wanting?) depends on your cathode resistor as well. Kerry B gave me this formula last year - (1/2.pi) x resistance x capacitance = freq. So:

        (1.571) x 1.9 x 0.68 = 2Hz

        (1.571) x 1.9 x 0.47 = 1.4Hz

        (1.571) x 1.9 x 0.33 = .98Hz


        Someone is bound to chime in if I've dropped a decimal somewhere, but that's my take on it.
        Last edited by tubeswell; 10-09-2008, 09:23 AM.
        Building a better world (one tube amp at a time)

        "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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        • #5
          The cathode resistor is in parallel with the tubes cathode output impedance, which for a 12AX7 with 1k5 cathode resistor basically halves it. The actual cathode impedance will depend on the operating point of the tube ie plate voltage/current. With your higher resistances therefore lower currents I would guess the cathode impedance is higher. So reckon on the effective total R being half the cathode resistor and you shouldn't be too far wrong.
          See aiken for the theory
          http://www.aikenamps.com/CommonCathode.htm
          Peter.
          My band:- http://www.youtube.com/user/RedwingBand

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          • #6
            Try this formula and yore pretty set.
            You get the center between the fuly bypassed and the unbypassed.
            Maybe its not the correct but it gets very close.


            You cant realy apply the 1/(2piCR) because its a shelf filter. If the resistor is low like 820 it wont make a 3dB boost anyway.
            Last edited by Tubis; 10-09-2008, 11:26 AM.

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            • #7
              I think regardless of teh formula, they all come out to rolling off way under the guitar freq range.
              Education is what you're left with after you have forgotten what you have learned.

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              • #8
                Tubeswell:- the formula that your using is incorrect.
                Should be the reciprocal of 2 x pi x R x C, with R in ohms and C in farads, making the sum:- 1/ 2, 3.142, 1900, 0.00000068
                giving 1/0.00812 = 123

                Hope this makes it clearer.

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                • #9
                  Put that into my formula. Lets say the Ri is 1k R is 1k9 and C is 680nF.
                  That will come to a centerfreq at 209Hz. How high this boost is in dB you get by calculating the unbypassed dB and the bypassed dB and then half of the boost you get from that. The bigger the cathode resistor, the bigger the difference in dB between the unbypassed and the bypassed. The lower the cathode resistance, the less audible difference you will hear.

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                  • #10
                    I can't seem to equate those numbers to what i'm hearing, so in effect chuck was right ! I had to try them all. The difference was obvious, tho funny enough i tended to like the .68 best because the other 2 seemed to lose too much character and sound less complex. So i'm going to try going in the opposite direction than what i thought would be best.... a 1uf

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                    • #11
                      Remember that theres more roll off parts in the amp that work together so just one of those bypas caps dont realy tell you the freq truth
                      And what about distortion harmonics....say 200Hz turning into 400Hz...
                      Thats why its sounds different where in the circuit you cut or add.

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                      • #12
                        I tried 5 different caps now and it's very apparent the .68 was the right one in the first place. It really makes a big difference in how "right" the tone sounds to me, tho i still think somewhere in the amp i need to change something to get a less metalish midrange. Tried some things in the the tone stack already, but theres more i can try there.

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                        • #13
                          Originally posted by Wakculloch View Post
                          Tubeswell:- the formula that your using is incorrect.
                          Should be the reciprocal of 2 x pi x R x C, with R in ohms and C in farads, making the sum:- 1/ 2, 3.142, 1900, 0.00000068
                          giving 1/0.00812 = 123

                          Hope this makes it clearer.
                          Hmmm... I had wondered about that, having seen it written several ways.

                          1) On the Valve Wizard site - it is written as; f(half boost) ~ 1/(2Pi*Rk*Ck).

                          2) The page on the Aiken site has had that part of the formula removed, with a note saying:

                          "Appendix C: The math behind the lower cutoff frequency due to a partially-bypassed cathode
                          This section under construction - thanks to Jean-Pierre Trolet for pointing out an error in the original document. I will update it when I get some time to review and rewrite the document."

                          3) Amp Kat gave me the formula that I used last year, and he also gave me a worked example, and I referred to those last night in deriving my numbers. I really trust Amp Kat.

                          However, having had this discussion, I still am not clear which is correct.

                          Cheers

                          :-)
                          Last edited by tubeswell; 10-09-2008, 07:29 PM.
                          Building a better world (one tube amp at a time)

                          "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

                          Comment


                          • #14
                            Originally posted by daz View Post
                            I tried 5 different caps now...
                            That's the key of course. The numbers correspond with real things that are happening, but the cap you want is the one that sounds good. Run the numbers to get you in the ballpark then listen. I suppose over time you could develop the ability to hear the numbers.

                            Ya a good 12ax7 swag is to halve the cathode resistor for your calculation.

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