Questions on grounding in a painted chassis???

Blueslover88

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Hey y'all,

I bought a 12x8x2" Hammond chassis for my EF 80 micro build. I wanted it black but just realized it's dipped in like undercoating of some sort so the inside is not clean metal. I know I'll have to clean areas for ground bolt which will go to PT mounting bolt, but do I need to clean anywhere else? Perhaps where the input or output jacks go or any of the pots? Or will connecting them to the ground bus wire to a PT bolt suffice?
 

Phrygian77

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The only reason someone might want to use a PT mounting nut as a ground point is to avoid drilling the chassis. You're already drilling everything.
 

RetiredUnit1

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PT bolt as grounding is ok if the machine screw is long enough for 2 nuts. To be sure the PT is secure, all four nuts need to be up against the chassis.

So after those four nuts are tight, then add the ground bus tab, then another nut. So chassis/nut/ground/nut.

I buy 3 1/2" #8 SS machine screws with kep nuts (from marshfasteners.com ) and replace the standard machine screws with those on the grounds.

However it is cheaper to just drill a hole in the chassis and use a shorter machine screw and kep nut for that.

I usually have one for the power cap section and a different one for the inputs to keep the noise from the power cap section from feeding back into the input signal.
 

Phrygian77

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If you don't want to mess with sanding or grinding paint away, get some 8-32 swageform screws, #8 locking solder terminals, and drill 9/64" holes for them. The screws will form their own threads and make solid contact with the chassis.

 

owlexifry

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- any grounding point should be clean bare metal.
- #40 grit sandpaper. that paint will come off no worries.
- drilling another hole is not difficult
- if your input jack is going to be a grounding point - this needs to be sanded off on the inside contact/fastening point.
 

sds1

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I've bought those chassis before, I don't remember the insides being non-conductive.

Does your meter give you good continuity between 2 random points on the inside of the chassis?

If the inside coating is truly non-conductive, you'll want to remove it from pretty much ALL places where hardware touches chassis. Pots, jacks, sockets, power switches, fuse holders (if conductive bodied), pilot lamp... all of that hardware should be at chassis potential or it poses a potential noise/safety issue, and most of the noise issues are very easy to reproduce.
 
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chas.wahl

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With respect, I'd like to question the above assertions about pots, sockets, power switches, fuse holders, pilot lamp, and especially jacks. All of these, except some jacks, don't have their internal workings (the parts "wired up" to one's circuit) electrically connected to their parts in contact with the chassis, so unless it can be demonstrated that such non-circuit parts are designed to provide a shielding effect (I'm thinking of pots, maybe, but I don't know if that's really the case with them) then I suspect that whether these are chassis-connected or not doesn't matter.

In the case of jacks, there are two types: the open-frame Switchcraft type where the sleeve is integral with the threaded bushing that goes through the chassis, and the plastic-bodied Cliff/Rean type, which are naturally insulated from the chassis by their design and materials, and have a sleeve connection separate from the bushing. In that case, you have a choice to make, at least for the open-frame type: do you allow the plug sleeve to reach ground at the chassis mounting (which in a minor way can contribute to ground loops, like any feature that allows more than one path to ground) or do you isolate all of those jacks from the chassis using insulating washers, and connect the sleeve to a single aggregated ground with other like items.

My point is this, when building a circuit, one should always be thinking about where one wants to have a ground, and where one doesn't. A connection of conductive material to chassis may either be a (n intentional) ground, or a (n intentional) shield, or not matter at all.

With respect to painted chassis: in my opinion, this is a bad idea generally. At least with a bare metal or a completely non-conductive chassis you know where you stand. With something that's painted, you don't -- and I wouldn't rely on "conductive paint" to be part of my grounding scheme. Probably the first thing I would do is get some stripper and remove the paint, then go from there. Or, go piece by piece: treat the chassis as if it were conductive for things you don't want to be grounded, and as non-conductive with respect to the things that you do want to be grounded.
 
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sds1

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I had a longer posted typed out and then erased it because I misunderstood his post, so I deleted it, then reconsidered it's relevance and re-typed an abridged version, so I apologize @chas.wahl I'm not sure which version of my post you are replying to.

With respect, I'd like to question the above assertions about pots, sockets, power switches, fuse holders, pilot lamp, and especially jacks. All of these, except some jacks, don't have their internal workings (the parts "wired up" to one's circuit" electrically connected to the circuit, so unless it can be demonstrated that such non-circuit parts that are chassis-connected provide (and are designed to provide) a vital shielding effect (I'm thinking of pots) then I suspect that whether they are chassis-connected or not doesn't matter.
Well I can't speak to design, but yes I'm specifically talking about non-circuit parts in most cases here. In the case of an Alpha pot I have observed it's potential to couple noise into the circuit when the metal housing is not grounded.

I was more specific about jacks in my original post. Firstly, I completely omit anything other than Switchcraft jacks because those are all I know! :) But my point on those is merely to have a good chassis connection unless you are specifically isolating them. So, to just be aware that those bushings etc. are circuit-connected.

Power switch bodies I think should be on chassis/ground/earth for safety reasons (can the isolation fail??). Beyond that, I have an example here of a standby switch whose body does not have a good connection to chassis, you can measure the radiated AC on the switch body with a meter and when you touch the switch bat you can hear a little pop.

Tube sockets are especially important to ground to chassis when they have accommodations for a shield. You've got extra metal there, plus of course when the shields are installed they won't be useful.

Any other metal bodies parts in there have the potential to receive/transmit noise if not grounded. There are some very strong emissions especially on the less-filtered HT side of things. I mentioned the pilot lamp holder, I've not personally observed any issues with this part but in my designs but... in some layouts I reckon it could be problematic?

Anyhow, at this point when it comes to masking it's harder to exclude parts than just include them all, my strategy/advice would be to simply mask off the entire inside of the chassis when painting/powder coating.

P.S. I should mention that it wasn't until a high gain design came along that I started noticing that powder coat was causing me problems. Also, when assembling to a powder coated chassis the powder gets chipped/rubbed away more time than not (from locking washers, screw threads, etc.) so you end up with an assembly that has a mix of grounded/ungrounded chassis hardware. Maybe I would have noticed problems on previous designs had there been more floating hardware? For example, even on the high gain design only I think 4 of 17 pot bodies ended up actually floating... but in this case a couple were particularly noisy.
 
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corliss1

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In my very first post here:


I show my grounding plan, and how I just barely touched the spots I used for ground connections with an angle grinder to get through the powdercoat.
 

schmee

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Hey y'all,

I bought a 12x8x2" Hammond chassis for my EF 80 micro build. I wanted it black but just realized it's dipped in like undercoating of some sort so the inside is not clean metal. I know I'll have to clean areas for ground bolt which will go to PT mounting bolt, but do I need to clean anywhere else? Perhaps where the input or output jacks go or any of the pots? Or will connecting them to the ground bus wire to a PT bolt suffice?
Yes, you need a grounding point near the inputs for the pre section as well, and you need to make also make sure all the components get grounded well.
Often connecting the pre grounds near the PT ground results in some bad humming.
 

chas.wahl

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I had a longer posted typed out and then erased it because I misunderstood his post, so I deleted it, then reconsidered it's relevance and re-typed an abridged version, so I apologize @chas.wahl I'm not sure which version of my post you are replying to.
Heh, heh: I couldn't tell you exactly which version of your previous post I was replying to, but I think it was consistent with what is shown above, now. For my own part, in re-reading what you've quoted I realize that I made a dog's breakfast of what I was trying to say in the first para, so edited now, and restated here, with edit in red:

"With respect, I'd like to question the above assertions about pots, sockets, power switches, fuse holders, pilot lamp, and especially jacks. All of these, except some jacks, don't have their internal workings (the parts "wired up" to one's circuit) electrically connected to their parts in contact with the chassis, so unless it can be demonstrated that such non-circuit parts are designed to provide a shielding effect (I'm thinking of pots, maybe, but I don't know if that's really the case with them) then I suspect that whether these are chassis-connected or not doesn't matter."

I apologize for the non-sensical portion originally posted.
Well I can't speak to design, but yes I'm specifically talking about non-circuit parts in most cases here. In the case of an Alpha pot I have observed it's potential to couple noise into the circuit when the metal housing is not grounded.
Experience point taken, but I've not heard before of the pot housing operating as a shield, as a thing, though it may in fact be one.
I was more specific about jacks in my original post. Firstly, I completely omit anything other than Switchcraft jacks because those are all I know! :) But my point on those is merely to have a good chassis connection unless you are specifically isolating them. So, to just be aware that those bushings etc. are circuit-connected.

Power switch bodies I think should be on chassis/ground/earth for safety reasons (can the isolation fail??). Beyond that, I have an example here of a standby switch whose body does not have a good connection to chassis, you can measure the radiated AC on the switch body with a meter and when you touch the switch bat you can hear a little pop.

Tube sockets are especially important to ground to chassis when they have accommodations for a shield. You've got extra metal there, plus of course when the shields are installed they won't be useful.

Any other metal bodies parts in there have the potential to receive/transmit noise if not grounded. There are some very strong emissions especially on the less-filtered HT side of things. I mentioned the pilot lamp holder, I've not personally observed any issues with this part but in my designs but... in some layouts I reckon it could be problematic?

Anyhow, at this point when it comes to masking it's harder to exclude parts than just include them all, my strategy/advice would be to simply mask off the entire inside of the chassis when painting/powder coating.

P.S. I should mention that it wasn't until a high gain design came along that I started noticing that powder coat was causing me problems. Also, when assembling to a powder coated chassis the powder gets chipped/rubbed away more time than not (from locking washers, screw threads, etc.) so you end up with an assembly that has a mix of grounded/ungrounded chassis hardware. Maybe I would have noticed problems on previous designs had there been more floating hardware? For example, even on the high gain design only I think 4 of 17 pot bodies ended up actually floating... but in this case a couple were particularly noisy.
I'm happy to defer to wiser, more experienced people. Safety, yes of course; I omitted consideration of that. But the above examples give me the general takeaway: why create heartache for yourself? Stick with bare-metal chassis, or completely non-conductive! And the corollary: tighten it up, and make sure it doesn't loosen!
 

Phrygian77

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Input jacks don't necessarily need to be grounded to the chassis right at the input. However, you may want to include a cap from sleeve to the chassis close to the jack for RF shielding, but even Blenclowe says it's rarely needed.
 

sds1

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I'm happy to defer to wiser, more experienced people.
Well I'm probably not in either of those categories, unless if you're specifically referring to use of painted/coated chassis, to which I would say (as I mentioned in my original post) this is a funny topic that you don't find much information on. Not sure how many pages you'd have to go back on this forum for example to find a non-conductive chassis finish. It's rare, especially in DIY context.

It's also one of those conversations like grounding, where some things you can get away with in some designs, but you can't get away with all things in all designs. And of course noise floor is always a shades of grey kind of thing.

But the above examples give me the general takeaway: why create heartache for yourself? Stick with bare-metal chassis, or completely non-conductive!
This is fair. Pros and cons all around.

In my case I'm getting the chassis laser cut and folded up in a sheet metal shop, the powder coat (or some other finish) is recommended to hide laser marks/dings/scratches. This leads to the conductivity issue but thankfully was an easy out for me -- the few shops that I've talked to are all accustomed to taking requirements for masking the enclosure prior to powder coating. I just have them apply one big mask over all cutouts from front panel to back.
 

tubeToaster

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I have built 4 amps using Hammond painted steel chassis. I have never removed any paint for grounding. When you drill a hole in the chassis you are exposing enough steel to make a good ground for any bolt ,pot, or jack that goes into the hole. I’ve never had any grounding issues. Just don’t drill oversized holes.
 

Blueslover88

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I have built 4 amps using Hammond painted steel chassis. I have never removed any paint for grounding. When you drill a hole in the chassis you are exposing enough steel to make a good ground for any bolt ,pot, or jack that goes into the hole. I’ve never had any grounding issues. Just don’t drill oversized holes.
That's exactly what I wanted to know. If the bare metal from the hole is enough contact to ground them. I will grind of some paint on the PT bolt and Ac ground just to be safe.
 

Blueslover88

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I have built 4 amps using Hammond painted steel chassis. I have never removed any paint for grounding. When you drill a hole in the chassis you are exposing enough steel to make a good ground for any bolt ,pot, or jack that goes into the hole. I’ve never had any grounding issues. Just don’t drill oversized holes.
I just checked and I have continuity between bare metal of one of the holes and all four input and the one output jacks.
 

Lowerleftcoast

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Plan for the worst case scenario. Power must make it to ground through the safety ground. Stuff happens. Be prepared.
 

corliss1

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^^^^I would also not rely on a screw making contact on a hole. What if you somehow magically perfectly center it? Take the extra few seconds and clear a spot for a nut or washer to dig in to. You'll see most any major maker that uses a powdercoated chassis (Peavey comes to mind) has clear spots where they put down tape during the powdercoat process to allow clean metal on all ground points.
 

Blueslover88

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^^^^I would also not rely on a screw making contact on a hole. What if you somehow magically perfectly center it? Take the extra few seconds and clear a spot for a nut or washer to dig in to. You'll see most any major maker that uses a powdercoated chassis (Peavey comes to mind) has clear spots where they put down tape during the powdercoat process to allow clean metal on all ground points.
I have a few spots ground down for safety ground near PT mounting holes. I was mainly concerned about input/output jacks and pots not touching clean metal chassis
 

Blueslover88

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^^^^I would also not rely on a screw making contact on a hole. What if you somehow magically perfectly center it? Take the extra few seconds and clear a spot for a nut or washer to dig in to. You'll see most any major maker that uses a powdercoated chassis (Peavey comes to mind) has clear spots where they put down tape during the powdercoat process to allow clean metal on all ground points.
This is just dry fitting things still. I will add a grounding lug for IEC and another for power amp ground. Should I make one more for the preamp ground bus?
 

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