Why Bus Wire Back of Pots?

[miallen]

The housing of a pot is clamped around the front onto the bushing and the bushing is bolted onto the chassis and the chassis is grounded.

So why is there bus wire soldered to the backs of pots?

 

[Micky]

To provide a failsafe ground.
Some pots are insulated from the front panel chassis.

 

[Garrett]

Grounding as well as securing them in addition to the hardware.

 

[miallen]

Grounding as well as securing them in addition to the hardware.

Ok, so if the pot housings are already grounded through their bushings and I'm not worried about them spontaneously coming loose, then it's not necessary.

 

[Ken]

Anyone who's had the ground fail to the 4/8 ohm jacks on a DSL has discovered the jacks are not chassis grounded; I assume the pots are the same way in amps with the buss wire.

Ken

 

[biggs]

It also helps stop the pot from turning inside the chassis if the nut loosens at all.

 

[danman]

It is necessary unless you create a different grounding point for everything attached to it. There are many different grounding schemes that you can use but the pots make a quick and easy method of mounting the ground bus and everything that is attached to it gets grounded through the pots casings. I prefer the "Larry Grounding" method myself but I have also had good success with a bus style either mounted to the pots or by itself on standoffs.

 

[miallen]

I just don't think the pot housings or even the chassis should be used for any other than a shield. So if the pots are like some Alpha pots that I have that have continuity through the bushing, they should not be used as a ground. Some grounding methods I've been looking at violate some basic rules about low noise amplifier design. And in a high gain / high current amp like a Marshall, I would think a star of stars style grounding scheme would be important. Meaning there should be separate grounds for each filtered section, separate ground for the chassis / shielded wires and of course the most important is a single thick ground wire from the power tube cathodes to the star. And using shielded cable in strategic locations (like inputs) might also be important. But I'm asking because this will be the first high gain guitar amp build for me. So as Fonzie would say, I could be "wro-wro-wrong".

 

[Micky]

Maybe you should have started with that fact...
The fact that you were gonna build an amp and had concerns. Your OP sounded like you were questioning a tried-and-true method of grounding the pots on a tried-and-true top selling amplifier. Oh wait...

Grounding in and of itself implies a common zero-volt reference, 'separate' grounds or in your case, grounds of different potential, suggest something different.

You should possibly elaborate on your thinking or methodology so others could chime in with their opinions as well. If we had a clear reason why you think this way it may help us understand the end by which your means are trying to accomplish.

 

[ampmadscientist]

The housing of a pot is clamped around the front onto the bushing and the bushing is bolted onto the chassis and the chassis is grounded.

So why is there bus wire soldered to the backs of pots?

Because oxygen, between dissimilar metals:
works it's way in over time and causes loss of continuity.
After a few years the pot is no longer solidly grounded. The noise starts to increase.

The buss wire doubles the ground, assuring the back of the housing remains grounded, and shielded.
(The back of the pot is crimped on, and it's not a dead solid connection)

The chassis is galvanized, the pot is plated with (something) or anodized.
The non-compatible metals encourages corrosion.
Same thing happens to all electrical connections when metal contacts metal.

If it was welded together, you might have a better chance.

 

[ampmadscientist]

I just don't think the pot housings or even the chassis should be used for any other than a shield. So if the pots are like some Alpha pots that I have that have continuity through the bushing, they should not be used as a ground. Some grounding methods I've been looking at violate some basic rules about low noise amplifier design. And in a high gain / high current amp like a Marshall, I would think a star of stars style grounding scheme would be important. Meaning there should be separate grounds for each filtered section, separate ground for the chassis / shielded wires and of course the most important is a single thick ground wire from the power tube cathodes to the star. And using shielded cable in strategic locations (like inputs) might also be important. But I'm asking because this will be the first high gain guitar amp build for me. So as Fonzie would say, I could be "wro-wro-wrong".

FYI
star grounding is not the quietest method.
This is because of AC ripple in the ground connections of the power supplies.

The chassis itself is the safety ground.
It's connected to the green wire on the power cord.
That's required, that's the law.

It's not just a shield, it's for safety.

And, the connections, screws, are hipot tested with 5000 volts.
If it's doesn't pass - it's illegal to sell it.

Noise is the secondary concern. Safety compliance is the first.

Whatever you do to reduce noise can't interfere with the safety requirements.

 

[miallen]

Because oxygen, between dissimilar metals:
works it's way in over time and causes loss of continuity.
After a few years the pot is no longer solidly grounded. The noise starts to increase.

Now this makes sense.

So then pot housings should have a soldered wire connecting them to ground.

But I would still argue that it should only be on the shield ground.

 

[Ken]

ground.

But I would still argue that it should only be on the shield ground.

Not being a tech, I'm a little confused. For a pot to be a shield against noise, doesn't it have to be grounded? And aren't all the grounded components common to each other even if they're connected to the chassis at different points? So what's a "shield ground"?

Your comment implies separate grounds...

Ken

 

[miallen]

FYI
star grounding is not the quietest method.
This is because of AC ripple in the ground connections of the power supplies.

I'm not sure I understand this. The major current sources / sinks would be primary members of the star. This would include the center tap of the PT. Actually that might be the most important connection to the star. So in order of importance (meaning they should be large thick short wires), the list might be:

• Center tap of PT
• Cathodes of power tubes
• Ground ends of all filter caps

If the current on the chassis is reduced to 0, then it should not be important where the chassis is connected to the star. The wire on the pots can be connected anywhere on the chassis and then the chassis can be connected to just about any ground. The pot housings are just an extension of the chassis.

The other grounds that should have little or no currents on them are:

• Chassis
• Mains earth ground
• Input jack grounds

So these could be secondary star ground connections. Of course, as you point out, the chassis would be directly connected to the mains earth ground for safety reasons and in practice the star would probably be something electrical bolted to the chassis with a good toothed washer.

So if this is not the quietest method "because of AC ripple in the ground connections of the power supplies" I would very much like to hear why.

 

[miallen]

Not being a tech, I'm a little confused. For a pot to be a shield against noise, doesn't it have to be grounded? And aren't all the grounded components common to each other even if they're connected to the chassis at different points? So what's a "shield ground"?

Your comment implies separate grounds...

Yes, there are separate grounds. But they all run to a central "star ground". So using a multimeter there should be no resistance measured between any of the "separate" grounds because they're all connected together.

But the trick is that they all have to be connected together back at *ONE* point. The reasons is because any length of wire has inductance and resistance and if there is significant current on that wire the ends will not be at the same potential and you could get noise in the system.

For example, if you disconnected the first preamp filter cap ground (the filter cap for the first gain stage of the preamp) and instead connected it directly to the ground at the cathodes of the power tubes, they would all electrically be connected to "ground" but you would get horrible horrible noise.

 

[mickeydg5]

It is best to think of grounding schemes as levels. The same scheme that one might apply to voltage supply levels.

Keep things simple. Most of these amplifiers are simple.

We all learned sets and subsets. Within an amplifier are sets and subsets all part of the whole.
Look at networks and circuits as those subsets being parts of sets again making up the whole.

Each stage or compartment of an amplifier can be looked at as a subset which is divided from other subsets or sets by connections and components of the power supply. Those subsets should derive their positive and negative references from the same points. Then the subsets are tied to the next group or set and so on.

The ultimate ground in an amplifier like these is chassis ground. Chassis ground connects to Earth ground which is part of the external power supply. Other ground busses or stars setups usually end up connected to chassis at some point. Good design and layout make for best outcome.

The key in the amplifier is compartmentalization of circuits and stages (subsets) with their connections made to the same reference points and configuring them into sets in an appropriate order.

I see the wire buss on the back of potentiometers as a common ground acting as a star ground point or accepting grounding connections from other star grounds.

 

[Guitar-Rocker]

Quote "If the current on the chassis is reduced to 0, then it should not be important where the chassis is connected to the star. The wire on the pots can be connected anywhere on the chassis and then the chassis can be connected to just about any ground. The pot housings are just an extension of the chassis."


"Chassis connected to just about any ground", can be a real good way to end up with ground loops, and increased noise.

 

[miallen]

"Chassis connected to just about any ground", can be a real good way to end up with ground loops, and increased noise.

It's not a ground loop if the circuit ground and chassis are only connected in one spot.

But a ground loop is a potential issue with using the pot ground for signals and not just as a shield. If there is any other place where ground connects to the chassis, that *will* make a ground loop.

However, I suspect the current in said ground loop would be comparatively small and that as long as the power tube cathodes, center tap of the PT and filter cap all come together at the same spot, the amp will be quiet enough. A little hum is cool anyway.

 

[92lx50]

What about the grounding method used in this build? Looks really neat and cool.

Rig-Talk • View topic - A Wondrous Adventure: SLO Clone Build (Now with Clips)

 

[mickeydg5]

What about the grounding method used in this build? Looks really neat and cool.

Rig-Talk • View topic - A Wondrous Adventure: SLO Clone Build (Now with Clips)

I am going to say that is a Clown build or someone who went to the same school.