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763 Blackvibe distorsion

Discussion in 'Shock Brother's DIY Amps' started by Baol1969, Oct 1, 2020.

  1. Baol1969

    Baol1969 TDPRI Member

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  2. tweedy_woodpecker

    tweedy_woodpecker Tele-Meister

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    Great to hear you might have solved the problem, there’s lots of knowledgeable and helpful folks here!

    And I like the red front panel with the white knobs, che bel colore, come una Ferrari!
     
    Last edited: Oct 2, 2020
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  3. Nickfl

    Nickfl Friend of Leo's

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    You might also check your bias, it looks to me like it's probably biased pretty cold, with your B+ voltages a -55 grid voltage seems like it would be somewhere around 45% dissipation so you might try getting it a little warmer.
     
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  4. Bergheim

    Bergheim TDPRI Member

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    I've tried the Type 3 MV with poor results, especially at low volumes. The Type 3 works by cancelling the outputs of the PI as they are out of phase. If the PI isn't perfectly balanced (which it isn't), it can cause a weird raspy distortion at lower volumes (higher level of cancellation). I can only imagine wrapping nfb around a type 3 will complicate things even more.. Does the problem go away with the MV dimed?
     
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  5. Baol1969

    Baol1969 TDPRI Member

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    I don't know what to say, yesterday everything was right and sounded great,today I turned it back on and sounds terrible, distorted and with a lot of noise, with sudden volume drops... it's really frustrating
    I turn it on, it sounds good for a few seconds then starts to "fuzz" a little, then volume drops, Then I shut it down 'cause I fear I could damage it (more than is it)
     
    Last edited: Oct 4, 2020
  6. Bergheim

    Bergheim TDPRI Member

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    With the 500+ voltages you had initially, it wouldn't surprise me if some of the filter caps are starting to fail. Other than that, I would inspect every solder/connection and swap some tubes.
     
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  7. Baol1969

    Baol1969 TDPRI Member

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    Yesterday I disconnected the master volume and the amp worked all day with no problems. However it also worked well for a couple of days with the master connected.
     
  8. sds1

    sds1 Tele-Afflicted

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    Wonky tube would explain this one away. The intermittent nature of the issue leans towards: intermittent tube, heat-induced fault, vibration-induced fault.

    It doesn't look like you've rolled tubes yet. Easy and natural to assume on a new build it's your fault, but it's also very easy to have a bad tube. Rule out the tubes out so you don't go mad chasing a ghost.
     
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  9. Baol1969

    Baol1969 TDPRI Member

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    Since I disconnected the Type 3 Master Volume a few days ago the ampli is working properly, at home not too loud. This evening I'll try to push the volume. Maybe later, if that was the problem, I'll try a different master. Maybe the Frondelli MV.
     
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  10. Nickfl

    Nickfl Friend of Leo's

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    I agree with @sds1 that you need to rule out a bad tube. It also sounds like you could have a bad connection somewhere. When you are having problems, does it include static and noise like a bad instrument cable or similar to the sounds you get when you plug into or unplug form an amp that is on? That would likely indicate a bad solder joint somewhere. You pictures aren't sharp enough to judge the soldering, but a couple of the joints look like they might be questionable.
     
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  11. sds1

    sds1 Tele-Afflicted

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    Yeah the fact that he is getting good results with "light play" is still consistent with vibration-induced fault. It's also still consistent with heat-induced fault.

    In that realm, on a new build, I think we'd usually be talking about tubes or an assembly issue like you say...

    The tubes will operate hotter, some resistors will heat up, the vibrations induced on the entire amp from the speaker will be greater... this is all happening when you push the amp to the max. And it should easily handle all of this and more.

    EDIT: if it's a head cabinet this is helpful to rule out the vibration-induced faults.
     
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