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



18 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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  2. Replies
    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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  5. finally fixed the labling. sorry for the deal

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  6. 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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  7. 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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  8. Hey Zach,

    Can you post the bias voltages?

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  9. 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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  10. 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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  11. 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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