Yet another ground debate

[Randall]

I am taking up where my customer left off 5 years ago on a Hoffman 5E3 build. Per the Hoffman site, he instructs to solder a bare ground bus wire across the back of the pots and connect it to input jack ground. I understand we want to ideally keep inputs and mains grounds seperated, and I have run a few ground buss wires myself, but I would never solder them to the pots, they are already grounded by the chassis. What would that accomplish? Customer wants the extra mojo he thinks the hoffman scheme will give him, but I think it's wrong. What I have done on my builds is run seperate ground wires from the pots to a ground lug on an input jack. Or sometimes I have drilled a hole next to the jacks and run the pot grounds, preamp cap ground, and cathode grounds to that point.

You have to click on the word Grounds in the link to get to the right page for some reason.

[Enzo]

There are lots of ground schemes. I don't care for that one, but IF IT WORKS, then it is OK.

Not all pots have a metal to metal contact between the rear cover and the metal bushing. A ground bus across the backs insures (ensures?) that all rear covers will be grounded and thus offer some shielding. Many do, but not all. I would install such a ground wire and ground it to the chassis at the jack, but personally I wouldn't also use it for circuit grounds. Others do.

He says he has removed star grounds to cure hum problems. In my humble opinion, all he did was remove POORLY DONE star grounds. Star grounds work.

Grounds generally cause/cure hum problems for the most part. Your customer wants mojo? Just what does he think this will improve, and how? If the amp works now and is not hummy, changing the ground scheme might just INCREASE hum.

If it ain't broke, don't fix it. Just my opinion.

[Randall]

The amp was only partially assembled, he populated the board and then it sat for 5 years. So it's not functioning yet. I'm wiring it now.

These pots do indeed ground the shells via the chassis with toothed washers. They are Alpha 1 Meg. What purpose would a wire between them serve?

I also question the idea that star grounds are bad. I wouldn't connect everything in this case to a single ground, preferring to keep preamp and mains/power amp grounds apart, but Hoffman is essentially using two star grounds at opposite ends of the chassis, which I agree with.

[uneumann]

So if the man wants pot cases wired - give him wires... they're not doing anything since the pots are already tied to chassis. Keep the wires short and tight to the pots so you don't create significant ground loops. Connecting the wires to the input jack will also have little or no effect - assuming the jacks are also tightly connected to the chassis already. The only thing the wire will do is duplicate the mechanical connection. As long as you're not going to actually tie circuit grounds to the new wires, they are harmless and he'll feel better. Just make sure you charge enough for the added Mojo...

[Randall]

Yeah, I'm not going to install a folly that has my name attached to it, regardless of what someone wants. Reputation an all that. I'm really just looking for educated opinions on whether this is a solid concept or not.

[Leo_Gnardo]

These pots do indeed ground the shells via the chassis with toothed washers.

What's the chassis made of, and how is the surface finished?

I've seen examples - and had to fix supposedly "grounded" pots - on painted chassis where the tooth washers didn't penetrate the paint layer. Same with anodized aluminum chassis. Some manufacturers mask the area around the potentiometer holes before painting, in an effort to keep paint out of the way. A couple times I ran across this on painted and anodized chassis, and although the paint was not interfering with making a good ground, the anodizing was. In one memorable case the result wasn't hum, but the amp picked up Radio Japan broadcasting from French Guiana, loud and clear.

If you're going to count on the tooth washers to ground the pots, do make sure there's nothing in the way. Sand or wire-brush the chassis contact area. Also consider the metal on Alpha pots. It's coated with something - you can't solder to them unless you sand/scrape the surface. Whatever that finish is, may also interfere with grounding, tooth washers or not.

BTW I remember that copper ground bus you put behind the pots in the Deluxe Reverb you rebuilt point-to-point. I still think that was outstanding.

[Helmholtz]

Also consider the metal on Alpha pots. It's coated with something - you can't solder to them unless you sand/scrape the surface. Whatever that finish is, may also interfere with grounding, tooth washers or not.

For the same reason one should not rely on good electrical contact between the crimped-on covers and the chassis (see Enzo's post above), even with star wahers.

[pdf64]

I had a friend’s DrZ EZG50 in that kept going crazy - squealing oscillation; an Enzo thump would temporarily resolve it. The issue ended up being that it relied on that style of pot case ground bus for the circuit board’s 0V return, and every damn pot must have had a bad connection, due to the aluminium chassis surface layer. Even with all those pot to chassis fasteners re established, it came back after a while with the same issue. The proper fix was to drill and fit a local chassis 0V tab using a dedicated bolt and star washer etc, that could be torqued up.

[uneumann]

Yeah, I'm not going to install a folly that has my name attached to it, regardless of what someone wants. Reputation an all that. I'm really just looking for educated opinions on whether this is a solid concept or not.

It seems my attempt at humor failed and obscured the point - sorry about that. Since the other posts have already recounted cases where the added pot case wires can help, let me reiterate that adding the wires also can't hurt if the pot cases are mechanically well grounded. So there is no downside or folly. He's happy since he gets what he wants and you have added insurance against mechanical contact problems... its win - win.

[pdf64]

I suspect that pot manufacturers don't take any particular consideration to make pots be resilient to having solder connections made to the case.

[eschertron]

The amp was only partially assembled, he populated the board and then it sat for 5 years. So it's not functioning yet. I'm wiring it now.

To really deliver the mojo, have him store the amp for another 50 years or so, THEN finish the build.

[Mick Bailey]

Also consider the metal on Alpha pots. It's coated with something - you can't solder to them unless you sand/scrape the surface. Whatever that finish is, may also interfere with grounding, tooth washers or not.

Alpha pots are zinc plated. Zinc isn't easy to solder to unless you use a zinc-specific flux and regular cored rosin flux doesn't work very well. The coating is very thin and scraping it gets down to the steel. Zinc corrodes in moist air, so coatings are often passivated - that's the yellow finish you get on parts with chromate passivating -- particularly on automotive parts. There are black or green passivation products as well. Sometimes a polymer coating is used but this is a surface treatment - passivation chemically reacts with the surface to protect the zinc.

Passivation increases electrical resistance. Either way, whether bare metal or treated, a crimped assembly cannot be guaranteed to conduct - especially where you have a plated shell crimped onto a diecast bush. Even old tinned Switchcraft sockets that have been damp will oxidise and show a high resistance between the threaded bush and the ground tab. Sometimes I need to clean these up and solder the joint to preserve originality as much as possible. Pots go the same way and the worst are probably the Vox cadmium plated items that need special handling to avoid breathing or touching the furry yellow cadmium oxide (once used as a pigment 'cadmium yellow')
.

[Randall]

The chassis is steel, either nickle or chrome plated, not sure. I brought the grounds from the pot lugs back on a wire and grounded to the input jack, along with the cathode grounds as I have done on my own builds. Electrically the same as a bus wire, except for the soldering the pot shells to ground in case the plating or toothed washer failure argument. I'll take my chances on that point that it is not necessary
.

[Leo_Gnardo]

Alpha pots are zinc plated.

Thanks for the explanation! Yes, time was, say 25-30 years ago, Allphas were the dickens to get solder on even when sanded or scraped. I think back then they had that polymer coating. Not anymore.

pdf64 - I've always found CTS pot housings to accept solder readily. Too bad the quality of recently made CTS makes me cross them off my shopping list. Maybe they sell the "good ones" to Fender & Gibson. We covered this in another thread recently, bemoaning the current state of CTS bought thru the usual suppliers. Ah, but you can solder to them... cold comfort, that.

[soundmasterg]

Hoffman's scheme seems to work well for most people, though I suspect most of the people using those techniques are building lower gain amps. I don't agree with that type of grounding, especially the buss soldered to the back of the pots, but if it works then it is fine, though unnecessary wasted effort in my opinion. I tend to use Kevin O' Conner's Galactic Ground scheme, which is similar to Merlin's scheme also. It works perfectly fine for me.

Anything is an improvement over the original tweed deluxe ground scheme. Trust me, I own a real 1956 5E3. It hums a bit more than I'd like but I won't modify the heaters where one side is grounded because it would mess with the value of the thing. I'll just build myself a clone if I want to do that.

Greg