Ground Loop???

[Sowhat]

Got a 1967 Sunn 200S in for refurbishment. Many techs have left their mark inside this thing over the years. I'm doing a lot to this amp... new caps, new sockets, remounting the transformers, going 3 wire mains and stuff like that. In examining the wiring I noticed a ground loop as shown in this picture. This can't be something Sunn did at the factory but crazier things have happened before. I'm drilling out the terminal board rivets and doing a screw, nut lockwasher treatment to them because they all are pretty loose and wiggle around a lot. This wire goes from the ground lug on the bias terminal strip to one of the mounting tabs on the multi-cap.



I'm thinking that some tech stuck this wire in there to try and solve a intermittent ground problem with the bias supply sometime in the past. I am going to remove this wire because it a stinking ground loop! Who in their right mind would put one of these in... I hope it was not Sunn.

My big question is does anyone out there have or have looked at an untouched vintage Sunn 200S and can confirm that this wire has been added or not... It's history as far as I'm concerned. The customer has complaints of hum... no wonder why.

[g1]

So without the wire, the bias circuit grounded through a rivet?
Does removing the wire solve the hum issue? If not, I'd consider it more of a fail-safe rather than ground loop.

[Sowhat]

I've finally had a look at other 200S heads and it looks like Sunn put those wires in there, as you say probably a fail-safe measure. Still it does not give me a warm fuzzy feeling having redundant ground paths in an amp, those can cause so many problems with the quietness of an amp. I do a lot of recording studio related work and one of my main jobs is to make gear quiet. Sunn is not the only manufacturer that does stuff like this, most of them do. I guess it's inevitable in the high pressure world of electronics manufacturing. Riveting those terminal strips and trying to rely upon that connection to conduct ground is not a very smart thing to do but a rivet is cheaper and quicker than a screw, nut, washer arrangement when you are building big batches of amps. There are 2 other place in the amp that do this as well up in the preamp so this looks like a factory sanctioned design. I still plan on removing those connections but I will check as to what effect they have if any. My terminal mounting scheme will assure good tight ground connections and any ground loops I can eliminate, the better.

[Chuck H]

So without the wire, the bias circuit grounded through a rivet?
Does removing the wire solve the hum issue? If not, I'd consider it more of a fail-safe rather than ground loop.

+++
The circuits that are redundantly grounded there shouldn't cause audible problems if properly decoupled. It may have been an insurance measure in the stock design.

[Sowhat]

+++
The circuits that are redundantly grounded there shouldn't cause audible problems if properly decoupled.

There's no decoupling there, it's just the physical chassis connection and a parallel grounding wire... actually, I don't know exactly what you mean about decoupling a DC potential ground path or how that is done. Decouple what from what?

[Enzo]

And remember too that "ground loop" is not a thing, it is a condition.

[Chuck H]

If there's nothing to decouple there's no threat of a ground loop problem! My point was that if any decoupling circumstances associated with those ground leads is sufficient or non critical a redundant ground may have been added for safety or reliability.

[soundguruman]

yes, the wire was added.
Manufactures unfortunately, use stuff like rivets and screw posts, sheet metal screws, to make circuit grounds.
These eventually oxidize and fail or become intermittent. Some of them overheat and actually burn.
That's why the wire was added.

Yes, a nut and bolt, lock washers, soldered AND crimped ring terminals, is the correct way to fix it.
It would be even better to solder the wire to the chassis, although this is not practical in many cases (you can't solder to aluminum, etc...)
This is a problem with many amps, including Sunn, Crate, "Ampeg" Peavey, and on and on and on.
Every time I see one, I install a permanent wire to the chassis...with a very tight screw nut and lock washer.

For two grounds that are closely located together, the ground loop is not an issue.
The loop usually occurs when the grounds are located much further apart.

[Chuck H]

Excellent point on the distance between the grounds. Of course there is resistance in the wire, but I think the actual distance between ground points is more relevant. How much voltage differential can there be across that distance? Another tip for grounding to aluminum... Use some sort of electrical contact grease and a lock washer with teeth. Aluminum forms an oxide layer that's almost invisible so clean the mounting surface with fine (very fine) sandpaper.

[Sowhat]

yes, the wire was added.

Not to contradict you because you make some good points but the wires were added at the factory when the amp was first made back in 1967 from my close examination of this head and several other 200S's. After tracing the whole grounding system it is clear that Sunn did this because the wires are routed inside of the tight factory laced wire bundles and they are that old brittle 22GA solid wire, nobody snuck those wires in there after the fact. There's indeed some distance across the chassis they all span... maybe 10 or 12 inches total. These definitely pose a ground loop potential of considerable proportion... at least they do in my eyes.

[Tom Phillips]

Not to contradict you because you make some good points but the wires were added at the factory when the amp was first made back in 1967 from my close examination of this head and several other 200S's...

Thanks for this is interesting information. Could you post a clean copy of the photo you posted at the beginning of this thread? I'd like to see an unedited view of the original wiring.
Thanks,
Tom

[g1]

It may seem like a potential ground loop is high priority, but try to also consider potential bias problems/failure. That could be catastrophic to reputation. From a manufacturer's perspective, reliability and warranty claims will trump a potential noise issue. My guess is when they installed that wire there was no big noise introduced. If it was objectionable, they probably would have looked for a different solution.

[soundguruman]

If there's nothing to decouple there's no threat of a ground loop problem! My point was that if any decoupling circumstances associated with those ground leads is sufficient or non critical a redundant ground may have been added for safety or reliability.

It's reliability. The safety ground is entirely separate.

[soundguruman]

It may seem like a potential ground loop is high priority, but try to also consider potential bias problems/failure. That could be catastrophic to reputation. From a manufacturer's perspective, reliability and warranty claims will trump a potential noise issue. My guess is when they installed that wire there was no big noise introduced. If it was objectionable, they probably would have looked for a different solution.

For some reason, manufacturer still uses UN-reliable circuit grounding, such as screw post to circuit board, with black anodized screw.
Many of them are just too stupid to make it reliable -- despite the fact that it damages reputation for reliability.
Like I said, many manufacturer does not understand HOW to make a reliable guitar amplifier, Even Today!