Ground questions

Discussion in 'Amp Tech Center' started by Porkchop Jones, Oct 16, 2020 at 6:31 PM.

  1. Porkchop Jones

    Porkchop Jones TDPRI Member

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    I'm building a Mojotone Studio 1 amp which is my first instructionless build. I just finished building a Champ and a Princeton Reverb that went quite well. However, this amp is more like a Marshall type build as I gather, and there are some differences. I've got the amp built up to the stage of wiring (see below) My question/s are about the ground wiring.

    The diagram shows that there are 14-different ground points. The chassis is aluminum. Am I supposed to float the ground over the chassis and connect it all to the ground connector on the power cord receptacle? Or, do I use the chassis as a ground?
     

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  2. dan40

    dan40 Tele-Afflicted

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    Marshall always used a ground buss attached to the back of the pots. I would personally run a ground buss along the top edge of the board and connect all preamp ground wires to that. Connect the end of the buss closet to the input jack directly to the chassis. You will also connect the ground wires from the pots and input jack to this buss. The ground point for the power supply should be attached to the chassis close to the PT. This method keeps the sensitive preamp grounds at one end of the chassis and the power supply grounds at the other. The safety ground from the power cord should have it's own grounding point with nothing else attached.
     
  3. dan40

    dan40 Tele-Afflicted

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  4. Burning Fingers

    Burning Fingers TDPRI Member

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    Hi Porkchop

    Your pics are too small for my old eyes to read any detail in...can you post larger versions of the pics or give us a link to the schematic ?

    You want to avoid the dreaded ground loop hum in your build...that can easily happen if earth ( ground ) points are here there and everywhere.

    You need to keep the ground points for the power supply on a separate point from the ground points for the preamp/rest of the circuit ground point.

    Don't connect all those ground points in the circuit to one point on the chassis and don't use the power receptacle as a ground point.

    The ground from the power receptacle should be securely connected to the chassis with a nut and bolt and solder lug washer.

    The power transformer ground ( assuming you are using a center tap transformer ) is connected to the chassis with a nut ,bolt and solder lug washer.

    The power supply filter cap grounds for the power amp section can be connect to the same point as the power transformer ground.

    You can make a preamp ground buss from thick solid wire ( transformer type wire with the enamel stripped off is good ) and have it sitting above the preamp section of the board..connected at either end to an ground point on the board .

    Then it is a simple case of short wires from the other points on the board that need grounding to the buss wire which is connected to the chassis either by a wire to the back of a pot or by a lead that is connected to the chassis with a nut and bolt.


    The preamp and power amp supply grounds should be kept at opposite ends of the chassis.

    Cheers
     
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  5. Porkchop Jones

    Porkchop Jones TDPRI Member

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    Thanks for the killer responses Dan40 and Burning Fingers. That was super helpful!

    To Burning Fingers, I think you should be able to enlarge the photo's by zooming in on them by using crtl++ once you have selected the photo. I tried to get better pictures but wasn't able to make them better so I let it go.
     
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  6. Porkchop Jones

    Porkchop Jones TDPRI Member

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    Ok, then another question regarding the PT ground. I appears to me that there isn't an actual ground line from the PT. It seems to me that the ground for the PT is the line that runs to ground from the right most corner of the diode quad. Is this correct? Or is it the ground that comes off the pilot light?
     
  7. dan40

    dan40 Tele-Afflicted

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    You are correct Porkchop...the ground from the bridge would be your power supply ground. Attach this to the chassis somewhere near the PT with a bolt, nut and lockwasher to assure a solid ground. The ground from the filter cap can and the cathode resistor from pins 3 and 8 of the power tube can also be grounded to this same point. As for the virtual centertap on the pilot light, I would install a ground lug near the pilot light and attach the end of the two resistors directly to that.
     
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  8. Porkchop Jones

    Porkchop Jones TDPRI Member

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    Got it, thanks. That's was super helpful!
     
  9. Burning Fingers

    Burning Fingers TDPRI Member

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    Doh ..I figured out how to get the pics bigger without loosing resolution..nice clean build you have going there Porkchop..well done.
     
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