Tuesday 2 October 2018

Marshall Plexi 1987x

Next, t Marshall Plexi 1987x.






With a switch to go between channels


With 2 separate inputs like the amp


With both channels jumpered




38 comments:

  1. These are very interesting Zach and I think I will try at least one of them. Would you not add a filter cap to the circuit? Just wondering if a 100uF cap from 9V to ground might help?

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    1. BTW I think the "bass" label on the top left grey wire on the top drawing should read "treble". :-)

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    2. Well it’s really normal and treble, but forgot to change it from the model t preamp layout. I’ll change that when I get back to my apt today and update the layouts. To be honest the Model T preamp didn’t need it, both when used with the class d amp or as a stand alone pedal, so I don’t think it’s really necessary, but you can always add on if you’re concerned.

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    1. Nope. It’s a FET conversion of the preamp from the actual tube amp. Just look it up online and you’ll find it. Honestly, you really shouldn’t need it.

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  3. Zach

    Look forward to starting this one this weekend. On the switchable layout, should the input on the left be treble? On the current layout it says "Bass" on both sides of the switchable layout.



    Just making sure. Thanks again for the layouts.

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    1. I didn’t get a chance to fix it yet. One side should be normal the other treble. I’ll fix it later today.

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    2. Sh*t - I didn't notice that someone had already asked earlier. Sorry!

      Thanks

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  4. finally fixed the labling. sorry for the deal

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  5. Can you explain why you went with what appears to be a common gate/base amplifier and not a common drain for Q4? The voltage divider on the gate is confusing, shouldn't it be grounded?

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    1. hey man. if you take a look at the amp schematic the anode and grid of V2a and V2b are tied together. when converting from tubes to FETs the anode = drain, and grid = gate. hence, Q3 and Q4 represent V2a and V2b in the amp and they are connected as such. also, it's not a voltage divider, as a voltage divider turns a large voltage into a smaller one, typically using two series resistors and an input voltage.

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  6. what are the trimmers for?

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    1. They’re too adjust the bias. I forgot to make the voltages and will add it when I get home.

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  7. Hey Zach,

    Can you post the bias voltages?

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  8. Hey mate,
    I'm breadboarding this to see if I like it better than my other plexi emulations ;-) a couple of questions though:
    1 - Shouldn't the 2k7 from Q1 source to ground be 820R, as per schematic?
    2 - what's exactly the purpose of the 220pF caps from gate to ground? can't see them on the schem...
    Thanks in advance Zach, and thanks for those nice little preamps!

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  9. I marked it verified because I've built the other preamp layouts which are identical aside from value differences due to being from different amps.

    btw, alex i rechecked the scehamtic and you're right that 2.7k resistor should be 820r, so i fixed it in the layout. also the 220pf caps are miller caps, and help the FET sound more tube like.

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  10. Thanks Zach!
    I'm having A F***** HELL OF A TIME with this on my breadboard! ;-) I'm changing values from Source to Ground on each channel (plus some caps from Drain to Vol), combining values from the Sunn, JCM800, Laney SG100 and GH100, SLO, and different plexis, to achieve different sounds. Hard to choose which combination I like more... It's a pity that each one substantially varies the D bias, otherwise I'd build a big-ass preamp with a rotary switch on each input fet to choose the channel's flavour. Those preamps are just awesome!

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    1. Cheers man. I hear you on that. I’ve got 3 mini amp builds that are based on these layouts: Sunn Model T, Laney Supergroup, and a Superbass. Each one sounds so different. It goes to show how small changes can make just differences in tone and gain. The amazing thing is that depending on which one I plug into causes me to play differently, solely because of the feel of the amp. I bought a bunch of other chassises just so I can build more mini amps. I also built a mini 100watt Orange OR120. Nothing like having a shit tone of 100watt amps that take up no space so I can swap at will.

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    2. Agreed, the sound I'm getting also makes me play one genre or another. Those mini amps are freakin funny to mess around with... As I'm on a budget, I did a separate poweramp (you may have seen the pictures on the show your guts thread) and build the preamps separately for more versatility. I should also build an Orange sim for when I'm in the mood for some crushing doom riffing :-)

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    3. Yep. Same. I used a 100watt class d amp similar to the one you posted.

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    4. Any chance ether of you could lead me to the plans of this mentioned 100w class D amp? I have built a couple of these preamps and that would work great!

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  11. rechacked the schematic an noticed that when i made the last correction i made a few errors. layout is updated and doubled checked against the schematic. it's all good now.

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  12. Hi Zach.. I'm not an expert in electronics but I have some skills building stuff by understanding diagrams.. What I don't know is how or where should I connect the speakers or the output of the sound? Thanks a lot!

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    1. hey man. the output of the preamp comes from lug 2 of the volume pot, it's written in the notes below the layout. if you're wiring it as a pedal then take a wire from lug 2 of the volume pot and connect it to the appropriate spot on the bypass switch. if you're going to use it as a preamp to an amp then the wire from volume 2 goes to the input of the amp.

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  13. This comment has been removed by the author.

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  14. look on ebay tpa3116d2 100 watt mono class d amp board

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    1. Less than 5$, seriously ? Are they that good ?
      Now I am considering building a super compact backup amp using one of these and a preamp stripboard from here, although I already bought a EHX Magnum for this exact purpose x)

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    2. Also, this is kinda off-topic, but do you share the power supply between the preamp and the TPA31XX power amp ?
      The J201s can take the 24V needed by the power amp, but I guess I would have to adapt the trimmers value.

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    3. Funny I seen these a couple weeks ago and considered them, but I would rather have sumthing I built to go with the rest.

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    4. According to the forums, the Magnum 44 and other Mooer/Hotone/... alternatives use class D chips from the TPA31XX family. The Magnum is great as a backup amp, but it really lacks some equalization, so I guess a DIY alternative using a similar chip plus onboard pre and eq might be at least as good as the EHX.

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  15. Is 9V the only acceptable value? Or can I go somewhere higher? Like sharing the 24V of a class D amp?

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    1. I run them at 18V or 24V for more headroom and amp-like response. As long as your caps are rated 50V or more, it'll be fine.

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    2. Awesome, thanks! I was looking for more headroom too but since I have never built a preamp before I was afraid to turn the voltage up so much!

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  16. You have to check out how these old Marshalls are configured. They have 2 separate volume controls: a "treble volume" and a "normal volume" + presence, bass, mids and treble controls. The treble volume offers a brighter sound than the normal one. Each of these volumes (or channels) have separate input jacks. Most players use the bright volume more often. Added to that, there's the EQ section to tailor the tone further.

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  17. Oh I get it now ☺
    Can this preamp be pluged in the amp fx loop or just in front of a amp?

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  18. Built and it works... but with The treble pot turned up it loses volume, becomes very ice picky and the highs totally overwhelm the signal. Is that supposed to happen or is there a flaw in my build? The only differences are a 1uf and 470n in place of the 680nf caps.

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  19. Nevermind. It’s the amp... a Marshall Origin 50. With the Tilt control on anything but full CCW, it gets very raspy. Fully CCW, it sounds great but the rest of my pedals sound dull. I guess I need to figure out a way to balance it out or find a different build. Sounds great in general though.

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  20. Hi, I don't see any difference between the layout " With 2 separate inputs like the amp" and the layout " With both channels jumpered" . Someone could help me I 'd like to build the " both channels jumpered". Thanks !!

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