Pots grounding

[Lambertini]

Hello!

Do you know why Fender grounded pot ground pins to the copper plate at multiple points in this 76 Twin Reverb instead of grounding them directly to the pot case already gounded to chassis ? Grounding to pot case seems much easier to maintain, right?

Cheers

[dwinstonwood]

I'm not sure. Maybe?

[Lambertini]

I just checked: some washers are black but metal.

[tdvt]

Grounding to pot case seems much easier to maintain, right?

Not sure WHY they changed it, but I would disagree.

A soldered ground wire would eliminate some potential failure points.   

[acheld]

I've always been interested in why Leo used a brass grounding plate, but have never seen a definitive answer.

That said, his priorities were to make guitars and amplifiers as affordable as possible, and this applied both to components and manufacturing.  You can bet that "ease of maintenance" was not a high priority.  My guess has been that the brass plate eased manufacturing at the time.

Grounding signal path through the pot body sets the stage for possible ground loops.  My opinion only.

[Lambertini]

All right, thank you! I'll stick with grouding ground pins to something else than the pot cases.

[HotBluePlates]

Do you know why Fender grounded pot ground pins to the copper plate at multiple points in this 76 Twin Reverb ...

The pots aren't the only thing soldered to that brass plate, right?  Do you also see where there are wires from "ground eyelets" on the fiberboard also running to the brass grounding strip?

This is about expediting production:

   - There are places over near the power transformer that get soldered to the chassis.
   - Soldering to the steel chassis requires a 150+ watt iron, that's about 10x as big as the iron used to solder eyelets or pot lugs.



   - The big iron for soldering to the chassis can't fit in/around pot lugs without burning wires.

   - Fender soldered the eyelet board & pot wiring to the brass strip outside of the chassis.
   - This prevents the chassis from acting as a heatsink, and requiring the big 150w iron.
   - The board, brass strip, pots, etc, all get loaded into the amp at one time, and then the fiberboard is wired to the sockets.

   - Ultimately, the pots sit at a point in the circuit, and would ideally ground at the "ground of that circuit."
   - The brass strip is an approach towards this ideal.
   - Random-grounding on a chassis is generally less-good, though you might not notice outside of high-gain amps.

[Beezerboy]

guessing you could have parallel production too. people building the guts would be independent of those building the chassis, it splits the work into smaller pieces... each worker only has to worry about a small bit of the puzzle

but I could be wrong

[WimWalther]

All right, thank you! I'll stick with grouding ground pins to something else than the pot cases.

I was shown to run an 18ga or larger solid wire buss soldered to the rear of the pots to use in the same way as Fender used those brass strips.

Is there something wrong with this approach?

(BTW, I was out of large buss wire.. so the one you see in the photo was previously the handle from one of those folded paper Chinese takeout boxes. =)

[Lambertini]

Is there something wrong with this approach?

Very close to my own question. Can't tell.
I've understood that grounding ground pins to this bar would be a bad idea because of ground loops.
I also see this as an horrible hassle if you have to change a pot.
Finally, why grounding pot cases together as they are already grounded to the chassis by the washer? Unless you used isolated washers?

[tdvt]

Is there something wrong with this approach?

Very close to my own question. Can't tell.
I've understood that grounding ground pins to this bar would be a bad idea because of ground loops.
I also see this as an horrible hassle if you have to change a pot.
Finally, why grounding pot cases together as they are already grounded to the chassis by the washer? Unless you used isolated washers?

I will offer my own ideas on it, arrived at after sifting through the endless amount of discussion out there, but remember there is always more than one way to do something.

The pot cases ARE grounded by virtue of being mounted to the brass plate/metal chassis, barring an issue with corrosion under the nut or intentionally isolated using fiber washers (not sure why you would do that, but there might be an instance).


Adding a bus wire soldered to the backs of the pot shells creates a redundant ground path, parallel to the chassis/brass plate.

I suspect that for that reason, soldering a bus to the pot shells has fallen out of favor.

The more common practice these days, if you don't replicate the "classic" Fender method, is to use a bus for control grounds that is NOT connected to any ground point except immediately at the input jack.

I usually have all the pre-amp circuit grounds land on this bus, along with the pot grounds. This leaves the pot cases essentially as shielding for the pots themselves, as the cases are not connected to any circuit grounds, other than the chassis.

Check out this control bus on Sluckey's web page.

https://sluckeyamps.com/supro/s-10.jpg

[Lambertini]

Check out this control bus on Sluckey's web page.
https://sluckeyamps.com/supro/s-10.jpg

Why are the wiper resistors on pot 2 and 4, and the ceramic cap on the control pin of pot 3 grounded on pot 3 case instead of the ground bus in the photo?
Doesn't this contradict the "case is shielding for the pots themselves"?

[sluckey]

Why are the wiper resistors on pot 2 and 4, and the ceramic cap on the control pin of pot 3 grounded on pot 3 case instead of the ground bus in the photo?

They are not grounded to the case. That is an insulated turret terminal. The two resistors and cap are just connected together with a shielded cable that runs to a tube pin. Here's the rest of the story...

[Lambertini]

One can call that a neat build!

[acheld]

All right, thank you! I'll stick with grouding ground pins to something else than the pot cases.

I was shown to run an 18ga or larger solid wire buss soldered to the rear of the pots to use in the same way as Fender used those brass strips.

Is there something wrong with this approach?

Not wrong.  It can work fine . . . It IS a compact solution if you're really tight for space.

But if you decide to change a pot it is a real PITA if all of the pots are soldered onto one wire on the back.  So I don't  . . .

As for "large,"  I've used that square wire, and 12 up to 18AWG, and all I can say is that the larger the wire, the more heat you have to put into it to solder well.   Although larger gets you less resistance in that short length of wire, the difference is truly negligible and makes no difference with the small current being passed in our tube audio circuits.   I've settled on 18AWG as large enough to get some rigidity in the wire run, and small enough not to need so much heat when soldering. 

[dude]

I've seen many grounding methods, buss soldered to pots, brass plates used, etc that all worked find. The most important process in grounding a guitar amp is, IMO, keep the power section star ground as far as you can from the pre-amp section. I used the buss-bar as in Sluckey's picture and never had a problem as long as that bar is grounded at the pre-amp end only, and preferably bolted near the input with a bolt to the chassis. I've used this method many times with great success, also have not use it with great success. Just keep the power section separate from the pre-amp section. The buss used this way is floating on opposite end of the pre-amp. I think this grounding method has been mentioned here many times...

[mresistor]

One of our members, P2P amps, has developed his own style of mounting a copper ground buss, probably at least 18guage behind the pots and then he attaches (laces) all the pot wiring to the semi-rigid ground bus wire. It makes for a very beautiful and sharp looking build.   


Take a look.

[HotBluePlates]

I was shown to run an 18ga or larger solid wire buss soldered to the rear of the pots to use in the same way as Fender used those brass strips.

Is there something wrong with this approach?

It's a pain in the butt to replace 1 pot when there is a buss wire soldered to all of them.

I did the same thing with buss-wire on pots when I started out.  I don't do it anymore because it's not helpful; the ground bass can be mounted somewhere other than the back of the pots.

I've understood that grounding ground pins to this bar would be a bad idea because of ground loops.

When "loops" are small, or very-low-resistance, they don't matter.
When current around the loop is very-small, the loop doesn't matter.

Any ground loop formed by the buss wire on the pot casings has all 3 factors for "doesn't matter."