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Thread: grounding lugs advice

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    grounding lugs advice

    Hello. How can I connect some tined cooper lugs to a raw metal chassis in a very professional way, please? I wanna start a project which runs signal and power returns through chassis. Unfortunately the convenient chassis for project is powder coated and not galvanised so I have to deal with raw metal for connections. I prefer to ansamble with nut and screw but not sure how can I get minimal contact resistance without fear it will be oxidate in time or will become loose. Any advice appreciated, thanks.

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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by catalin gramada View Post
    Hello. How can I connect some tined cooper lugs to a raw metal chassis in a very professional way, please? I wanna start a project which runs signal and power returns through chassis. Unfortunately the convenient chassis for project is powder coated and not galvanised so I have to deal with raw metal for connections. I prefer to ansamble with nut and screw but not sure how can I get minimal contact resistance without fear it will be oxidate in time or will become loose. Any advice appreciated, thanks.
    While the 'usual' approach was to mask off the unpainted chassis with adhesive circular 'donuts' so the grounding locations aren't painted over, that's no longer what you have. Does your 'Grounding' location already exist (appropriate size hole thru the chassis for the grounding screw/nut/star washer)? If so, you could tape over that area, then carefully cut out a circular hole pattern thru the tape. Then, carefully scrape away the powder coat paint from the chassis with a suitable sharp blade. I find X-Acto # 16 blades suitable for this sort of operation, though any sharp narrow cutting edge that you can control will suffice. You're looking to get to bare metal surface.

    I'm assuming # 10 screw size being used here for your grounding lug/hardware. First, install the grounding pan head screw thru the opposite side, then install a Star Washer under the first nut (or Hex Keps nut will suffice), and tighten firmly. Now, your grounding lug(s) can mount to the top of this nut. I'd actually use a flat washer, then another star washer on top of that, the lugs, then lock washer and nut. Keps nuts to secure the lugs tend to spin the lugs, whereas separate nut/washer don't have that tendency. For long term protection of the now-exposed bare metal, once the grounding screw has been installed, you could apply some clear lacquer over the expose metal (not over the grounding screw hardware).

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    Hi, Thanks for You input. It is not a custom chassis, but a standard one. Unfortunately the one is convenient for my plane ground ideea, regards placement of stages, decoupling caps, in a word distribution currents, it is painted, and very well done, I have to drill holes in respect with grounding points in my project. But have no ideea how to connect better the lugs to chassis to minimise contact resistance. It will be blank raw iron spots, this one will get oxide in time. As well have no ideea how tin with iron react and not tried to start autoprotection oxide at surface. I saw for earthing application as solution a spike washer in between that means double contact from this intermediate washer. Ok, maybe is best compromise but still not clear how this washer assure physical stability as time cooper is plastic deformable and asked for an elastic washer to keep constant contact.A stacked sandwich washers pack for each contact point ruin the ideea to keep it simple against classic return wiring. I wonder how they made it 50 years ago ? The only amp saw it grounded as so was a German one made in '60. They effective spot welded lugs to chassis which I can't.
    You're talking about same thing I suppose. The standard test , at 10 amps current, pass this type of connections for 0.1 ohm contact resistanc I read it. This is good enough to be used as return connection into audio amplifiers, please ?

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    Last edited by catalin gramada; 01-22-2019 at 07:09 AM.
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    Quote Originally Posted by catalin gramada View Post
    Hi, Thanks for You input. It is not a custom chassis, but a standard one. Unfortunately the one is convenient for my plane ground ideea, regards placement of stages, decoupling caps, in a word distribution currents, it is painted, and very well done, I have to drill holes in respect with grounding points in my project. But have no ideea how to connect better the lugs to chassis to minimise contact resistance. It will be blank raw iron, this one will get oxide in time. As well have no ideea how tin with iron react and not tried to start autoprptection oxide at surface. I saw for earthing application as solution a spike washer in between that means double contact from this intermediate washer. Ok, maybe is best compromise but still not clear how this washer assure physical stability as time Cooper is very deformable and asked for an elastic washer. I wonder how they made it 50 years ago ? The only amp saw it grounded as so was a German one made in '60. They effective spot welded lugs to chassis which I can't.
    Why don't you proceed as nevetslab suggests? Once you fasten screw and nut with the serrated washer on the bare chassis, the contact points of the teeth will be airtight. Then cover/seal with laquer or conformal coating.

    http://www.boltfast.co.za/washers-serrated-washer/

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    Maybe is better ideea to solder directly to chassis like Fender did, but no quite. I think more to drill two small holes 0.5mm spaced in between and to use that 'bridge' between holes as hooking point.The thermal dissipation into chassis mass will be more reduced to get nice soldering spots with minimal thermal effort. What are You thinking...?

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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by catalin gramada View Post
    Maybe is better ideea to solder directly to chassis like Fender did, but no quite. I think more to drill two small holes 0.5mm spaced in between and to use that 'bridge' between holes as hooking point.The thermal dissipation into chassis mass will be more reduced to get nice soldering spots with minimal thermal effort. What are You thinking...?
    Proper hardware firmly tightened into the chassis as a threaded stud, then attaching your circuit grounding terminals to that point will be as good, and far more convenient. Granted, Fender, Traynor and many other mfgrs in years gone by have done it by soldering directly to the chassis. I've had great success going the hardware approach. You'd still have to get back to bare metal with your chassis, and applying that big 100 to 300W American Beauty soldering tip to your chassis, it will change the surrounding power coat paint surface (I think). As long as it's not an aluminum or stainless steel chassis.

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    Lifetime Member Enzo's Avatar
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    And that is the whole point of the famous Fender Brass Strip. It provides a virtual chassis that is easy to solder to.

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    You're all far above my level, and Im still working on this same issue, but the last few trials I cleaned off the inside of the chassis just big enough an area under a lug. The outside of the chassis can be painted. I cleaned off the lug, and held it in place with nylon screw and bolt, then using a big iron soldered the lug to the chassis. Then took the nylon hardware off and bolted the stainless one on. Your soldering skills are way better than mine, so you can get it to look very nice. Mine is still a little messy but getting better. The lug is attached so well that if the screw/bolt loosened up, the connection would still be electrically solid. I did put a tiny bit of loctite blue on the bolt before tightening the nut down as well.

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    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

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    This is a true professional aproach. Electrical connection is one problem, mechanical the second one. Solutions like two in one, three in one etc. are just compromises as you see...I stucked with ideea to drill row of small holes (1mm) at small distances in between (1mm) and to use that "bridge" between holes as a hook point for wire(s). Thanks.

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    Last edited by catalin gramada; 02-08-2019 at 10:12 AM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    First, some possible technical ways to proceed. Making a tidy de-painted spot on a chassis can be done easily by drilling a pilot hole for the eventual bolt, then making up a small spot sander. You cut a length of wooden dowel, and in one end insert a nail or screw in the center, down the axis of the dowel. Remove the nail/screw, glue a small disc of sandpaper onto the end, and finally reinsert the nail/screw through the sand paper. Chuck the dowel into a hand drill, insert the nail/screw into the pilot hole in the chassis, and run the drill to sand the paint from around the hole. Should only take a few seconds once the sanding dowel is set up.

    Next, are you SURE you want to build something on this principle, using the chassis for power ground and signal ground? It is possible that there are some situations where this works OK, but using a chassis as all three of signal ground, power ground, and safety ground (you ARE using three wire AC for your power, right?) is a classical example of what not to do for grounding. The currents flowing through the chassis metal cause "ground" to be different voltages at different points on the chassis. This is especially worrisome for a guitar amp where there is high gain, high input impedances, and big speaker currents flowing through the chassis.

    Using the chassis as a ground plane as you mentioned might be OK for RF stuff, but even there, one would generally want to enclose the whole mess in a grounded shell to exclude external RF, and put an explicit ground plane only where it was needed.

    There is a lot to consider in grounding, but you always have to be aware of what currents flow in "ground", from where to where, and what that does to your signal.

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    Quote Originally Posted by R.G. View Post
    . . . <clip> . . .
    Next, are you SURE you want to build something on this principle, using the chassis for power ground and signal ground? It is possible that there are some situations where this works OK, but using a chassis as all three of signal ground, power ground, and safety ground (you ARE using three wire AC for your power, right?) is a classical example of what not to do for grounding. The currents flowing through the chassis metal cause "ground" to be different voltages at different points on the chassis. This is especially worrisome for a guitar amp where there is high gain, high input impedances, and big speaker currents flowing through the chassis.

    Using the chassis as a ground plane as you mentioned might be OK for RF stuff, but even there, one would generally want . . .
    R.G. Your comment struck a note. I've read hours and hours of grounding threads but your question above hit on a description I have not heard described this way before. Don't the signal grounds, e.g. 'ground' side of the cathode resistor/cap for the preamp tubes, need to be tied to the chassis someplace? I got as far as sort of understanding that the cathode resistor/cap should be tied to the return side of the power supply cap that feeds that tube. But what happens from there? Isn't there supposed to be a wire that goes from that junction to the chassis?

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    Mike, the signal ground of a stage should be, and it is related to the decoupling cap of this stage which ideally decouple any a.c. down to 0 Hz. The noise problems comes from output returns. This stage have a load which is happen to be the grid leak resistor for the next stage. In a logical way have to consider to tie this load also to the stage which generate the output voltage and it is...but, the input of next stage is more sensitive and prone to quick pickup noise so we have to move the load of first stage directly into the input of second one. What is happen next is output signal of the first stage will return to the source,due to the physical distance, carrying with it all garbage all the way. This path should be managed as well to not interfere with sources of interferences and to have as low impedance as possible to not be able to create currents between poles.Any a.c. noise will appear here will be picked up into the grids and amplified in a measure depends of sensitivity of the stage

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    Last edited by catalin gramada; 02-09-2019 at 08:07 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    .I stucked with ideea to drill row of small holes (1mm) at small distances in between (1mm) and to use that "bridge" between holes as a hook point for wire(s).
    Donīt. Period.

    Peel paint to bare metal , put a sturdy soldering lug there attached with a bolt and star washers, and to rust proof it cover area with some kind of lacquer, thatīs the safe approved method.

    Also remember to make the green/yellow pigtail at least 1 inch longer than hot and neutral wires, so if somebody strongly pulls mains wire (supposing you donīt use an IEC connector) ground wire is guaranteed to be the last one to snap off, and by a generous margin.

    If you canīt be bother to properly unpaint a safety ground chbassis area, I donīt understand how/why you would consider chassis a suitable grounding medium anywhere else.

    I suggest in that case you run a thick (say 1.2mm) naked copper wire where needed, with one end reaching main supply caps negative terminals (the "source" of supply ground), itīs called a "bus wire ground" and used to be fairly popular.

    Not the most modern way but definitely miles ahead of using chassis as an "everything" ground.


    Which specifically requires that, and used *only* for safety chassis grounding.

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    Usually I did, (sent also from the same layout as yours)

    Click image for larger version. 

Name:	bus.jpg 
Views:	28 
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ID:	52443

    but found chassis grounding as quiet as anything else, so why not ?

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    ..and Mike, you can easy spot on how noisy a grounding point to chassis it is just tie a shield from a input shielded wire to that point, the noise will be direct coupled to the input. I saw a lot of input shielded wire from pots running with shield tied to the pot case which was happen to be a noisy spot

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    Quote Originally Posted by mikepukmel View Post
    [...] Don't the signal grounds, e.g. 'ground' side of the cathode resistor/cap for the preamp tubes, need to be tied to the chassis someplace? I got as far as sort of understanding that the cathode resistor/cap should be tied to the return side of the power supply cap that feeds that tube. But what happens from there? Isn't there supposed to be a wire that goes from that junction to the chassis?
    Grounding is a very slippery subject. There is only one One True Ground, and yes, it is the planet itself; in fact, not every place on the planet is a good ground. For fun, you might want to look up the grounding arrangements for large radio transmitting antennas. We're not going to be dealing with the OTG in amps, so we have to look elsewhere. It gets really tricky to say what "ground" means in a world where we don't use OTG, but instead "the point we have decided is the reference point for voltage measurements".

    Yes, the cathode resistor/caps of preamp tubes, and eventually the output tubes as well, must connect to the chassis someplace to get quiet operation. Exactly how that happens is an exercise in Ohm's Law. We want the signal ground to be at the same voltage everywhere. If it's not, then "ground" here amounts to being a signal at someplace else, different from the remote circuit's "ground". That difference gets amplified.

    The trick to making some electrically remote place have a zero voltage difference between it and your local "ground" is to make the current between them be zero. The simplest way to do this is to make sure that one and only one wire connects them. That makes it impossible for current to flow, so the voltage difference must be zero, no matter what the resistance of the connections is.

    So we want the chassis connected to signal ground. We want the circuit's cathodes, cathode resistors, cathode caps, bias resistors, etc to be connected to signal ground. We want the power stage to be connected to signal ground, and often we want the output voltage to be connected to signal ground. Same for power supply ground. ACK!!

    This can all actually be done. It's called star grounding. You pick one place that you define as the One True Ground, and connect all other grounds to that one by one and only one wire each. The wires may each have a current running down them, making each remote point be different from the One True Ground, but the fact that these are offset from one another doesn't make any difference to the circuits as the offsets from one ground to another can never be supplied to another stage for amplification.

    In this sense, you want the chassis connected to your signal ground (and by extension to your One True Ground) by one and only one wire. Since no return path exists for current, no current can flow between chassis and signal ground, so the chassis IS at signal ground. If the chassis forms a Faraday Cage shield, incoming radiated EMI can cause circulating currents on the chassis itself, causing local spots on the chassis to be different from signal ground, but those differences never get transmitted to the circuits to be amplified.

    So yes, you want signal ground connected to chassis. But only once.

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    To alleviate long term corrosion in adverse locations (eg. near sea-side or in a chemical plant), it is common practice to apply a light smear of vaseline, or a commercial interface compound, to slow down long term air and moisture ingress to the metal interfaces in a bolted connection. The tight clamping pressure and toothed washers provide a stable low resistance connection, where the vaseline only acts to fill any minute voids that can show up as rust spots over years.

    If you were somewhat paranoid, you could use brass hardware, or a conformal coating over the top, but in reality such a joint does not have self-heating or micro-arcing from high currents, and if safety was really a concern then you would be doing a PAT (portable appliance) type test periodically.

    You can set up your own test (ie. pass 1-10A DC through the joint and try and measure the uV/mV drop), and yes most compliance checks require an earth continuity test with resistance no more than 0.1 ohm, but using a commercial tester with 10A operation can be a pain if it uses kelvin terminals.

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