Why terminate circuit ground(s) to chassis, rather than directly to mains ground point?

[Tele_Tom64]

Thank you!

 

[58Bassman]

Based on what I know about RF and gain circuitry, I think direct grounding to the chassis reduces the opportunity to induce RF into the signal because any length of wire acts as an antenna. It's the same reason why it's good practice to keep all wires--signal and ground in a guitar circuit--as short as possible.

In my amp builds I carry all grounds from the board to a common point to cancel any differential in ground potential from various points on the board. But for pots, jacks and tubes, I direct ground to the chassis.

My builds always have very quiet operation.

Are you using metal jacks and control shafts? If so, do you use a Brass strip, like some amp builders? If so to both, they all should be at/very close to chassis potential and as long as the connection for the safety/power cord ground has good integrity, no current should flow between these.

How do you make the ground connections from the board- wire with a soldered ring terminal, secured between the washer on a control?

 

[chas.wahl]

Are you using metal jacks and control shafts?

Yes, jacks isolated though, and "control shafts" are part of pots.

If so, do you use a Brass strip, like some amp builders?

No.

How do you make the ground connections from the board- wire with a soldered ring terminal, secured between the washer on a control?

End of ground bus wire is extended to a dedicated ring terminal; the last thing that ground bus is connected to, before the terminal, is the (primary) input jack's sleeve connection.

 

[58Bassman]

Something that needs mentioning- if the building's electrical service is sub-par, noise rejection will never happen because the amplifier/pedal chain ground connection to earth isn't at low impedance and it's certainly not at earth potential.

 

[chas.wahl]

Then I'm cooked. I live in a (large, multi-family) 95-year-old building and my apartment is wired with cloth-and-(dried-up, cracking)rubber-insulated 14 gauge solid; hot and neutral wires only. The grounding is only by virtue of steel conduit and boxes, (newer, NEMA 5-15) outlets given grounding only through the #6-32 screws fastening them to the boxes. The kitchen was renovated in the '80s, and some outlets and a circuit were added, using BX cable. Recently I checked outlets, and found 2 that were wired with reverse polarity, and 2 others that had no apparent ground (one of those supplied by the 80s BX cable). The former I fixed easily enough; on the latter I disconnected the outlets, jostled the boxes with a large screwdriver, and got some kind of ground, that has 120 V between hot and neutral, and only 112 V between hot and ground -- so I'd say that's an "imperfect" ground.

Edit: I'd written NEMA 15, but meant NEMA 5-15

 

[Peegoo]

Are you using metal jacks and control shafts? If so, do you use a Brass strip, like some amp builders? If so to both, they all should be at/very close to chassis potential and as long as the connection for the safety/power cord ground has good integrity, no current should flow between these.

How do you make the ground connections from the board- wire with a soldered ring terminal, secured between the washer on a control?

I don't use a brass strip.

I solder a terminal to the chassis on the preamp side of a split ground buss. I know it's really unnecessary, but I have had to disassemble front panels on enough older amps a bunch of times over the years to fix poor grounds between the control pot and the panel. I'm not implying my way is better, it's just a different approach to improve reliability over time. It doesn't add noise, I know that.

 

[King Fan]

Then I'm cooked. I live in a (large, multi-family) 95-year-old building and my apartment is wired with cloth-and-(dried-up, cracking)rubber-insulated 14 gauge solid; hot and neutral wires only. The grounding is only by virtue of steel conduit and boxes, (newer, NEMA 15) outlets given grounding only through the #6-32 screws fastening them to the boxes. The kitchen was renovated in the '80s, and some outlets and a circuit were added, using BX cable. Recently I checked outlets, and found 2 that were wired with reverse polarity, and 2 others that had no apparent ground (one of those supplied by the 80s BX cable). The former I fixed easily enough; on the latter I disconnected the outlets, jostled the boxes with a large screwdriver, and got some kind of ground, that has 120 V between hot and neutral, and only 112 V between hot and ground -- so I'd say that's an "imperfect" ground.

Given that you live in a building that could be nicknamed "Old Sparky", sir, I think your signature, "I'm calling it Shock Brothers" is both witty *and* wise. Noise rejection is the least of your worries. Even so, starting a post with, "Then I'm cooked" may be tempting fate....

 

[chas.wahl]

Well, fingers crossed, we've lived here for 36 years and raised two kids from infancy without losing either one; and I've never shocked myself using outlets all over the apartment to run power tools while making improvements. But my impetus for checking the outlets was concern that I not get neutral and hot crossed when hooking up amps -- though I realize I've always connected amp to (newer) outlets that I wired myself, being careful to observe correct polarity and check ground -- not by design, just happenstance.

What's gnawing me now is that I realize there are another 112 apartments in the building, occupied by people who are mainly even less savvy about this than I am!