Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Swampertech on December 20, 2018, 04:01:27 pm
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Hey sluckey, I was looking at Doug's 5E3 grounding scheme, and had a question about star grounding. V1 A-B, V2 A-B, and the B+3 filter cap ground are all referenced to a Preamp Ground, which is bussed back to the chassis, PT CT, 6.3VAC CT ground lug connection. Only V3 and V4 are referenced to Power Ground, which includes the B+1 and B+2 filter caps grounds and the ground of the secondary of the output transformer, and those are bussed back to the chassis ground lug. Is this the quietest method for grounding?
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V1 A-B, V2 A-B, and the B+3 filter cap ground are all referenced to a Preamp Ground, which is bussed back to the chassis, PT CT, 6.3VAC CT ground lug connection.
That's not quite right. The preamp ground is the input jacks ground lugs. That pot buss ground is not routed back to the power ground point. I think Hoffman just showed the ground buss ground symbol to the right of the pots because it would be to cluttered to show it over at the input jacks. This is exactly how I would do it. Only difference, I would float the ground buss over the pots and not solder to the back of the pots. Does this look better for you?
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My input jacks (Switchcraft) are isolated from the chassis, and all of their grounds, both input jacks and Lead/Bright channel pots are grounded together and run to the neg. lead of the B+3 cap. I did want to ask you about the NFB I have coming from the 16 ohm side of the output transformer secondary up to the cathode of V2A. Wouldn't that feedback resistor signal be referenced to the ground of the OT secondary, which is B+1 filter cap ground?
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My 5E3 doesn't have a NFB circuit. Maybe you should show us a schematic of your actual amp?
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I'll change that preamp ground!
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Haha! Now I remember. What I said about Hoffman's circuit still stands.
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Steve, I went back and changed the ground problem, and now have a ground screw near the input jacks which I run to the jacks/controls ground.
Just to verify my grounding situation, I pulled the input jacks and Lead/Bright controls out of the chassis, and still had a connection to ground even before installing the new ground connection at the input jacks. I pulled the receive/send jacks out of the chassis, and the grounding of the input jack ground and chassis went away. I had isolation washers on these 1/4" jacks, but one must have become uncentered on the washer. Anyhow, put the s/r jacks back in and that fixed the input jack ground to chassis problem.
I also changed the V2 cathode ground reference to the preamp ground.
I thank the man
From Saraland!
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You do have two isolation washers on each jack, right?
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Yes
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And I checked the resistance between the jack ground and chassis after I re-installed them just to make sure.