grounding question for rackmount chassis (tube preamp)

[JGR]

Hi,

I just finished another tube bass preamp built into a 1 U rack mount Hammond chassis.

It is this style:
Hammond Mfg. - Rack Mountable / Table Top Enclosures (RMCS & RMCV Series)

All of the panels have a coating on them - the only thing that conducts are the four corner brackets that all of the panels attach too. I scraped off the coating on a small area of the bottom panel of the chassis where the ground lugs are mounted for the star grounding scheme. I used switchcraft open frame jacks with shoulder washers so that they are isolated from the chassis. Pots are RV4 PEC's.

So I found that I was still getting some hum after everything was done, and it was alleviated by connecting any of the pot bodies or front panel switches to the star ground. So I eliminated the hum by running a ground buss across the back of all the pots and then connected it to ground.

Just wondering why I ran into this issue since everything is isolated (pot lug grounds are not connected to the body of they pots, they run in star scheme as well). Any ideas? I know the pots and switches touch the exposed chassis through the holes, but since none of the circuitry is grounded to them, I am wondering why it caused a ground buzz.

Also, would I get better shielding if I sanded off the panel coating at the corners where they meet the support brackets so that the entire chassis is electrically connected? With the way the chassis is constructed, all of the panels (top, bottom, sides, front, back) are isolated from each other due to the coating.

Thanks!

JR

[kg]

not all grounding related issues are caused by ground loops.

in fact, in the perfect grounding solution, having loops all over the place would cause no ill effects.

the loops cause the issue when the ground buss impedance is non-zero. which is of course every real circuit.

you are probably seeing the effects of shielding more than anything. shielding a circuit means to surround it with a faraday cage, also grounded to the earth. a cage relies on electrical conduction to do its shielding, so yes, you should try to eliminate any resistance between panels.

[Dave H]

So I found that I was still getting some hum after everything was done, and it was alleviated by connecting any of the pot bodies or front panel switches to the star ground. So I eliminated the hum by running a ground buss across the back of all the pots and then connected it to ground.

I'm guessing your front panel is floating which is not good for either shielding or safety. As Ken said there should be no resistance between the box panels. The bodies of the pots and switches should also have zero resistance to the panels. If you do that the ground buss across the pots won't be required.

Dave H.

[JGR]

Guys, thanks for the replies. You confirmed what I was suspecting - I had been second guessing myself thinking why would a manufacturer make a chassis like this if you have to scrape the finish off?? Anyway, I removed finish on all the panels so that everything is grounded, and all is well. I removed the ground buss from the back of the pots and things are even quieter. Much better shielding too.

Cheers,

JR