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Bassman Micro LTP master volume not working properly

Discussion in 'Shock Brother's DIY Amps' started by dantahoua, Sep 27, 2020.

  1. dantahoua

    dantahoua TDPRI Member

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    Hello!
    I just finished the Bassman Micro LTP from Robrob. Great amp.

    First problem I had was squeal when master-volume pot was past noon and the Presence pot was working reverse. So after searching this forum I found it seems to been a non negative feedback problem so I reverse the two wire going to the OT and now the amp sounds perfect!!! No squeal, no hiss. :)

    Note that I did not found any 47pF capacitor in my stock so I did not put the "stability cap".

    The only "problem" I have now, the master volume pot seems to be not effective. It goes from no sound to full sound directly with a pinch of a turn. I double check the value and the taper, everything seems ok. Not a big issue but maybe someone could enlight me on this and maybe it's because of another problem in the amp...
     
    sds1 likes this.
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Seems pretty straight forward. Just to reaffirm... using your meter, the sweep (the side of the pot you are using) of the MV pot does not *jump* in value as it is turned?

    Send some gut shots of the MV part of the circuit please.
     
    sds1 likes this.
  3. dantahoua

    dantahoua TDPRI Member

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    The pot does not jump, I tested it...
    The master volume is the pot near the hole on the pic. It's very simple, so that's why I do not understand why it do not work as it should!
     

    Attached Files:

  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I don't see anything out of place. Is it possible the two 82k grid leak resistors are not grounded? Clip in a jumper from their junction to ground just to be sure.
     
  5. dantahoua

    dantahoua TDPRI Member

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    Yes they are from under the board... Not such a big deal, but you know how it is... We always want it to be the most perfect as possible... ;)
     
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  6. andrewRneumann

    andrewRneumann Tele-Holic

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    What resistance does that MV pot show when you have the knob at 50% rotation?
     
  7. dan40

    dan40 Tele-Afflicted

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    I have used the same "crossline" MV in my build and it also worked exactly the same. I have tried this MV in several amplifiers and it always seems to act this way.
     
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  8. andrewRneumann

    andrewRneumann Tele-Holic

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    I think @Tom Kamphuys also had this problem on his last build. Ended up pulling out the post-PI MV.
     
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  9. andrewRneumann

    andrewRneumann Tele-Holic

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    Assuming everything else is right, try temporarily lifting one end of the NFB resistor (to remove NFB) and see if the Post-PIMV plays nice. I’ve begun to think there shouldn’t be any volume controls in a feedback loop, but that’s me and my limited experience.
     
    dan40 likes this.
  10. dan40

    dan40 Tele-Afflicted

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    It seems like this particular type of MV will work perfectly well in some circuits but in every amp I have tried them in they seem to act more like an on/off switch.
     
  11. dantahoua

    dantahoua TDPRI Member

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    So if I understand this problem is recurrent with this design! Anyway, I forgot to make a hole for a master volume on my build, so i'll just remove it, simple as that. (there was two additional hole for another gain and another volume, I wanted to make kind of two channel, but finally I prefer the simpler one channel!).
     
  12. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    I had a problem with the post PIMV in a jcm800. That is a different beast as the original design has a pre PIMV which limits the signal going into the ltp. My ltp was clipping all the time. (If I recall correctly). Multiple (if not all) builders had the same problem and the post PIMV has been replaced by a pre PIMV by Rob.

    A bassman normally has no pre PIMV and the signal going into the ltp is lower by design. I think my jcm800 problem should not exist here.

    I'll reread my own thread and see whether I can remember some more.
     
    robrob likes this.
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