Cascading Non-mv Plexi?

Discussion in 'The Workbench' started by mAx___, Apr 8, 2019.

  1. mAx___

    mAx___ Active Member

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    Hi guys,

    I was thinking in cascading V1a and V1b of a 1959-style preamp for a future build.
    I read and watched videos of the One-Wire mod and it's said to work only (or, as intended) with PPIMVs installed.

    Mine will be a Non-MV build. What would be the best way of cascading inputs for this one?

    I'm trying to come up with the way the factory would do it on an existing Plexi rather than a rethinking of the circuit 2203 kind of thing. I looked up everywhere but found no specifics.
    Would the Randy Rhoads mod be what I'm after? Couldn't find any schematics for that one.

    Thanks!
    M.
     
  2. neikeel

    neikeel Well-Known Member

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  3. mAx___

    mAx___ Active Member

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    Thanks Neil! So this mod would give me a cascaded 2203 circuit on one side of the switch and a 1959 on the other? I couldn't zoom in enough to see well.
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    a 2203 / 2204 "is" a modified plexi. They just take the 2 preamps and put them in series.
    (call it cascade if you must)
    The difference being is that the low frequency of the input is rolled off more to prevent the preamp from getting too muddy in the lows.
    This is accomplished by changing the cathode bypass cap of the first stage, to limit subsonic response.

    You will need to shield the input jack, and shield the volume control wiper wire to avoid wild oscillations in the preamp.

    If you don't use a master volume you need to control the loudness another way. (like an attenuator)
    Because this will make the amp quite a bit louder...

    If you look at 2203 schematic you are looking at the way the factory "would" do it.
     
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  5. mAx___

    mAx___ Active Member

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    How did the factory modded the Randy Rhoads SL? I gather they kept it a four holer while doing a small change in the circuit that kept everything almost the same (expect now the inputs are cascaded) without going the 2203 route. Is this correct? If I could find that schematic...all the old links to photobucket I found on internet forums are not working anymore.
     
  6. DesolationBlvd

    DesolationBlvd Member

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    Found the 1959RR schematic here: http://www.musiciansroadhouse.com/viewtopic.php?t=11593

    The big change is that the normal channel's mixer resistor is routed to the bright channel input instead of V2A. The bright channel coupling cap is also increased from 2n2 to 22n.

    I did the mod to my Studio Vintage. Very hard to dial in, way too much gain and bass introduced early in the signal path. From there, I lowered the normal channel bypass cap to 1u from 330u, did not increase the bright channel coupling cap, and put a 470k resistor to ground after the 470k/470p bright channel mixer.
     
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  7. ampmadscientist

    ampmadscientist Well-Known Member

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    If you cascade a plexi you are going the 2203 route.
    There isn't any real difference.
    You don't have to use a master volume but it will be very hard to play because it's so loud.
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    .
    Comment deleted due to brain failure.
     
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  9. ampmadscientist

    ampmadscientist Well-Known Member

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    https://drtube.com/schematics/marshall/jcm800pr.gif
    This is the schematics of both preamps side by side. As you will see there isn't a lot of difference between them.
     
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  10. mAx___

    mAx___ Active Member

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    Not a lot of differences but significant for what I intend to do. I'd rather not have a MV, and I'd like to keep the four inputs instead of two like the JCM800.

    Do you know the reason for the 10K cathode resistor in the JCM800 replacing the regular plexi resistor/bypass cap solution? Less gain?
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    the 10K lowers the sensitivity
     
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  12. mAx___

    mAx___ Active Member

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    Thanks so much! So it looks to me like a "one wire mod" kind of thing. Easy to try and remove if it doesn't work right.
     
  13. ampmadscientist

    ampmadscientist Well-Known Member

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    One wire mod isn't really doing it the best way so don't expect it to be so great.
    If you want to do it better use the 2203 schematic.

    For one thing you will get too much subsonic / low frequencies, which muds the sound up.
     
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  14. stickyfinger

    stickyfinger Active Member

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    Whats great about the RR mod is you can leave one channel stock and the other will hot rod (cascade) in to the stock channel you choose. The hot rod V1 can be tweaked to what ever you like.

    Neils special looks very interesting an seems like that would be the best for classic Marshall in on package.
    Need to try this.
     
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  15. neikeel

    neikeel Well-Known Member

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    I did a couple of RR mods. I found like Desolationblvd that they were hard to dial in and the other 1ww mods a bit of a one trick pony at least in my hands.
    The 2 in 1 works really well (ok I am biased!) the layout as shown is to mod a HW type 1987 with minimum changes. In a 100w chassis I have a better way that involves keeping v1 as a shared cathode and adding an extra tube that is 2203 spec. I put 120k plate resistors on this tube.
    Regarding 10k on second stage, it is referred to as a cold clipper stage. It maintains that Marshall Kerrang definition. You can lower it for more gain but it will lose definition and send you in the SLO direction. Lowest is 6k8 IMO. You can bypass it with anything from 0.1 to 0.68uF but I prefer 8k2 unbypassed.
    I have done all this experimentation myself but others may prefer different tones and need to do this themselves with trimmer pots and cap boxes to come to your own conclusions as I am just a hack hobbyist.;)
    (With a bit of background)
     
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  16. purpleplexi

    purpleplexi Well-Known Member

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    I un one wire modded my plexi. I hoped it would give a plexi but moreso but what I got was something else that I didn't much care for. This looks interesting though - I might give this some thought.
     
  17. DesolationBlvd

    DesolationBlvd Member

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    More like "needs to be tweaked". If the normal channel is the one being made into the booster, the cathode bypass and/or coupling capacitors need to be reduced to get rid of the excess bass.

    I went through with sacrificing the normal channel, because I found that various boost/overdrive pedals in front of the bright channel got me almost what I wanted. After playing the amp yesterday evening (I only finished the mod late Sunday night, too late to turn it up even attenuated), I can say it was worth it. The boost from the extra triode blended into the stock tone better than the pedals ever could.
     
    Last edited: Apr 9, 2019
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  18. mAx___

    mAx___ Active Member

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    From your previous post I gather that you ended up with 1uF instead of 330uF?

    My problem right now is that what I like for the Bridge pickup on my Strat, sounds too muddy on the Neck pickup. And if I adjust values to remove the excess bass, the bridge pickup sounds too brittle. I need to find the right balance...
     
  19. neikeel

    neikeel Well-Known Member

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    It’s up to you.
    I did a fair amount of experimenting so that you don’t have to!!
     
  20. mAx___

    mAx___ Active Member

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    That's great. IIRC you ended up with a 1uF bypass cap for the second triode too? I can barely read the values in the Neil Special layout...
     

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