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Thread: Slope resistor or coupling caps?

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    Slope resistor or coupling caps?

    I need to cut bass outta this tube amp. I think Iíll change the slope resistor from 33k to 56k.
    Unless you guys know of a late stage bass cut trick? Is there a opposite of a resonance control that cuts vs boosts low frequencies?

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    what amp

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    Senior Member vintagekiki's Avatar
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    https://robrobinette.com/How_The_TMB_Tone_Stack_Works.htm
    How The TMB Tone Stack Works

    Click image for larger version. 

Name:	TMB_Tone_Stac_Calc1.jpg 
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    The higher the value of R1 (100k) the more bass.
    Lower value R1 (56k and less) less bass.

    The higher the value of C1 (470p and more) the less bass.
    Less value C1 (250p and less) the more bass.


    http://www.duncanamps.com/tsc/download.html
    Download Duncan Tone Stack Calculator and understand how the tone controls really work

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    Last edited by vintagekiki; 01-02-2019 at 10:04 PM.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by vintagekiki View Post
    The higher the value of R1 (100k) the more bass.
    Lower value R1 (56k and less) less bass.

    The higher the value of C1 (470p and more) the less bass.
    Less value C1 (250p and less) the more bass.
    As always thanks for the reference vintagekiki!

    In my experience, the lower the value of the slope R, the more bass. Above 100K tone gets brittle.

    About the treble cap value, true, but only because more treble is passed with the larger cap value, making bass weaker by comparison.

    RobRob sure tries hard, and his website is voluminous. But IMHO one must take info found there with a grain of salt. Kind of like the Piazza schematic series.

    For our OP, it's easy enough to swap values of the tone stack slope resistor to satisfy your ears. There are other ways to roll off excess low end, like reducing the value of interstage capacitors including those that feed output tube grids. And reducing the value of cathode bypass caps.

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    Enjoy. Every. Sandwich.

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    This is on a extra gain stage 2204. First stage coupler is .0047 2nd is .002. 33k slope .

    I’m wondering how Bogner and some others do the “tight/loose switch?

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    On a tube amp, generally the first gain stage has a (large) cathode bypass cap value of 22 - 25uf.

    I have used as small as .47uf or .68uf to cut bass. Works for me.

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    The world is full of people that are right.

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    Itís at .68uf. With this amount of gain, (lots) itís kinda tubby on the low end

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    The change of R1 shifts the center frequency of the middle tones, which is subjectively felt on the character of the tone.
    With R1 = 100k (Vox, Fender) the tone is soft, and with R1 = 33k (Marshall) the tone is hard.

    The Marshall JMP 4150 Club and Country Bass 100W went a step further and instead of the fixed value R1 introduced the Mid Sweep control with potentiometer VR3=100K

    https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-JMP-4150-Clubncountry-Bass-100W-Schematic.pdf

    An interesting solution of sonic bass (C11-C14, R18-R20 and VR6 Slope control)
    https://drtube.com/schematics/marshall/1986.gif

    Personally, I am an adversary of reduction the value of the coupling capacitors in the output stage.
    If there is a need to reduce the bass, the can be reduced values coupling capacitor in preamp.

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    what about 1uF cathode bypass cap on the added gain stage and .02 coupling cap.

    How big is your cathode bypass cap on the added gainstage?

    The added gain stage comes before the tone stack?

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    So my issue with dropping the coupling caps can be that when I back the gain down the amp gets VERY anemic. So thatís why I was looking for a idea to cut bass in a feedback loop or something late stage

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    Last edited by vintagekiki; 01-05-2019 at 05:24 AM.

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