Why did those Plexis sound different

Discussion in 'Marshall Amps' started by wonderingape, Jun 1, 2020.

  1. ampmadscientist

    ampmadscientist Well-Known Member

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    Get your scope and try it.
    I always use a scope for just about everything.
     
  2. ibmorjamn

    ibmorjamn Well-Known Member

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    If the preamp tubes 12ax7 common ( just assume not lower amplification tubes). The configuration of the preamp stages does overdrive less than the master volumes that followed . I don’t know the specifics of each resister or capacitor but the schematics are everywhere. A little research will tell all but schematics tell the difference. The vintage parts and tubes help .
     
  3. SmokeyDopey

    SmokeyDopey Well-Known Member VIP Member

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    I don't have a scope, that's why I ask. And I haven't even biased an amp yet. Still learning...
    Don't want to poke around inside an amp just yet.
     
    Last edited: Jun 1, 2020
  4. wonderingape

    wonderingape Active Member

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    I'm simply asking because there are so many different answers regarding where the distortion come from. But I think even in NVM amps, all the components play parts in producing distortion.

    I don't think there is such thing as "the old amps from the 60's are pure power tube distortion".
     
  5. SmokeyDopey

    SmokeyDopey Well-Known Member VIP Member

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    The videos I've seen where they show a clean signal and when it begins to distort is having the probes connected to the output speaker jack with a dummy load. I haven't seen where they monitor the preamp and power amp distortion as the gain is increased.
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    That first initial shock....you'll be addicted for life.
    Seriously, once you get a good 500 volt jolt :lol:, you learn electrical safety very fast.

    I think I started at age 3.
    My mom told me to never put a bobby pin into an electrical socket.
    So I did.
    I wanted to find out what it was like.
    There was kind of a burning smell.
     
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  7. SmokeyDopey

    SmokeyDopey Well-Known Member VIP Member

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    I've been shocked many times. Electric fences, 220V wall sockets, and cap discharges. That didn't scare me away :fingersx:

    As a kid I tried to revive a toad with an electric fence.
    When I was 13 or 14 I attempted to make a lightning rod I put up in a tree out of some scrap metal, and wanted to connect to an old 12V car battery (never finished that project). And I enjoy arc welding.

    So I've learned a bit (the hard way in some cases), and would like to understand an amp end to end before I have one open and live in front of me.
     
  8. wonderingape

    wonderingape Active Member

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    Aren't the PI and the preamp tubes also clipped? If I remember correctly, I saw someone put the Trainwreck Express on the oscilloscope and it showed the speaker output is the most hard clipped but the PI and the preamp tube on the third stage output were also clipped but not as hard. So, it could be interpreted as 40% of the distorted sound come from the power tube, 30% from the PI and another 30% from the third stage output. Isn't it?
     
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  9. ampmadscientist

    ampmadscientist Well-Known Member

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    I never sat down and calculated it that way.
     
  10. ampmadscientist

    ampmadscientist Well-Known Member

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    Yeah I had a wire antenna when I was 12. It was struck by lightning.

    Then later I had a tower, 355 feet.
    Lightning struck that sucker and ball lightning rolled out of the antenna match cabinet, then rolled across the floor, and down the drain.
    I was like: FUCK! :run: Did that just happen?:rofl: holy sheepshit.
    I was like 18 inches away from being vaporized.
     
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  11. wonderingape

    wonderingape Active Member

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    So to clear any confusion, in conclusion, every components play parts in producing distortion and there is no such thing as pure power tube distortion just like there are no such thing as pure preamp distortion, PI distortion, etc.
     
  12. SmokeyDopey

    SmokeyDopey Well-Known Member VIP Member

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    Oh man!
    When I was a kid we moved into a very old house in the countryside where there was an old powerline that was touching the wall (no one paid attention and noticed it), there was a lightning storm and it hit that power line. Since it was touching the wall it blew and made a hole in the wall. I was in the next room when it happened and probably jumped a few feet. Went running to see what the hell happened and there was debris all over, smoke, and a big hole in the wall. The cat was just lying on the bed as if nothing.
     
  13. Lukas

    Lukas Well-Known Member

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    Cause they were made by little elves in Santa’s work shop.
     
  14. South Park

    South Park Well-Known Member

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    The phase splitter does not have to clip because the signal is clipped . Now when You get preamp tubes clipping and power tubes clipping What kind of harmonics do you get
     
  15. ampmadscientist

    ampmadscientist Well-Known Member

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    That's my tower right there. I say "my tower" because I designed and built a lot of it.
    It was the same height as a Saturn 5 moon rocket.
    Well, technically 357 feet if you measure real close.

    tower.png
     
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  16. BowerR64

    BowerR64 Well-Known Member

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    I always thought the parts used in those older marshalls were way over speced also.

    Like if it neded a cap at 10-15 volts they used caps rated at 50 also the resistors, look at the resistors used today and those monsters used back then. The leads used in them were beefy and made of real metal not some mix of crap that had just enough to make a connection.

    I have a handful of parts my dad collected back in the 70s-80s he worked on CB radios and TVs and these are the same parts from the best marshall era and they are just different big and beefy.

    I also noticed that the same value parts are WAY off from each other not near as tight on tolerance as more modern parts. Could be the way they were made or the age its hard to say
     
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  17. ampmadscientist

    ampmadscientist Well-Known Member

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    No
    It is power amp tube distortion. And the preamp is still clean.
    Yes the PI output is distorted when the preamp is clean.
    "pure" distortion just from the power amp yes.
     
  18. wonderingape

    wonderingape Active Member

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    Then the only components producing the distortion are the PI and the power tubes (how about the cathode and caps?). Is it at half volume or max? I think at max the preamp tubes started to clip too. Scope images of this will be interesting.
     
  19. ampmadscientist

    ampmadscientist Well-Known Member

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    Ummmmm
    the components are there to make the circuit work.
    I don't think you attribute distortion to the components.

    When the power amp clips (FULL power), the preamp is still clean.
    At full power, when you reach that point, the preamp tubes are not distorting.

    When you push the power amp beyond full power, there is power amp clipping.
    But at that point just at clipping, the preamp is still clean.

    Sure you can push the preamp to distort if you drive it hard enough.
    That's why yahoos use boost pedals.

    But personally I like no pedals.
    I just set the power amp into distortion. To me, that's when it sounds glorious.
    I don't really like muddy preamp (that's the American way mud in the preamp)
    The British way is avoiding the muddy sound from the preamp.
     
    Last edited: Jun 2, 2020
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  20. SmokeyDopey

    SmokeyDopey Well-Known Member VIP Member

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    I still find that hard to believe. It may be true, but I've never seen proof. I was under the impression that both stages distort.
     

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