The UN-official 4100 mods thread

Discussion in 'The Workbench' started by ampmadscientist, Jan 26, 2015.

  1. ampmadscientist

    ampmadscientist Well-Known Member

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    YOU are advised that the modifications described should be installed by qualified technicians.

    You should have
    A. Professional tools and soldering equipment
    B. Test equipment
    C. Safety training
    D. Knowledge and experience to use all of the above.

    UN-qualified persons may cause damage to the circuit board, or the amplifier.

    I have been working on guitar amps 40 years, you asked.

    You will need to be qualified to work on circuit boards and to do experienced PC soldering.
    If you do not have the professional tools, always hire a Marshall Tech to do the work.


    The circuit board can be damaged by an inexperienced person.
    If you do not have experience working on PC boards, I strongly urge you to NOT attempt these modifications.


    1. When you change the 4100 and install the mods, you may also need to also adjust levels of the effects loop.
    The effects loop does not match normal effects pedals, and will probably need to be modified.
    The input/output levels of the effects loop may need to be changed.

    2. The reverb pre driver IC2 may be overloading the reverb springs. You may need to decease the reverb pre drive level to lower the reverb distortion.


    3. Also, somebody reminded me yesterday:
    This amp needs some preventative maintenance for the low voltage power supply.
    (That's the 15 volt power supply)

    C22 and C21 filter caps for the 15 volt supply should be replaced.
    These are 1000uF 16 volt.
    But you should probably replace them with 25 volt caps. These 2 caps go bad faster...because they are for the 15 volt supply rails.
    When those 2 caps C21 C22 go bad:
    There will be high frequency oscillations in the effects loop.
    (you won't hear it, the frequency is too high)
    This will kill the audio and overload the power amp...
    So, it's a good idea to replace them before they go sour.

    You should also measure across D4 and D3 to confirm that there is 15 volts across each diode.
    If the voltage is lower than 14, the diode is probably going bad.
    These are 15 volt zener diodes...use big ones if they fit! I would use 5 watt to replace the 1 watt diodes, if they fit on the board.
    (I don't know if 5 watt will fit)

    4. To get this started, you solder a bypass capacitor across R2.
    You can just solder it in without removing the circuit board.
    This increases the gain, and allows low frequencies to pass into the tone controls.
    The capacitor I used was 6.8 uF, 35V.
    (see below)
    The + of the cap faces the cathode of the preamp tube...
    The - of the cap is grounded.

    The cap to bypass R2:
    .68 uF stock Marshall Gain, less bass for sure.
    1.0 uF more bass and a bit more gain than stock
    4.7 uF noticeably more bass and gain
    6.8 uF about as far as it can be pushed, without sounding muddy. Good for metal.

    Reference for experiment:
    http://www.ampbooks.com/home/amplifi...ode-capacitor/
    Use this calculator to predict bass response and gain in your experiments, when bypassing R2 with a capacitor.

    The bigger the value of the R2 bypass capacitor, the more gain, and the deeper the bass will go.
    But, don't over-do it.

    Do you have the schematics?
    Do you understand what I am talking about?

    5. To clean up the clean channel, remove LEDs 1-4 and remove C33.
    This allows the clean channel to be clean.

    6. Oscillations, ringing, squealing, on overdrive channel:
    The reverb tank wires must be moved.
    The reverb tank wires need to be moved away from the input jack. These wires are creating a positive feedback loop...
    Moving the reverb tank wires (away from the input jack) stops the whistling oscillations in the preamp.
    The problem is that the reverb tank wires are too close to the preamp.
    Too close to the input jack. This causes a loop oscillation to form, and the squealing occurs.
    This causes oscillations and whistling noises when you crank up the overdrive boost gain really high. __________________

    Photos:
    1. Piggybacking a bypass capacitor across R2 on the circuit board.
    2. Moving the reverb tank wires away from the input jack.


    To be continued...
     

    Attached Files:

    Last edited: Jan 26, 2015
    reddog22, aldo, Kunnz and 2 others like this.
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    7A. Replace R35 with 68K Prevents guitar from overloading preamp input stage.

    7B. Replace C8 with 100p. Allows more ultra high frequencies into the overdrive. (Classic Marshall Crunch increases)
    8. Take out C35. Allows more high frequencies to enter diode clipping for overdrive.
    9. Replace C 25 with .1 uF. Allows more low frequencies for clean gain control.
    10. Install a 500p cap across R54. Allows more high frequencies for clean gain control.

    Pre EQ Overdrive Tone Stack:

    11. Replace R43 with 10K. Revoices Pre EQ.
    12. Replace R7 with 6.8K. Revoices Pre EQ
    13. Install a wire jumper across C24. Makes IC4B unity gain.
    14. Replace R37 with 10K. Decreases loop overload of IC4B.

    At V1 Preamp Tube:

    12. Take out R 13. Removes negative feedback from V1, increases sustain.
    13. Install 6.8 uF cap across R2. (as above directions)

    Tonestack –converts the tone stack to Plexi Style


    Warning: Qualified Technicians Only Will install this Modification.
    DO NOT attempt this if you are not a Qualified Technician.

    Do these steps all at once, and don’t skip any of them:


    • Install a wire jumper across C11.
    • Change C37 to 470p 500Volt. I used silver mica.
    • Change C38 to .02 uF 600 Volt I used Orange Drop.
    • Change C 39 to .02 uF 600 Volt I used Orange drop.
    You are welcome to use Mustard Caps....or whatever............Just make sure the voltage rating is 400 volts or higher.


    C37, C38, C39 must be rated 400 Volts or higher. I told you so.
    You better CONFIRM the capacitor voltage ratings. I told you so.

    Failure to follow the voltage ratings above will cause damage to the amplifier.
    I told you so.

    Last Steps to be continued...
     
    reddog22, Snake_j and ricksteruk like this.
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    The next issue is the balance of Bass mid / and treble,
    this will adjust the tone controls (tone stack)...

    To do this I change the slope resistor R61.
    The stock value is 33 K, however it's probably going to be too muddy in the bass.

    If the value (33K) is decreased (lower) the bass and mid will be stronger.
    If the value is increased, The bass and mid will be less powerful.

    The trick is to let the treble dominate (treble control),
    The mid and bass will work, but "just" enough.
    Too much mid and bass will make the amp sound muddy.

    I wound up changing R 61 to 470K. I have no idea why, I just liked it better.
    But YOU set it where you like it the best. It's your Marshall.

    The trick is test it with a GUITAR:

    Treble should be crunchy
    Bass should be clean (or much cleaner)
    AND if you set R61 just right, that's the ideal spot.

    Should sound like Plexi on Steroids....
    If you get that, mission accomplished.

    Otherwise the amp will no longer sound like a Marshall.
    If R 61 is adjusted wrong - the amp does not sound like a Marshall anymore.
    If R61 is adjusted right - it will still sound just like a Marshall.
    Plenty crunchy in the top end, and clean in the lows.
     
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  4. Snake_j

    Snake_j Member

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    Thanks Ampmadscientist!
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    The Effects Loop

    Method 1:
    The first pedal in the effects loop MUST have an input level volume control.
    The last pedal in the effects loop must have an output level volume control.

    Otherwise, it will be too difficult to adjust the levels for send and return of the effects loop.
    The effects or amp may overload with too much signal. Distortion will occur.

    Method 2:
    The effects loop is modified to adjust the levels correctly.
    This prevents the effects pedals input from overloading...
    and prevents the amp return from overloading with too much signal.

    Either method will work OK.

    Effects Power Supply - Effects HUM
    Hum from ground loops is a big problem when several effects are running from the same power supply.
    Especially when many effect pedals are using the effects loop and the front input jacks, powered from 1 power supply.

    This makes the high quality isolated power supply a necessity.

    The effects loop ground, and the front input jack ground, will produce this ground loop HUM when combined with a non-isolated power supply.
     
    Last edited: Jan 26, 2015
  6. tschrama

    tschrama Well-Known Member

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    Don't you wanna decrease R60 to 47k or about?.. You know.. to protect IC5 from destruction?
     
  7. chuckharmonjr

    chuckharmonjr Well-Known Member

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    Would these work on a 4500 as well?
     
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  8. tschrama

    tschrama Well-Known Member

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    I am sure you meant well, but I cannot agree at all:

    4] will only have an effect if the feedback loop around V1 is disabled. Furthermore a cap around the last cathode always sounds nasty to me. Further furthermore, You now have an enourmous voltage swing being fed to IC5. Not only would I expect IC5 to fail prematurely, but you now have a clipping IC5. You just made a opamp clipper instead of a diode clipper. Sounds much better?

    This also feeds enormous voltage swing into FX-loop and Reverb drive.

    You need to change R60 to 47K like other Marshalls and be done with it.

    5] the clean channel can already be clean, if you set the chA gain pot low. You just prevent it to be used as a moderate overdrive now.
    6] I had that problem. Are you sure it's the reverb wires? Aren't they shielded?
    7A] will only add noise to the amp and will do nothing for protecting the input stage.
    7B] will add squeals and oscillation, unless you have other way for preventing those?
    8] Will certainly ad more noise/hiss, and oscillation. You NEED some treble reduction in a high gain amp.
    9] it add more bass in the clean/crunch channel. Isn't there enough bass already?
    10] Idem, does the clean channel really need this? I don't think so. It's fine as is.
    11,12,13,14] Just Snipe one end of C27 and be done with it. No more icepicky highs, more bottom available if you want to.

    12] Aha, now this should be mentioned at 4]
    13] Allready discussed: A cap around the last cathode will allways give harsh overdrive.

    Aren't the cap for the Tonestack already rated 400V. They should!

    Isn't the tonestack no pre-clipping, since you drive IC5 into clipping with you unaltered attenuator after the now free-running V1? That's why you needed such weird value for R61. To prevent mudd. It's just a pre-clipping bass reducer.

    Congrats with you opamp clipper sound jjjjjjjust like a Plexi on steroids. Clips?
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    "Allready discussed: A cap around the last cathode will allways give harsh overdrive."

    T
    hat's really not true in this circuit. It's a matter of personal taste.
    The circuit was voiced by an experienced guitar player.

    "will only have an effect if the feedback loop around V1 is disabled...."

    That's step 12 (above). Already covered it.
    Removing R 13 disables the feedback loop at V1.

    BUT...it has an effect whether or NOT the feedback loop is disabled!
    Try it yourself.

    P.S. Take out R13 BEFORE you modify the tone stack. That is covered above.

    R60 change to 47K?
    No, but if you want to, go ahead.
    I didn't have any problem with IC 5 being destroyed.
    It's been running about 3 years like that...


    I did mention that the effects loop levels need to be modified or adjusted.
    But that is true for all Marshall amps, not just this one.

    Marshall effects loop has never been a good level match for USA effects pedals.
    That's why
    the first effects pedal in the loop should have an input level volume control.
    The last pedal in the loop should have an output level volume control.
    OR, you can modify the loop levels permanently, inside the amp.
    I did cover that effects loop level description, above.

    I did also mention (above) that the reverb drive needs to be adjusted, to prevent the reverb from distorting.
    Yes, already said that too.

    If you like to change R60 to lower the effects loop level, that's one way to do it.
    I did it a different way....
    Either way is possible. Either way works just dandy.

    A. The clean channel has no headroom. It's very muddy (diode clipping).
    That's why we cleaned it up. Very big difference. HUGE difference.

    B. Reverb tank wires:
    The shielding is not enough. The wires are too close to the input stages.
    A steel or aluminum shielding plate works even better. (we tried all methods).
    ON later models of 900: Marshall installed steel shielding plates, at the factory.
    They reached the same conclusion that I did! For the same reasons.
    MOVING the wires is the "poor man's method." BUT, it worked pretty well.

    If you are happy with the stock sound of the amp:
    Don't modify it.

    The modification was designed to please professional guitar players...without the need for distortion pedals.
     
    Last edited: Jan 28, 2015
  10. ampmadscientist

    ampmadscientist Well-Known Member

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    I'm not sure yet.
    I will check that out...
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    So anyway...add your own suggestions, and personal tweaks.

    If you have some good ideas for mods:
    by all means, add them to this thread!
     
    tubes likes this.
  12. tschrama

    tschrama Well-Known Member

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    Look, I think I came over a bit harsh, sorry.

    I wont rebuttal you reply, as I think that it would take too much positiveness away from your initiative to publish you mod tips. It's a great initiative to publish your mod tips! I think you could do better, and I gave my input for you (and the forum readers) to learn and use to their advantage, albeit a bit harsh.

    Cheers,
    Thijs
     
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  13. tubes

    tubes Well-Known Member

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    Thanks ampmads.

    Very good of you to offer all that information.

    I have made a note of those suggestions.
    (I will not be messing around inside my amp myself, but all interesting anyway - something to think about at next service...)

    I like my more-or-less stock 4100 as an 'all round' amp for covers. Seems to me I get good clean sounds, almost like a Bassman sometimes.
    Also, I get creamy overdrive tones for solos, which I like. And I don't play any metal. So 'creamy' is usually where I want to be.

    But I know there are other regions in which an amp should perform....

    Meanwhile: I was wondering...
    Do you have any thoughts about the range of opinion about the bass response of the 4100?

    I mean, I hear people say the amp is by nature 'thin' and has no bottom end.

    But that's not my own experience.
    From experience I'd say the amp has enough bass to embarrass a bass player.
     
  14. ampmadscientist

    ampmadscientist Well-Known Member

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    It's obvious to a trained ear that this amp has poor bottom end.
    But there is a reason for this, and a solution for this.

    Reason: The Marshall sound has crunchy treble and clean bass. Otherwise, it won't sound like a Marshall!
    This is accomplished by building a pre set EQ curve into the amp.
    The treble is deliberately boosted, and the bass is deliberately attenuated.
    This EQ curve is intentional, and installed at the factory.

    The EQ curve looks like this: / treble on the right, bass on the left. You can't change it unless you redesign the voicing...
    The Result: is that the high frequencies (much louder than bass) distort and compress far easier than the low frequencies.
    The treble crunches, it's louder - and distorts FIRST.
    At the same time, bass is softer - and distorts much less, Distorts LAST.
    Hence, crunchy top end, and clean bottom, The Marshall Sound.

    However: in this 4100 design
    The bass is attenuated so much, it's almost a mistake. The result being that the amp screeches treble with little mids and bass to balance it out.
    It sounds UN-natural, because the treble is overpowering the mids and bass. It's not balanced out enough to sound good!

    Solution: (go to step 4 above)
    At R2 we are adding the bypass capacitor. .68 thru: 6.8 uF
    The amount is adjusted according to taste.
    This extends the low frequency response of the first preamp tube.
    Before, the low frequencies were blocked off...
    Now, low frequencies are allowed to enter the rest of the amplifier.

    Balance of Bass and Treble: (last part of step 13 above)
    The slope resistor R61 is adjusted to preserve the Marshall sound.
    The treble is allowed to be crunchy, as usual.
    The bass and mids are adjusted to remain much cleaner.
    When this adjustment is correct, the amp will sound "just like a Marshall."

    If the slope is adjusted incorrectly:
    The Bass will be overpowering, and muddy sounding.:nono::nono::nono:
    It won't sound like a Marshall.

    Then you can see that adjusting R61 slope resistor is a pretty important step in the process, to make the amp sound : "Just Like A Marshall."
    There IS a sweet spot - And it WILL sound just right, when set correctly!

    I am not trying to change the Classic Marshall Sound. I am trying to preserve it.
     
    reddog22 and tubes like this.
  15. ampmadscientist

    ampmadscientist Well-Known Member

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    If you have a better mod, or better ideas-
    You are most welcome to offer all of them.

    This modification was developed by guitar players - for guitar players.
    The sound it produces was developed by ear - using guitars, and speakers.
    The sound it produces was not developed using "theory" or "test equipment."
    It was done this way - to please musicians - for music.
     
  16. tschrama

    tschrama Well-Known Member

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    Neh, that's not the real reason. For that amount of gain you need at least such bass attenuation pre-clipping or you'll fart out. For higher gain I usually opt for 8-to-12dB more bass attenuation at 100Hz than this 4100. The heaviness, the bottom and huge feeling for modern tones come from bass boosting after clipping, and this is where the 4100 is lacking. And I've always suspected the output trannies to be bass light.

    Solution: Snipe one leg of R17, solder an 4.7nF cap between the leg of that R17 and the PCB where you just sniped it off. You now have a depth mod installed. (use 10nF if R17 is 47K).

    This gives a much more open , heavy feel to the amp.
     
  17. tubes

    tubes Well-Known Member

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    Much appreciated ampsmad.

    But, I dunno why, but some of the 'sonic' issues mentioned here just don't ring true in my experience.

    Sure enough, if I'm not careful, the 4100 can screech, as you mentioned.
    Sure enough, if I'm not careful, the 4100 can perform other unwanted tricks that you have mentioned - osscilation in the loop for e.g.

    But meanwhile it DOES normally have loads of bottom end.

    It's a tricky business: Of course I'm mainly interested in getting the perfect equalisation for our usual line-up here and the usual venues.
    It's not easy, we have a large band and, for some reason, I always think the sound should be well equalised up front, live.

    An equaliser in the loop does not always achieve the right mix.
    On the other hand, I don't really imagine a different amp would do any better neither.

    Actually, both of my Marshalls seem to sit themselves into a mix very well. I'm tweaking the details.
    I don't know about circuits but I appreciate those who do.
     
  18. ampmadscientist

    ampmadscientist Well-Known Member

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    C 15 should be a 400 Volt Cap, or higher.
    I never realized that some amps have a 250 volt cap installed for C15. That was probably a mistake.
    I would change C15 to 400 volt.

    Update: there is TWO C15s on my schematic!
    One is in the phase inverter.
    The other is at the input of IC8.
    You should change both C15s to 400 volt or higher.

    Thanks to members of this forum for the heads up concerning C15.
     
  19. Snake_j

    Snake_j Member

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    I took apart my 4100 to start a grocery list of caps/pots and to see what the general state of things are. It appears to be unmodded in any way and all in good condition;) The odd thing I noticed is that C21 and C22 were 25V, 1000uF already, they appear stock as they are the same brand/color, etc as the other caps on the board. My 4100 was made June of 90' and has the PC0112/JMP50B and PC0189/JMP52A rev boards. So was my amp already serviced at some point? The schematics show a 16V cap as stated in this thread.

    My JMP50B PCB has FS5, FS6, and FS7, which are all blanked. By FS6 and FS7 it states heater fuses...The schematic shows 6.3A on the heater filaments...Don't most heaters when bad just blow open as opposed to shorting? I could not find FS5 on the schematic I have, maybe I missed it. I saw pics of some boards that had them and some that do not...is there any benefit to adding them?
     
  20. ampmadscientist

    ampmadscientist Well-Known Member

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    The caps could have been changed, because they tend to fail faster.
    16 volt cap in 15 volt supply.
    The factory may have updated the parts...
    Seems like you don't need to change them if they are already 25 volt.

    Sometimes heaters arc to B+. The incorrect impedance speaker can cause arcing from screen or plate to heaters.
    That's what the fuse is for.
    They are hoping when it arcs, the fuse will save the PT.
     

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