Hi/Lo input ground wiring

[daveyajd]

I have never put 2 inputs on any previous amps so I’m a bit confused. Looking at Doug’s hookup info: I get the cliff jack wiring, but the switchcraft wiring seems weird just because it doesn’t show any connection to the low jack ground lug. The shunt and ground are connected on High. Do I connect the low ground to the high ground and then run a wire to my input end ground lug?


Aaron

[sluckey]

I have never put 2 inputs on any previous amps so I’m a bit confused. Looking at Doug’s hookup info: I get the cliff jack wiring, but the switchcraft wiring seems weird just because it doesn’t show any connection to the low jack ground lug. The shunt and ground are connected on High, the tip and shunt on the low. Do I connect the low ground to the high ground and then run a wire to my input end ground lug?

Look closely. The Hi jack doesn't have any ground wire either. Hoffman relies on the jack bushing/nut to provide chassis ground for both jacks. Fender does the same.

[daveyajd]

Ok. So I don’t need any ground wire to my ground buss? Does having one affect anything? Because I think I have always run a ground wire from my input jack. Grounded in two places?


Aaron

[sluckey]

Just terminate your ground buss at the input jack sleeve/ground lug. Look here...

     https://el34world.com/charts/grounds.htm

[daveyajd]

Thank you so much Sluckey! So why does the output jack get wired to the power side chassis ground? Wouldn’t it be grounded at the jack as well?


Aaron

[tubeswell]

Thank you so much Sluckey! So why does the output jack get wired to the power side chassis ground? Wouldn’t it be grounded at the jack as well?


Aaron

Simple answer is the output jack is DC-isolated from the amp's power supply ground returns

[daveyajd]

Awesome. Thank you both.

[sluckey]

Simple answer is the output jack is DC-isolated from the amp's power supply ground returns

Not so if there is a NFB loop from the OT secondary.

[daveyajd]

So output still needs to be grounded in that case? At the same spot or somewhere else?This build is a JTM variant so it seems to have NFB.


Aaron

[sluckey]

Look at the link I provided.

[daveyajd]

I’m sorry. I don’t see any reference to NFB grounding on that drawing.


Aaron

[sluckey]

Where is the speaker output grounded?

[daveyajd]

Yes I got that the first time around. That is where I have always grounded my output. I thought your response to tubeswell was referencing a different ground. It seems it was just clarifying the “DC isolation” when including a NFB. Thanks for all your guidance.


Aaron

[tubeswell]

Simple answer is the output jack is DC-isolated from the amp's power supply ground returns

Not so if there is a NFB loop from the OT secondary.

Okay so the not-so-simple answer is the secondary is galvanically isolated from the amp’s power supply signal ground (that is) connected to the primary - but for the NFB. However, any hum (that might be ‘reverse-sourced‘ from the NFB output in the preamp) is a low current source working into a high load impedance. And so noise from such a source will be much less of a factor than noise that is already inducted through the OT from Pr to Sec. So the best place to clean up ground hum is the signal ground, and if doesn’t matter if you ground the OT secondary at the output jack sleeve