>

470K attenuator resistors...ever messed with them?

Discussion in 'The Workbench' started by Matthews Guitars, Jun 16, 2021.

  1. Matthews Guitars

    Matthews Guitars Well-Known Member

    Joined:
    Aug 17, 2019
    Messages:
    4,259
    Likes Received:
    6,405
    I just had an interesting modded amp across my bench for a while. It's a blackface Fender Bassman, 1965 by the serial number, and it had some simple mods done that made it a real screamer, but it could have been reverted to dead stock condition in 15 minutes if you wanted to.

    The drawback is that it was SO hot that you sacrificed almost all of the clean headroom.

    As I investigated it, to find out what the mods were and how they worked, I soon found a single resistor change that was responsible for most of its impressive level of drive.

    The 470K attenuator resistor on the grid of the second preamp tube had been dropped to just 22K.

    Marshalls share that feature.

    There's a 470K attenuator between V1B and V1A on a 2203/2204 and other models that share the preamp/master volume circuit, and two resistors of the same value that serve the same function in a 1987/1959.

    Have any of you played with reducing these resistor values for easy gain boosts?

    What were your results and conclusions? I realize that if you go too low, blocking distortion is likely to occur.

    Of course it's easy enough to do this mod. But I want to hear your opinions before I even bother to try it.
     
  2. neikeel

    neikeel Well-Known Member

    Joined:
    Feb 12, 2014
    Messages:
    6,017
    Likes Received:
    4,741
    On a 1987/1959 they are more a voltage divider, there should be similar arrangement on a 2203/4 plus the treblepeaker. It’s like swapping a 1meg pot for a 50k pot at mid point.
     
    junk notes likes this.
  3. johan.b

    johan.b Well-Known Member

    Joined:
    Feb 21, 2015
    Messages:
    1,932
    Likes Received:
    2,422
    Location:
    Södertälje, Sweden
    The Groove Tubes Trio preamp crunch mode was basically a 2203 pre with different values at that point.. and if I remember correctly, forum member AlvisX, talked about messing with those values and the peaker cap...
    J
     
  4. neikeel

    neikeel Well-Known Member

    Joined:
    Feb 12, 2014
    Messages:
    6,017
    Likes Received:
    4,741
    @johan.b iirc AlvisX likes removing the treble peaked cap.
     
    johan.b likes this.
  5. StingRay85

    StingRay85 Well-Known Member

    Joined:
    Aug 18, 2019
    Messages:
    1,336
    Likes Received:
    1,048
    Basically it decides if 50% or 100% of the signal is removed, depending on position of the preamp gain pot. It avoids overloading the next gain stage. But in theory you can experiment with these values no problem.
     
  6. johan.b

    johan.b Well-Known Member

    Joined:
    Feb 21, 2015
    Messages:
    1,932
    Likes Received:
    2,422
    Location:
    Södertälje, Sweden
    .. I've been meaning to try it for a long time but never seem to get to it... but you could try reducing the 470k paralleled with the 470p cap. It should make the treble slope less steep...or more gentle, if you will,... I'd try a 100k and go from there... same with the 470k/470p before the preamp pot... perhaps someone else already tried it..?
     
    Last edited: Jun 17, 2021
  7. Max Gahne

    Max Gahne Active Member

    Joined:
    Dec 1, 2020
    Messages:
    191
    Likes Received:
    244
    Take a look at an AB165 schematic which might be the amp you're working on - AB165 is a 1965 Bassman. The 470K resistor you're describing might be the one connecting the B voltage to the signal path before the coupling cap of the second preamp tube. Something I'm not familiar with. But it's not a typical 470K/470K attenuator like in a 2203/04.
     
    neikeel and Pete Farrington like this.
  8. Pete Farrington

    Pete Farrington Well-Known Member

    Joined:
    Jan 18, 2021
    Messages:
    367
    Likes Received:
    359
    Yes, a schematic / model ref for this amp would be great, I looked at a few but couldn't match any with the description in post #1.
     
  9. Max Gahne

    Max Gahne Active Member

    Joined:
    Dec 1, 2020
    Messages:
    191
    Likes Received:
    244
    here's the schematic I'm referring to. Whether it's the right one or not I don't know but @Pete Farrington take a look at where the 2 channels merge at the 470K resistor going to the plate voltage. What does that do? bassman_ab165_schem.gif
     
    Pete Farrington likes this.
  10. johan.b

    johan.b Well-Known Member

    Joined:
    Feb 21, 2015
    Messages:
    1,932
    Likes Received:
    2,422
    Location:
    Södertälje, Sweden
    If ab165 it could be the local feedback after the mixer. Yes reducing mixer values would change things..
    Screenshot_20210617-204100_Google.jpg
     
  11. Max Gahne

    Max Gahne Active Member

    Joined:
    Dec 1, 2020
    Messages:
    191
    Likes Received:
    244
    It does look like local NFB. If this is the right schematic and resistor @Matthews Guitars did they change it to 22K or 22M?
     
  12. Pete Farrington

    Pete Farrington Well-Known Member

    Joined:
    Jan 18, 2021
    Messages:
    367
    Likes Received:
    359
    Yes, that stage is configured as an inverting mixer 'virtual earth 'NFB amplifier.
    Gain = about 220k/470k so about 2.
    Altering the 470k to 22k would reduce the gain to 0.1, ie a 1/10 attenuator.
    Aiken outlines the theory https://www.aikenamps.com/index.php/designing-single-stage-inverting-feedback-amplifiers

    Whatever, it's not something that's akin / applicable to any regular classic Marshall.
     
    neikeel likes this.
  13. neikeel

    neikeel Well-Known Member

    Joined:
    Feb 12, 2014
    Messages:
    6,017
    Likes Received:
    4,741
    Sorry, from the description I was thinking the mixer resistors on a Blackface 6G6-B ('64) which has a 470k/470k pairing before the grid of the PI.
    [​IMG]
    Changing the either 470k after V2 (bass channel) or V1 Normal for a 22k would really give a very high signal boost for that channel
    [​IMG]
     

Share This Page