18w mini plexi mv

Discussion in 'Building the Classics' started by Marcomel79, Jul 17, 2021.

  1. Marcomel79

    Marcomel79 Well-Known Member

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    Hi, i just built a mini plexi clone from tube town and i installed the master volume nr.3 from the trainwreck pages. It worked well for some time and then it started sounding weird, like if there is some kind of delay and the sound is thin and buzzy.
    Anybody has any experience with it?
     
  2. _Steve

    _Steve Well-Known Member

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    Does the issue only occur with the MV installed? ie if you disable the MV does it work normally?
     
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  3. Pete Farrington

    Pete Farrington Well-Known Member

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    Do any other controls affect whether it goes into the ‘bad sound’ mode?
    Is there a ‘tipping point’ on some controls (eg volume, treble, mid), such that the sound is fine till a certain setting, but above or below that, it goes bad?
    A demo of the issue on YouTube may be useful.
     
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  4. Marcomel79

    Marcomel79 Well-Known Member

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    Hey Pete it just sounds very strange no matter what setting. Listening to some people talk about that mod, as i understand, both signals from PI get mirrored and cancel each other out. Or something to that extent. What i hear, its like a delay in the sound. Also the sound is distorted as soon as i roll the channel volume on, with the MV very low. Without the MV it works perfectly fine... ill try to get a video up on youtube tomorrow with alligator clips.
    It worked fine the first week and suddenly it didnt anymore...
     
    Last edited: Jul 18, 2021
  5. _Steve

    _Steve Well-Known Member

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    Sounds like you are running fixed bias? If so, that particular PPIMV method doesn't work well with fixed bias amps.
     
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  6. pedecamp

    pedecamp Well-Known Member

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    It was working and now its not. Check your work and the parts you used.
     
    Last edited: Jul 18, 2021
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  7. Marcomel79

    Marcomel79 Well-Known Member

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    It is. But the strangest thins is that it worked beautifully at first... heres a little clip from when i first tested the amp
    https://youtube.com/shorts/vAR7p4Xc1Is?feature=share
     
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  8. Pete Farrington

    Pete Farrington Well-Known Member

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    I’ve not come across that before. What’s the thinking / experience behind it?
    I suppose a lot of modern amps have independent bias (eg for each push pull side), so a Type 3 wouldn’t be appropriate in that scenario.
     
    Last edited: Jul 19, 2021
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  9. 2L man

    2L man Active Member

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    Check Biases!!! Tubes can drift when they are new.

    Also measure bias when you turn MV low to verify it does nor effect to biases.
     
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  10. pedecamp

    pedecamp Well-Known Member

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    I'm thinking one of the components you added went bad, test everything with a meter.
     
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  11. Marcomel79

    Marcomel79 Well-Known Member

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    Im not sure if thats the case because it works perfectly without the MV.....
     
  12. Marcomel79

    Marcomel79 Well-Known Member

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    The anp is a fixed bias so i checked it a few times afterwards and it was constant....
     
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  13. pedecamp

    pedecamp Well-Known Member

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    Correct, I'm talking about your master volume, check the components. I'm no tech so I shouldnt be giving any advice but my comments are based solely on logical assumption. Good luck with it, hopefully somebody will be able to help you here. :yesway:
     
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  14. Marcomel79

    Marcomel79 Well-Known Member

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    :):)
    I checked the pot before installing it and it read the correct values.... i dont mind playing it loud, but my wife sure might :)
     
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  15. 2L man

    2L man Active Member

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    I think someone recently wrote that "type-3 MV" is not good for grid bias and it would surprice if its use does not change the bias? Perhaps not when almost dimed but when approaching zero?
     
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  16. Marcomel79

    Marcomel79 Well-Known Member

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    When i was just done building the amp the MV worked perfectly with the volume very low, like on 1 or 2.... im still happy without the MV but it would be nice to be able to play it at a bit lower volume.
     
  17. 2L man

    2L man Active Member

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    I understand that and thats why I recommend messuring what are both tube bias currents when you turn MV to zero? If biases drift when you use MV away from 10 it can change sound worse?
     
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  18. Pete Farrington

    Pete Farrington Well-Known Member

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    What’s the hypothesis for the reason that might occur?
    The phenomenon was put forward in post #5, but despite my request for clarification, none has been provided :scratch:
     
  19. _Steve

    _Steve Well-Known Member

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    Ah that was me sorry I didn't come back. I had this issue on one of my very first builds and had someone point out the incompatibility at the time. I'll try to search for the post and share. The problem occurs when the PPIMV volume is set low.
     
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  20. 2L man

    2L man Active Member

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    Type-3 master volume shorts control grids when turned to zero. When g1 current is small perhaps this short between g1's change bias balance and then it might still change bias balance when MV is used on low settings?

    Perhaps when tubes were new they kept balance but now after some use they don't keep the balance when g1's interfere?
     
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