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big muff tone control

Discussion in 'Shock Brother's DIY Amps' started by Jewellworks, Mar 1, 2021.

  1. Jewellworks

    Jewellworks Tele-Meister

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    ive seen references to this saying the Supro Thunderbolt is also a mid scoop/tilt control, but ive also seen several variations of the Supro t-bolt schematic.

    supro t-bolt.jpg

    supro tbolt 2.jpg
    notice how the 100K resistor is in a different spot? seems to me the top one uses it as a grid leak, whereas the bottom one doesnt have a grid leak?
    or neither has a grid leak...?

    im forever tweaking my 1 knob tone control to act the way i want, and the TSC Calculator has the Big Muff, and it sure looks like something i want to try, exactly as-is

    big muff 1.jpg

    what confuses me about this schematic is what is the RL resistor? and when compared to the original pedal schematic (using transistors and not tubes), what does the TSC schematic mean when it says V OUT? -to the top of the volume pot?
    and i doubt the C3/.12uf cap on the tone wiper is necessary for tube amps? i dont see it in any other schematic. but that 100k is on the wiper, like a grid leak. like in the first Supro T-Bolt pic above... and NOT coming off the .003 or the side of the pot... like in pic 2

    big muff 2.jpg

    ive been doing some research on this forum and read many references to the Big Muff/Supro Thunderbolt Tilt tone control, but very few schematics. one mentioned adjusting the caps and resistors for high impedance.
    such as this...

    Big Muff Hi Imp.jpg

    and @printer2 was daydreaming one day, and came up with this switchable thing...

    TweedBigMuff.jpg


    my QUESTION is... WHICH ONE?! that and the aforementioned TSC RL question...
    and the high impedance adjustments?

    i PLAN on using this on a simple SE Champ 5F2a-type circuit, but with a 6SL7. i havent ruled out another gain stage either... but not likely

    then theres this from @FenderLover :
    also, VERY interesting, but no schematic. :(

    help me pic one to try. im also getting idea about my next amp after this one, and the Fenderlover tone control sounds great, just reading it...:rolleyes:
     
  2. mrriggs

    mrriggs Tele-Meister

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    [Sing along] One of these things is not like the other...

    The first picture you posted is a different circuit than the rest of the pictures. That schematic looks like a cut-and-paste job so I suspect that the 100k resistor in the 'wrong' spot is an error.

    RL in the Tone Stack Calculator means load resistance. In the first two pics, where the tone control is hooked to the grid with no grid leak, the load resistance would be the input impedance of the triode which is extremely high [if not pushed to saturation]. Telling the TSC RL = 10M would be a reasonable approximation.

    Those circuits don't need a grid leak resistor because the tone stack is at ground potential and acting as the grid leak.

    In the transistor circuit pic, RL would be 100k parallel whatever got cutoff coming from the top. The 0.12uF cap is necessary to keep the tone stack [at ground potential] from dragging down the bias voltage at the base of the transistor.

    In the fifth and sixth pics, RL would be 1M, the volume pot.
     
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  3. mrriggs

    mrriggs Tele-Meister

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    A 6SL7 with 100k plate resistor and bypassed cathode will have an output impedance of ~30.5 kOhms. Use that for the RIN value in the TSC. I assume you will have a 1M volume pot after the tone stack so use 1M for RL. Plug in the other values from the fifth pic then go from there.
     
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  4. Jewellworks

    Jewellworks Tele-Meister

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    i hope im not annoying, but... how are you coming up with that output impedance? right now, ive got 150K (and a bypassed cathode) on V1A, and 200K (and bypassed cathode) on V1B. so... im sure theres a magic number to find on the 6SL7 Data sheet.
     
  5. mrriggs

    mrriggs Tele-Meister

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    With a bypassed cathode all you need to know is the internal plate resistance and the external plate resistor. These appear in parallel to any load 'looking in'. Plate resistance listed in the 6SL7 datasheet is 44 kOhms, with a 150 kOhm resistor the output impedance is (44*150)/(44+150) = 34 kOhms.
     
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  6. Jewellworks

    Jewellworks Tele-Meister

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    thank you for that. the Big Muff circuit on the TSC is directly from the pedal, and not a tube, so the RIN impedance is only 1k. knowing how to figure out the impedance, and plugging the correct value into RIN made a huge difference in the tone curve.

    so how do you figure the impedance on a cathode follower? same way? internal plate resistance (44k) and the cathode resistor? (2.5k) so... (44*2.50)/(44+2.5) = 2.3k? that would be a low impedance load, much closer to the original 1k the pedal had.

    right?
     
  7. mrriggs

    mrriggs Tele-Meister

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    Close, but you don't use the internal plate resistance since it isn't the plate doing the work.

    The internal resistance is the inverse of transconductance. The data sheet says that the transconductance is 1600 microMhos which is 0.0016 Mhos (that's Ohm spelled backwards since it's the inverse of Ohms, get it?) 1 / 0.0016 = 625 Ohms.

    So it's that parallel your cathode resistor. (625*2500) / (625+2500) = 500 Ohms
     
  8. Jewellworks

    Jewellworks Tele-Meister

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    I just realized how wrong my cathode follower design was and upped the resistor to 150k. Using the formula above, it still came out to 622 ohms. Awesome
     
  9. NTC

    NTC Tele-Holic

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    Jewelworks, Can you post the updated schematic?
     
  10. Mongo Park

    Mongo Park Tele-Afflicted

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    Yah man if you triangulated it I would love to know what you ended up with.
     
  11. Jewellworks

    Jewellworks Tele-Meister

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    Deco 7B.jpg

    this is the latest dwg, reverting back to an earlier design

    there are several "optional" values for the coupling caps and Grid Block resistors for getting rid of my blocking distortion. i even have a 2nd, small value, Cathode Bypass Cap on my 6L6 to charge and discharge quickly, as a suggestion by @Lowerleftcoast . i havent tried it yet, but will soon.
    note: no NFB. not yet anyway... i want to hear this thing work and dialed in before i add NFB, if i add it at all.

    my big question is the .01 coupling cap after the Cathode Follower? seems to me it needs to be there, otherwise the Tone Pot will have DC on it, and that aint good... but i wonder how it will interact with the rest of the tone stack? i could go .02 to make it a bit deeper, but i already have a problem with blocking distortion, and im trying to trim the fat where i can...
     
  12. NTC

    NTC Tele-Holic

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    You can't put the volume control between a gain stage and the cathode follower . The cathode follower needs to be connected after a gain stage like in a 5F6A bassman, 5E8, 5F8, 5F4, 5E7 etc. You need another triode. That was probably one of the problems when you built this before. I am sorry I can't draw this up for you, but any of those schematics above have examples. The circuit will be similar even with a 6SL7. Fix that, and I think you will be on to something. This looks promising to me.
     
  13. mrriggs

    mrriggs Tele-Meister

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    Do you have Microsoft Excel? I put together a Big Muff spreadsheet that works like the TSC. I could modify it to account for the coupling cap and post a link if that is something you can use. Have company coming soon so I wouldn't be able to get to it until later tonight or tomorrow.
     
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  14. mrriggs

    mrriggs Tele-Meister

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    Good catch. The volume control is killing the bias.

    Never seen it done before but it should work if he moved the coupling cap from ahead of the volume control to the ground leg of the volume control. That'd allow the grid of the cathode follower to run at the same DC potential as the plate of the gain stage. AC would be grounded so the volume control would still work.
     
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  15. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The .01 coupling cap RC filter will have a knee at 15Hz. A .0047 will have a knee at 32Hz. The tone control will also be different due to the coupling cap being in series with the .0039 tone cap.
    I would go with the .0047 coupling cap. You can always try larger values by paralleling another cap with the coupling cap.
     
  16. NTC

    NTC Tele-Holic

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    That is the right phrase - that cathode follower has to have a high voltage on the grid, and having a standard gain stage before it gives the right bias.
     
  17. mrriggs

    mrriggs Tele-Meister

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    Last edited: Mar 7, 2021
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  18. Jewellworks

    Jewellworks Tele-Meister

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    ok...

    1) move volume to after V2, feeding the 6L6
    1a) keep the 220k grid leak? seems like i should... (in my 5F2A variant, i used a 500k pot followed by a 220k grid leak and it works great. this would be 1M? or stick to the 500k?)
    2) delete 1M grid leak/volume pot at the V1B Cathode follower, AND...
    3) delete the coupling cap between V1a and V1b, and tie the plate and grid together directly (that always confused me. i thought the grid didnt like all that DC on it)
    4) beef up my coupling cap to .1 or .22 (220nf) before the tone stack

    as for my blocking distortion issues, i posted in another thread that by adding a small cap (.05uf) in parallel with the bypass cap (22uf) on the 6L6 , i got rid of my blocking distortion. the fast charge and discharge time washed away the build up of electrons on the grid. beefed all my coupling caps and bypass caps back to normal values and ive got plenty of full bodied overdrive. thanks to @Lowerleftcoast for that one!

    i also discovered the source of my squealing. i fixed it by using all shielded wires to/from the volume and tone pots. i was able to lower my 700k grid stop on the 6L6 to 200k. this is the best this amp has ever sounded.
    i still have issues with the entire layout of the chassis, and im redesigning and moving things around, but if i get approval of the circuit above, with the fixes mentioned above, then im building that one in the new chassis. ill be sure to post my chassis layout before i start drilling and bending.

    new schematic coming soon. thanks for the tremendous help on this guys
    -assuming your all guys that is... ;)
     
  19. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Master volume??? Isn't that a usual place for a MV?
    Maybe 150K to 180K??? Clip in some resistors parallel to the 220K. A 1M would parallel to a 180K value. A 470K would parallel to a 150K value. Easy to try. Might be a better way to quell the blocking distortion??? IDK.
    It is easy to try the larger values by clipping caps in parallel with a smaller cap. The large coupling caps make for very low frequencies getting past the RC filters they create. Calculate your RC filters.;)
     
  20. Jewellworks

    Jewellworks Tele-Meister

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    left the 220k before the 6L6 alone, and instead put the 1M volume at the end of the tone stack where it looked like it belonged.

    Deco V7B.jpg

    this looks promising to me. i went back and checked all my other cathode follower designs, and ALL of them had the MV before the cathode follower, and with a coupling cap... all wrong.

    @Lowerleftcoast , assuming the RC works the same with the cathode follower, im getting 4.8hz. 150k/.047 is 22.5hz. 150k/.033 is 32hz

    cathode follower resistor works the same as the plate resistor, but in reverse (basically?) if i increase, do i get more gain? or is its value more for the RC factor? or impedance value? CF isnt about gain, but there is "some", isnt there?
     
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