Can anybody help identify the mods in my 1987X?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Random Hero, Jan 3, 2018.

  1. Random Hero

    Random Hero Member

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    So I know I have a switchable bright cap, cascade mod and coupling cap. It also has a PPIMV.
    Can anybody help identify which version of the PPIMV and cascade mod I have?

    Regarding the PPIMV; I find it HELLA bright and I think that's due to the "A1M" pot? I'm guessing that's an Alpha 1 meg potentiometer? The caps all read "223J 400A". Beyond that, which I'm gonna get switched out I think, what else is going on?

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    Also, here's the cascade mod. Can anybody tell me what they see here?
    Interestingly with this, when I switch it on, I have to switch to the normal channel inputs as it cascades normal into bright I believe. BUT, when I jump the channels as normal, input 1 still works? Can anybody see why?

    Also, why is the PPIMV deemed imperative with the cascade mod? Is it because it's extremely hot and you want to limit what's going to the power amp to keep that cleaner? I always see that you must have one with the cascade mod.

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    Thanks!
     
  2. stahlhart

    stahlhart Member

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    I'd probably go bigger than .022uF for the PPIMV coupling capacitors, for starters.
     
  3. TimmyP

    TimmyP Member

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    "... why is the PPIMV deemed imperative with the cascade mod? Is it because it's extremely hot and you want to limit what's going to the power amp to keep that cleaner? I always see that you must have one with the cascade mod."

    Turn the master all the way up and play away to see if you need a MV at all. If so, one assumes - based on "what they say", that the PPIMV gives a more 'power tube like' distortion.
     
  4. Avatar Tech

    Avatar Tech Member

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    Your PPIMV appears to be just a dual ganged pot with coupling caps on either side of it. I agree with @TimmyP. It's not an absolute necessity with a cascaded preamp, and removing it is not going to magically warm the amp up. You need to compare the values of the components in your amp to the original schematic. It's been modded so tiger's a good chance that some of the original values have been altered according to someone else's taste or needs.
    A cascade mod is often accompanied by some other general high and low end attenuation mods in order to make the amp sound good with the cascade mode engaged. If it were my amp, I'd figure out how far it's been taken away from the original schematic, then make some decisions about how to best address my tonal needs.
     
  5. Random Hero

    Random Hero Member

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    That's what the Rich Mod uses, no?
     
  6. stahlhart

    stahlhart Member

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    I'm not terribly familiar with the component details for mods currently being done -- my knowledge is somewhat aged. Back in the day they would typically put .05-.1uF caps after the phase inverter. Recognize that this is many years before the Internet, and the knowledge traveled differently back then.

    You did say "hella bright" in the OP, and my thinking was that you'd get a bit more low end response if you increased the value of those.
     
  7. gldtp99

    gldtp99 Member

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    http://www.blueguitar.org/new/schem/misc_mod/pi-m-vol.pdf

    This is similar to the OP's amp------ years ago many people would use larger value caps in the "after MV pot" position------ sometimes up to as much as 10x the value of the "pre MV pot" set of caps.

    When the MV pot is cranked to the loudest position the caps will effectively be in series------- if both caps are .022uF then you can see how they might limit bass response with two in series.

    Use whatever values work for you.
     
  8. MKB

    MKB Silver Supporting Member

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    The PPIMV is the standard Marshall type, where the bias circuit is left untouched and the signal is attenuated by the PPIMV pot. Marshall usually uses a 1M dual pot for this IIRC. All coupling caps tend to be 0.1uF. I'd suggest you make sure the caps on the PI side of the PPIMV pot (the ones not on the wipers) are rated at the correct voltage, I'd go for 630V there. The ones in the amp look mighty small.

    While you're at it, those PPIMV caps need to be secured better than they are, and the cable shields should be better insulated. And make sure any unshielded wires from the front panel switches\pots are kept well away from the PPIMV and phase inverter sections.

    If there are added wires with high level signals such as in the phase inverter near wiring on the preamp gain mods, you could be getting parasitic oscillation or parasitic coupling that is causing the treble boost. You have to be very careful with flying wires in high gain amps.

    IMHO that mod job wiring looks a bit poor. It needs to be cleaned up.
     
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  9. Random Hero

    Random Hero Member

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    Curious how the different value pots will sound in a PPIMV?
     
  10. Random Hero

    Random Hero Member

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  11. Avatar Tech

    Avatar Tech Member

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    Typically a lower value will slightly attenuate high end. However, if you need to tame the high end in general, IMO there are simpler and more effective methods available.
     
  12. pdf64

    pdf64 Member

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    How the pot value affects frequency response will probably differ according to the pot's setting.
    This due to there being several parasitic etc effects interacting with the pot resistances, and it's all inside a feedback loop.

    How do you normally set the presence control, on a regular 2203 type Marshall?
    As a Post PI MV is turned down, the presence control will become pretty much disabled, but revert to a native open loop power amp frequency response that is equivalent to a high presence setting, ie it will tend to track the speaker impedance/frequency curve.
    Maybe that is the cause of your issue.
    If so, it's basically unrelated to the MV pot value, it's just inherent to how a Post PI MV operates in such an amp.
     
  13. Random Hero

    Random Hero Member

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    I normally set the presence quite high... 6 or 7, sometimes a hair more.

    That said, I'm finding it better with the bright cap out - pretty much as I expected it to sound that way. With the bright cap in, it's super bright! This is using the PPIMV too.
     
  14. sickboy79

    sickboy79 Member

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    Sometimes, PPIMV designs can be bright at lower volumes. I would expect that goes away at higher volume levels.
     
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