Wednesday, 1 August 2012

MXR GT-OD

Some of you may be thinking this layout looks pretty familiar, and yes, it is extremely close to the ZW-44 Zakk Wylde overdrive.  So close in fact that I've included a switch that will select either the GT-OD or ZW-44 on the fly.  I've got an original ZW-44 and really like it, but watching some of the clips I think I like the sound of the GT-OD more so I'll certainly be adding this to the collection.

Info from MXR about their original:

Built from the ground up with the best available components, the MXR GT-OD Overdrive pedal creates smooth, warm, and classic overdrive without excess noise or tone coloring. The simple controls make it easy to dial in a great tone quickly-from crunchy rhythm tone with chord clarity-to sparkly and harmonic-rich lead tones with tons of sustain. You won't find an overdrive that is more organic, responsive or open sounding.

    A traditional overdrive that provides powerful, cutting, transparent overdrive tone with great definition
    Adds sustain and gain without adding excess noise
    Place in front of an overdriven amp for screaming lead tones
    Indestructible die-cast housing






31 comments:

  1. Looks great, thanks !
    If I don't want to wire a clipping switch, I guess I just have to connect the stock 3 diodes from gain 1 to sw1b right ?

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    1. Yes, or anywhere you can fit them in between IC pins 1 and 2.

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  2. Verified, as if there was any doubt. Surely a sign of serious addiction when i`m building this just to have something to build :-)

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    1. Nice one! What do you think of the difference between the ZW44 and GTOD modes? I've seen people say they were surprised at how different it sounded for such a small difference.

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  3. Thank you for the schematic.

    Just finished the pedal and it works like charm.

    But I ran into some problem that I solved using some schematics:

    1. The picture of the transistor is upside down, CBE is labeled correctly
    2. The layout showing the cuts is different from the upper one which is correct. See line 2 and 8.


    Thanks again and keep on bringing all those great pedals!

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    1. Thanks for the heads up, I've updated the layout. This happened because I re-jigged things slightly from the first version of the layout I released, but mustn't have updated the bare board layout. Doh!

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  4. I've attempted to make this...but couldn't get enough diodes from my local electronics store. Instead of the clipping switch, I connected an extra little vero board with 2 diodes in parallel. I also didn't have an SPDT switch, so I just connected sw2a to sw2b. This should produce sound, right?

    Is there any way of just checking that these are the reasons it doesn't work...?

    Many thanks

    ps. can you please do an MXR Distortion III - that is a great pedal.

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    1. You don't want to connect SW2a to SW2b, that will effectively make the first half of the opamp a buffer instead of a gain stage. Those two wire should just go to the little vero board at either end of the diode arrangement.

      I would still expect you to get sound out of it though, so you're going to need to check all the voltages between the transistor/IC pins and ground to see if that points us in the direction of the problem.

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  5. Hi! i built this one today but it doesnt work right.
    I used the RC4558P, but the rest is exactly like your schematic.
    The problem is that when i try to raise the Gain the circuit stops working and all i can hear its a very loud note ... something like a sine wave. This happens with the Sw2 engaged, if i disengage it the circuit works fine but then the gain knob has no effect whatsoever. I haven't found the error yet, do you have any idea on what i should check?
    Thank you for your work!

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  6. I'm a newbie to DIY...I'm a little confused with the dpdt wiring. is this applicable to 3pdt switch?

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  7. Another dumb question :
    Does the led wire mean that you don't have to wire the led to DC Jack ? like in the picture below ?
    http://1.bp.blogspot.com/-k5Sh68yVU18/TzGRFKbiALI/AAAAAAAAAlk/CbfaaduUjYQ/s1600/%21Offboard+wiring.png
    And do would you add a resistor ?

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    1. That graphic is just representative, to emphasise that a current limiting resistor is between the supply and the LED. When you see LED+ on a layout, that resistor has been included in a free space on the board and so you don't need to hang it offboard somewhere.

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  8. well no luck with that one

    First I tried to built a little check circuit witch looks like that
    http://alborea.free.fr/Telechargements/diy/IMG_0459.JPG

    everything ok when bypassed but when I plugged my transformer, I got a loooot of hum!!! Any idea to make that kind of thing correctly ?

    then I put the transistor in the wrong way ( drawing must be reversed for the mpsa14 !) I don't know if I kill it...
    Once Reversed strangely I had a signal almost clean with one of the spdt position and overdrive
    with an awfull loud hiss, and all that with the hum :(

    And eventually I messed up with the opamp so I broke one of the leg, and I don't have any spare one.

    So let it be, I hope I'll have more chance with my next to come Ep Booster :)

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  9. Hello here!
    A newbie question about the "Led+" wire : If I want to build this one with a millenium bypass, can I use this wire to drive the millenium? I'm afraid the resistor would lower too much the power... I guess the best way is to forget this "led +", as the borad seems too small to me to include the millenium on it. Right?

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    1. This is set up to be wired using true-bypass 3pdt switches. The millenium is usually used when you are using a buffered pedal. I would suggest using the method posted above under the off board tab. Here's a link to it:

      http://tagboardeffects.blogspot.com/2012/02/offboard-wiring.html

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    2. This layout and the majority of the layouts are setup to be wired using true bypass switching. Check this link, which will take you to the off board page of the site, which has a wiring diagram. Usually the millennium method is used for buffered effects, so it isn't not needed here.

      http://tagboardeffects.blogspot.com/2012/02/offboard-wiring.html

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    3. Thanks Johns for your answer, but as I only have DPDT by now, and as I also want a signal led, I would like to use the millenium bypass (wich can also provide a true bypass : http://tagboardeffects.blogspot.fr/2014/01/millennium-bypass-2.html).
      So my concern is about this wire, I guess I can forget it, but I'd like to get it confirmed...

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    4. in that case, yes man you can drive your millennium board by omitting the 4.7k resistor and the cut under it.

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  10. Hi again, another noobie question... I think I read about it somewhere here but don't remember.
    I started the build yesterday night, but ain't got the 18nF cap. I have 15 and 22nF in stock. Wich one would be the best to use in this build?

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  11. hola a todos... yo realice este proyecto igual como lo dice ahi.... y dio sonido de una y sin problemas con los 2 selectores, clipping y selector de modo.... simplemente GENIAL

    Hello everyone ... I make this project just as it says there .... and gave a sound and trouble with the 2 selectors , clipping and mode selector .... just GREAT

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  12. Now I will single version of ZW44 have such ....

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  13. Hi, Tnx for the layout
    Can you tell me when i select GT-OD and when ZW-44 in switch ?

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  14. Just finished mine and all I can say is WOW! Great overdrive...... the difference between the two model choices is very subtle..... The GT-OD was a little more open and had more edge than the ZW-44. But both settings are awesome. The sym. clipping is real good. Forget the tube screamer, build this instead.......

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  15. I have a question ..... to select the clipping part of the LEDs , they light ??? or light .....

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  16. Hi all ..... I built this project and only had to make a change that I forgot to comment before the transistor ..... must rotate 360 degrees, it's not like the image of rest say okay ... .. here I give you link the transistor data sheet ... to be guided and see .....

    http://diagramasde.com/blog/71817/MPSA13-MPSA14-pdf-ON-MPSA13-MPSA14.html

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  17. Does anyone have a schematic that corresponds to this layout?

    I'm not currently planning to build one, but I would like to look at the schematic for the circuit. Is there a reason why I never see schematics when I look at this site, only layouts?

    Turning a layout back into a schematic is not trivial - plenty of opportunities for making mistakes.

    Generally, I'd want to see the schematic before deciding whether to build any circuit. Having a schematic to refer to makes it a lot easier to debug any problems that might arise when building one too.

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  19. Anyone know which resistor value to increase to cut down the gain a bit when in ZW mode? I am wanting to use the pedal more as a volume boost.
    Is it the 33k resistor attached to the gain circuit?
    Thanks
    Jason

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  21. s it okay to post a link here? If so, let me know, but I won't for now. A schematic helps a lot. There's one on freestompboxes.org in the MXR - Zakk Wylde ( ZW-44 ) thread.

    The gain is set by a 1M Ohm pot, in series with a 33K resistor, on the same op amp stage as the clipping diodes. The 33K resistor sets the minimum gain, so the first thing you want to try is reducing that. Reducing it to zero would do no harm (it would just be very quiet at lowest gain, probably), so pick any value you like.

    If you've already built the circuit, it might be easiest to put a resistor in parallel with the 33K one - try a 10K, and go smaller, if you want it to start lower. Do that for testing, anyway - just hold one against it, without touching either leg to avoid adding noise.

    If you want to reduce the maximum gain, the best way is to replace the pot with a smaller value. Roughly speaking, a 100K pot will give you 1/10 of the gain, or if the 1M is a linear pot, the same gain as turning that 1/10 of the way - and a 500K or 470K pot will give you about half the gain.

    If you've used a linear pot, and you want the same max gain, but more control over the low end, you could try a log pot. There are two sorts - regular audio taper, and reverse taper (less common). If you can only get the regular one and it seems to have the opposite effect - bigger increase at the low end - swap the outer two wires on the pot and use it backwards - counter clockwise for more gain, clockwise for less.

    If you can't easily remove the pot, you can try either:

    a) A resistor in parallel with the track (outer two pins) - that works, but gives a bit of an uneven response to turning the knob, with little extra effect at the top end (e.g. past 12 o'clock).

    b) A resistor from the wiper to one outer pin (the one it connects to at minimum gain). That also makes it more "log" (more response at one end of travel than the other), which might be good.

    c) Two resistors, each going from the wiper to one outer pin. Depending on the values you choose, that can keep it working closest to how it was, just reducing the maximum.

    - by "wiper" I mean the centre pin or lug of the pot.

    I approve of reducing the minimum gain. I've done it to my MXR Bass Overdrive - with a resistor in parallel with the minimum gain setting one, and another in parallel with the track of the gain pot to reduce the maximum, and I've added a switch with a choice of clipping methods). I may do it to my Zakk Wilde too (same thing, without the switch).

    The volume knob (Level?) just works as an attenuator on the output, so it's fine to just turn that all the way up, and reduce the gain to get a cleaner sound. If it's the last pedal before the amp, that's the best position for that knob to be in anyway - just use the volume or gain on the amp.

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