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Thread: Tone circuit - schematic attached

  1. #1
    EFK
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    Tone circuit - schematic attached

    Can someone educate me a bit on this tone circuit? Is it a 'bandaxall' circuit? Pot 1 is Vol., Pot 2 Treb and Pot 3 Bass. Anyone have any comments on it etc? I'm trying to learn all I can about this amp one piece of the schematic at a time.... Any thoughts at all!
    Attached Thumbnails Attached Thumbnails chan1.jpg  

  2. #2
    daz
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    me thinks thou doth needeth this...

    Download

    I'm assuming you don't have the duncan tone stack calculator, so download it and you'll find it has a simulation of that very stack (tho it's called "james" which i believe is the first name of the designer of that circuit) that you can play with all you want and learn more about it than anyone can tell you.

  3. #3
    EFK
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    Alas! I useth a Mac. *sigh* However I will look into the 'james' circuit. Any thoughts on the values indicated on the schematic? Trying to slowly wrap my addled brain around this amp.

  4. #4
    daz
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    No, sorry. But see if you can get on someone's PC and try it. it's a tiny download. have a PC at work?

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    I don't think it will work very well as is, the 470k in series with the output needs to be between the wipers of the treb and bass pots, with the output then taken directly from the treble wiper.
    As it is, the bass control will bypass the treble control to some extent. Peter.

  6. #6
    Supporting Member tubeswell's Avatar
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    I think there's a resistor missing from your Baxandall circuit. (The last link gives you all the formulas)

    Adam's Amplifiers: Tone Stacks

    Passive Baxandall

    The James-Baxandall Passive Tone-Control Network

    There's some other guff on it here, which appears to attribute the origins of the design to someone called Michael Volkoff (in 1935) (- as opposed to E.J. James in 1949):

    http://ozvalveamps.elands.com/tonestacks.htm
    Last edited by tubeswell; 05-08-2009 at 12:29 PM.
    Building a better world (one tube amp at a time)

  7. #7
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    Quote Originally Posted by tubeswell View Post
    I think there's a resistor missing from your Baxandall circuit.
    +1, tw and Peter are right, the missing resistor should be placed between the wiper of the treble pot and the wiper of the bass pot ( output on the "treble" side ). Without it, as Peter wisely noted, there will be some mutual influence between the controls and the treble control will act less "linearly".

    Cheers

    Bob
    Hoc unum scio: me nihil scire.

  8. #8
    EFK
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    Hang in there with me guys - I don't completely understand all of this. I appreciate the help! The above 'Adam's amplifiers' link makes the most sense to me, especially given the neat little sketch of the pots i.e "actual installation."

    (1) Bob Martinelli and Peter: what would be the practical/audible effect of the "mutual influence" or the bass control "bypassing" the treble control? Are you indicating a heavy emphasis on all bass all the time?

    (2) This is in a commercial amp (Gibson GA40/42RVT), according to the turret board designed by the omnipresent (seems that way considering all the blackhearts, epi designs...) Pyotr Belov. Assuming the actual circuit in the amp matches the schematic - it seems to have been changed occasionally as production commenced so I need to verify it - why would he set up the tone circuit this way? Can there be any use for it being set up in this manner? Maybe tone knobs being decorative as opposed to effective?!!

    Also, why are the cap and resistor values in the schematic so far off the 'standard' values i.e the above links?

  9. #9
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    1) Without that resistor the bass control would extend his "domain" over some of the high frequencies, and the slope of the treble control would be different.

    2) "Assuming the schematic matches...." Uhm..... since I see no apparent reason to design a tone stack in such a flawed way, I' d say that the schematic is wrong ( and the resistor on the lower right is the main suspect IMHO, as Peter already noted ).

    It's not the first time it happens and it certainly won't be the last

    HTH

    Best regards

    Bob
    Hoc unum scio: me nihil scire.

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