Input Grid Resistor. Mod, Or Keep Stock? '81 2203

Discussion in 'The Workbench' started by JakeAC5253, Nov 30, 2016.

  1. JakeAC5253

    JakeAC5253 Member

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    I've heard some people recommend to lower the value of the input grid resistor on my 1981 JCM800 2203 from 68k to 1k and that this increases gain slightly and lowers noise at higher gain levels. I'm normally a guitar straight to amp guy, but I do like running some ODs (OD808, OD820, Modded SD-1), distortions (Japan HM-2) or fuzzes (Fuzz Face, Modded Big Muff) from time to time.

    Should I fuck with the value or not? I have researched a bunch of mods for the amp already, and most when you look deep enough, you find reasons to leave it stock.
     
  2. johan.b

    johan.b Well-Known Member

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    Might add a tiny bit of zizle in the highest highs but also risk adding in radio interference. I'd leave it. Or replace with same value metalfilm resistor if you worry about noise.
    Remember, people usually remove the bright cap to reduce highs. This would do the opposite.
    J
     
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  3. Jazz P Bass

    Jazz P Bass Active Member

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    Keep in mind that it isn't simply a 68K resistor.
    It's a circuit.
    There is also a 1Meg resistor to ground.
    That's refered to as a voltage divider.

    Personally, as the rest of the amps circuits rely on that first stage, I wouldn't mess with it.
    There is a reason most all amp input circuits use that 68K/ 1 Meg divider circuit.
    It works very well.
     
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  4. johan.b

    johan.b Well-Known Member

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    on the 2203/4 the 1M comes before the grid resistor, so only the miller effect plays in.
    even if it came after, it's a near 1/20 ratio. you loose less than a db...had it been 20/1, it would have been significant
    J
     
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  5. ampmadscientist

    ampmadscientist Well-Known Member

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    No, you really don't want to change that: maybe 33K at the least.
    All that happens is that it overloads the input, resulting in muddy-ness, especially w/ effects pedals.

    No, it does not "increase gain," at all. It does increase sensitivity, but too much is bad.
    Gain is limited by the power supply voltage.

    But what you can do: is use higher gain (selected) preamp tubes, and this makes a big improvement.
    (high gain tubes are actually high sensitivity, it's not the gain that is making a difference)

    "gain" and "sensitivity" are 2 different things...
    What you want is: increase sensitivity...the "gain" is not the actual issue.

    The amp has a fixed amount of gain...and you cannot increase that. (V2 preamp is a compressor stage, which limits the gain)
    No matter how much signal goes into V2....it cannot produce more gain...only more compression. The output of V2 is always LESS than the input.
    BUT what you can do is increase sensitivity...which is actually what you DO want.

    Marshall Plexi: sensitivity = 10 mv
    It takes 10 mv of input signal, to drive the amp to full 100W output.

    Marshall 2203: sensitivity = 5 mv.
    It takes 5 mv of input signal to drive the amp to full 100W output.

    So you see the difference?
    2203 is twice as sensitive as Plexi. But both amps are 100 watt.
    The difference is sensitivity....not gain.
     
    Last edited: Dec 2, 2016
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  6. stickyfinger

    stickyfinger Active Member

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    Its a grid stopper. The first one is the noisiest resistor in the amp. Making it smaller is always a good idea if Radio interference doesn't come in to play. Amps have had improvements since the typical vintage value of 68k. A bunch of newer amps (Vox/Fender/ect) use replace the 68k with a 10k/100p to ground divider. This gets ruffly the same frequency shaved off while allowing the resistor to be much smaller/less noisy.
     
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  7. ampmadscientist

    ampmadscientist Well-Known Member

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    I think resistor too small makes the amp sound (too hot) overloaded...I am frequently making the resistor larger, not smaller.

    But true, first resistor does contribute to noise the most, that is undeniable.
     
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  8. neikeel

    neikeel Well-Known Member

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    I agree with AMS on this, I find 33k works well (which is effectively what you get in a 4-holer with a pair of 68k in parallel) for not being too big, giving optimum sensitivity, effective as a grid stopper for noise and giving the pups a loading that works too.
     
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  9. JakeAC5253

    JakeAC5253 Member

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    I figured as much. I had reservations about it, pretty much everything mentioned here, just wasn't sure if it was a common modification for these amps or not as this is my first Marshall. I really have no issue with the sound of the amp currently. I am going to be adding a 3 position switchable gain mod to it in the next days as soon as parts arrive, and somebody recommended the grid stopper mod because apparently in modded gain ranges, noise can become an issue. I suppose I'll leave it stock (fantastic amp, no doubt worthy of its legacy) or put another 68k in parallel if noise really is very noticeable at higher gain levels.
     
  10. pleximaster

    pleximaster Well-Known Member

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    I know this is an old thread but I resently saw my third old Jtm45 with 47k gridstoppers instead of 68k

    I have previously always though divination from the 68k like 47k and 56k we’re replacements. But the h looked good

    Anyone else seen Marshall used off values in plexis, non 68k grid stoppers???
     
  11. stickyfinger

    stickyfinger Active Member

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    Nope but seen 82k replace 100k plate resistors.
    I'm sure they used another value when they ran out undeniably
     
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  12. FourT6and2

    FourT6and2 Active Member

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    I use 33K input grid stopper on V1. Any Marshall that had multiple inputs/channels with 68K input resistors were running two in parallel when using only one input. That's 34K. So when I build single-channel amps, I just use 33K.
     

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