More Bass For My Marshall Markiii 2100?

Discussion in 'The Workbench' started by twothemax, Jan 4, 2017.

  1. twothemax

    twothemax Active Member

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    came across this on the net.
    this guy stated

    Since this amp was too trebley I did a mod that affected the negative feedback to try to give it more bass response. Putting a .001uF capacitor in series with paralleled C6 and R17 in the negative feedback circuit creates a shelving filter that does not feed back lower frequencies. So when lower frequencies are not fed back, they are not phase canceled in the phase inverter, and the amp has more bass response. Still sounded like a Marshall gain type of sound but at least had some heavy bass to go along with it to fill in the sound.

    what does this guy mean by "in series with paralled c6 and r17".
    i understand the series/parallel thing, but this seems a bit hard to figure out what is meant.
    thanks
     
  2. m1989jmp

    m1989jmp Active Member

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    Imagine your c6 and r17 are tied across points A and B in parallel.

    You lift (desolder) one junction (lets say B) and place a cap (0.001uF) between this point you just desoldered and point B.

    So now the situation is: B point-> 0.001 cap ->r17 and c6 junction

    The 0.001 cap is now in series with the parallel combination of r17 and c6
     
  3. twothemax

    twothemax Active Member

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    thanks.
    so is c6 jumped to r17 in // ?
    then lift one side and add .001uf in series to the c6 r17 combination.
    what do you mean by c6 junction?
    would you have a pic of this or diagram.
    thanks
     
  4. Adrian R

    Adrian R Well-Known Member

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    Plug up the FX loop with a patch cable..if not use EQ in loop, boost front end, crank presence, balance with treble...different speakers, turn it up. All of this will produce more low end.
     
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  5. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Here is a link to a schematic.
    http://www.drtube.com/schematics/marshall/cd0192-iss7.pdf
    Have a look at the OT area and you will see C6 paralleled to R17 which connects to the Vio (violet) wire and to the 4 ohm tap of the OT.
    The .001uF capacitor resonance modification would be placed from the Vio point to the next point where C6 and R17 are connected. That would be in series with C6/R17 circuit.
     
  6. twothemax

    twothemax Active Member

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    :agreed:thanks. love my mark iii the way she is.
    i done a fair bit of work to her when i got it, but they were your normal can caps, electrlytic caps, power and preamp tubes, bias resistors i also replaced the ot, only because i thought it had a hot spot on it. i later found out that its not uncommon. so now i have a new ot in it too. she sounds sweet.
    i myself never had trouble with more bass, but i found that mod and was wandering about it.
    got myself some gt75s coming too. may as well match the beast with the rite cab.
    thanks again
     
  7. twothemax

    twothemax Active Member

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    thanks mickeydg5. figured it out. i was curious about this mod.
    your explanation was helpful.
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    Try put a capacitor across R7 in the preamp. Try .68 - 6.8uF 35V or higher. The + of cap faces the cathode, - goes to ground.
     
  9. RickyLee

    RickyLee Well-Known Member VIP Member

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    Ampmad, check the schematic you are referring to. R7 on the MK3 is a plate resistor.

    :D
     
  10. BowerR64

    BowerR64 Well-Known Member

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    R5 or R6?
     

    Attached Files:

  11. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    no way man
     
  12. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    There are different revision versions and each has different component callouts. Marshall is good for screw ups like that.
    I believe Ampmad meant place a cathode bypass capacitor on V2a 1.5k cathode resistor.
     
  13. BowerR64

    BowerR64 Well-Known Member

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    V2a? then it would be R8?

    I believe this is the correct schematic for my amp, not sure about the OP

    JCM900 2500 MK3.jpg

    Here is a different one, not sure the difference maybe a different version of the amp? But it does have the same 1.5K cathode resistor as R7

    JCM900 2100 preamp.jpg
     
  14. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    That is the one.
     
  15. ampmadscientist

    ampmadscientist Well-Known Member

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    I'm really sorry, there is a difference in the schematics!

    on some schematics, it's R7[​IMG]

    ...on other schematics It's a different number.

    Sorry about that, there is more than 1 schematic.

    Yes this is the cathode bypass cap for V2A. Installing that cap will increase bass.
    Try 0.68...to 6.8 but don't over-do it.

    Also at V1A
    The stock cathode bypass cap is .68, you can increase that. This will also boost the bass.

    The bass (or lack of bass)
    is really caused by the bypass of the preamp tube cathodes.
    Change that and you will get plenty of bass.
     
    Last edited: Jan 6, 2017
  16. RickyLee

    RickyLee Well-Known Member VIP Member

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    The more I look at the MKIII schematic, the more I want to dig mine out and get reaquainted with it again.

    I am digging the Gain 1 circuit where they are utilising two functions in one control pot. And then that .22uF cap there might be something I played around with in my amp but I can't remember.
     
  17. ampmadscientist

    ampmadscientist Well-Known Member

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    In the top schematic it's R8 that's right. V2A cathode bypass cap.
    In the bottom schematic it's R7.

    I played all around with 900s and the reason it has no bass is that cap is missing. I used 6.8 uF. But don't overdo it...

    https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/calculator/
    So using the calculator, 6.8 gives you a roll-off point of about 50 Hz.

    I put that cap in and I knew I was home, bingo Bob's yer Uncle.
    The + faces the cathode, if you are using a polarized cap.

    But the side effect is: when you boost the bass up, cheap effects pedals will be pretty noisy. (because of the power supply noise)
    You gotta use some pretty quiet pedals because the 900 preamp compresses noise just like the audio...so it will also boost the hum from the effects pedal.
    I think you understand.

    You gotta use isolated power supplies for the effects pedals.
    If you have effects in the loop, and you have effects at the input jack, and these are running off the same power supply, it will cause a ground loop hum.
    So you gotta use 2 separate isolated power supplies for this.

    Following that, A. effects loop (it's complicated) B. compression (it's complicated)
    at the effects loop you gotta adjust the level of send, because it can overload your effects pedal input...and some effects pedals do have input level adjust.
    But there is a level adjust control on the loop...BUT...the control is a dual pot.
    It turns the send up, and the return down, at the same time.
    This can make it seem like the control is doing nothing.
    (this may have been a design mistake)

    So I think what you gotta do is tweak effects loop.
    1. adjust level of send to match effects pedal input (it's too much signal)
    2. adjust level of return
    by changing resistors.
    as far as the level adjust pot, you may want to modify that adjustment direction / range to be more useful.

    B. Compression
    In a Marshall amp, the V2a is a compression stage which saturates the grid of V2b,
    More and more signal goes in, but there is no more gain, it just saturates more.
    (power supply voltage limit is reached, and gain can no longer increase)
    Output of V2 is cathode follower. The cathode follower is always less than the grid input.
    In other words, output of V2 is always less than the input, just more saturated / compressed.

    But wait there's more
    The input of the amp limits low frequency and allows high frequency to pass..
    (this limit is determined by the cathode bypass cap at V1a is 0.68uF)

    When the high frequency reaches V2, it is compressed (crunch),
    When the lowest frequency reaches V2, it does not compress (because) it was limited by V1a cathode bypass cap (above). See? This is key.

    If the low frequency compressed, it would sound muddy and dirty/gritty like a farting sound.
    But instead, the highs are compressed and the lows are clean, which is why a Marshall sounds like a Marshall.

    So you need to be careful not to overdo the lowest frequencies.
    Because if those lowest frequencies are compressing, then the amp is going to sound muddy. What you want to do is maintain crunch, without going overboard on the lowest frequency.
     
    Last edited: Jan 7, 2017
  18. twothemax

    twothemax Active Member

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    thanks for the info!! great read.
    anyway of adding a bit of sizzle to the distortion?
     
  19. BowerR64

    BowerR64 Well-Known Member

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    Couple questions, is it better to mess with the value of the bypass cap on V2a or V1a or both?

    Like i was planning to try a polarized 4.7uF over R8 but maybe it would be better with a 2.2uF and a 2.2uF at C3?

    Or maybe playing with V1a would over drive the low end at V2 and make it sound fartty?

    I already have a 2466 so i dont need the fartty low end that amp has all i want and more if i want it.

    Also you mentioned the effects loop is a double pot how can i tell which way to turn it so that it lowers the send level and raises the receive level? I have a lexicon alpha but it seems the send is still pretty hot at times.
     
    Last edited: Jan 8, 2017
  20. BowerR64

    BowerR64 Well-Known Member

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    I did it last night to my 2500 and it kinda ruined the sound of it.

    I ripped all the guts out of it to clean it up rolling back a bunch of the gain and the 4.4uF bypass cap just seem to put gain right back into it.
     

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