Mojotone 18 Watt TMB Ground Scheme

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MSVguitar
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Mojotone 18 Watt TMB Ground Scheme

Post by MSVguitar »

Ground Scheme mojo 18 watt TMB001.pdf
I'm a new member on this forum. I am building a mojotone 18 watt TMB kit and didn't like the grounding scheme. A ground buss on the back of the pots and no direct chassis ground for the inputs. I read the posts on this forum regarding grounding and made a sketch to make some changes. See my attachment. I have a couple of questions. 1) I want to add a chassis ground for the high gain input which is located near the center of the chassis. Should I add the chassis ground close to the high gain input to keep the lead as short as possible or should I move it as far away from the PT as possible? 2) I want to add a ground wire attached to the terminals as needed between the pots and eliminate the ground buss attached to the back of all pots. Does it matter where I attach the ground wire to the ground buss on the board as long as I keep it in the preamp section.

Mark
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Re: Mojotone 18 Watt TMB Ground Scheme

Post by JMPGuitars »

Welcome to the site!

It sounds like you might have already seen the grounding threads in my signature?

The input jacks, preamp, and pots should all go to 1 single ground point near the higher gain channel input jacks. Don't separately ground both input sets.

Thanks,
Josh
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Re: Mojotone 18 Watt TMB Ground Scheme

Post by MSVguitar »

Thanks Josh. Yes I read your grounding thread. I really appreciate your feedback.

Mark
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Re: Mojotone 18 Watt TMB Ground Scheme

Post by MSVguitar »

LAYOUT CHANGES FOR MOJO 18WATT TMB.pdf
I have reviewed your grounding thread again as well as some sections of Merlin Blencowe's 'Designing Tube Preamps for Guitar and Bass' and have made some changes to the Mojo Tone layout. There are 5 ground points in the build. I attached a file that shows the circuit and 5 ground points. My concern is the potential for noise in the 2nd & 3rd stage of the high gain pre amp because there are two ground points, one for the circuit components and one for the cap can, creating a ground loop. I think this is the best I can do with this layout. I would like your feedback. I will finalize the layout and make the changes to the build. Then I will post some pictures of the chassis build.

Thanks,
Mark
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Re: Mojotone 18 Watt TMB Ground Scheme

Post by JMPGuitars »

MSVguitar wrote:
Tue 11/22/22 9:13 pm
LAYOUT CHANGES FOR MOJO 18WATT TMB.pdfI have reviewed your grounding thread again as well as some sections of Merlin Blencowe's 'Designing Tube Preamps for Guitar and Bass' and have made some changes to the Mojo Tone layout. There are 5 ground points in the build. I attached a file that shows the circuit and 5 ground points. My concern is the potential for noise in the 2nd & 3rd stage of the high gain pre amp because there are two ground points, one for the circuit components and one for the cap can, creating a ground loop. I think this is the best I can do with this layout. I would like your feedback. I will finalize the layout and make the changes to the build. Then I will post some pictures of the chassis build.

Thanks,
Mark
You should only have 3 ground points:
Preamp near input jack at higher gain chain (covers all inputs, pots, preamp)
Power Amp star ground (includes filter caps, speaker output, etc)
IEC socket ground (and nothing else)

Thanks,
Josh
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Re: Mojotone 18 Watt TMB Ground Scheme

Post by MSVguitar »

LAYOUT CHANGES FOR MOJO 18WATT TMB Rev 1.pdf
MOJOTONE CHASSIS BUILD GROUND POINTS.pdf
JMPGuitars wrote:
Wed 11/23/22 10:41 am
MSVguitar wrote:
Tue 11/22/22 9:13 pm
LAYOUT CHANGES FOR MOJO 18WATT TMB.pdfI have reviewed your grounding thread again as well as some sections of Merlin Blencowe's 'Designing Tube Preamps for Guitar and Bass' and have made some changes to the Mojo Tone layout. There are 5 ground points in the build. I attached a file that shows the circuit and 5 ground points. My concern is the potential for noise in the 2nd & 3rd stage of the high gain pre amp because there are two ground points, one for the circuit components and one for the cap can, creating a ground loop. I think this is the best I can do with this layout. I would like your feedback. I will finalize the layout and make the changes to the build. Then I will post some pictures of the chassis build.

Thanks,
Mark
You should only have 3 ground points:
Preamp near input jack at higher gain chain (covers all inputs, pots, preamp)
Power Amp star ground (includes filter caps, speaker output, etc)
IEC socket ground (and nothing else)

Thanks,
Josh
I revised the circuit diagram to have 3 ground points. I also added a picture showing the chassis build with the 3 ground points. I will also revise the heater wire routing shown in the picture. This is the routing shown on the Mojotone layout. I didn't want to route the speaker output ground wire across the heater wires. I also think that this would be better overall because the heater wires will be as far away from the component board and all the AC lines will be at the far end of the chassis.

Thanks,
Mark
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Re: Mojotone 18 Watt TMB Ground Scheme

Post by JMPGuitars »

Looks like a lot of good choices!

I wouldn't go that route with the speaker ground wire though. You want your ground wires as short as possible.
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