Why ground rails / blades?

[antigua]

One thing I've noticed with all the rail / blade type pickups on the market is that they ground the steel blades, as seen in this picture:



Does anybody know why they do this? I've seen this applied to Bardens, DiMarzios, and Chinese imitations. I'm not sure what point there would be to having shielding on the inside of a circuit, as opposed to outside. It's certainly not vital to the operation of the pickup, and it must increase the capacitive coupling across the circuit by some non trivial amount. 

I'll test this more later, but I don't really have a good test method for gauging static interference in order to see how much noise reduction gained or lost by connecting or disconnecting the ground connection to the blades. 

[stratotarts]

A large grounded surface anywhere near a circuit does help to reduce electrostatic interference, whether it is inside or outside. When it's on the inside, the circuit part becomes the middle node of what is in effect, a capacitive voltage divider between the grounded part and the electrostatic source. The majority of the electrons collect on the grounded part, vs. the circuit, so that the interfering voltage signals to the circuit are greatly reduced. It's because the grounded part is so much larger, and also close to the circuit (compared with the electrostatic source).

A good demonstration of that is the V5 pickup tester circuit bench setup. Even though the wiring is open on the workbench for testing, the recommended sheet of conductive material under the apparatus works effectively to control hum. It's not as perfect as a shielded box, but it works amazingly well. I use a layer of tinfoil underneath a thick paper work surface.

For the same reasons, a well designed ground plane on a PCB can by itself, dramatically reduce interference. In some cases, so well that a shielded box is not necessary and plastic can be used - even though the conductors are not sandwiched between grounded layers, but are exposed on one or both sides.

[straylight]

Try touching the poles of a regular strat pickup. Shouldn't cause much if any buzz as even though they're probably not grounded they're closer to the ground side of the coil. Ususally. A Reverse Wound Reverse (magnetic) Polarity coil will behave similarly.You can spot if someone has cheated with a Reverse Connected Reverse Polarity middle pickup becasue it's quite noisy when you touch it. Humbuckers with a conductive baseplate tend to have the poles contact grounded but not always (airbuckers).

It tends to be the case (but is not always) that humbucker bobbins are wound consistently in one direction so one coil is RCRP. Not a problem with a conductive baseplate or concealed poles. I note the (early production) 1977 plastic cover and baseplate Dimarzio Super2 on my bench has the poles glued in a recessed position so you cannot easily touch them. In rail designs they tend to be quite exposed.

Try ungrounding it and have a listen.

I note that a gotoh hotrail in the neck of one of my strats for about 15 years. The bass side of the rails have had the black paint warn off the rails and have rusted some. I note that when playing pithout a pick and muting the wound strings with my thumb I was getting some rust transfer so there are situations where a player might make a lot of contact. I did have it set fairly high as it didn't induce wolftones as eagerly as some of my single coils and I wanted all the output I could get to balance with a very hot bridge pickup.

[antigua]

Jul 2, 2018 22:28:49 GMT -5 stratotarts said:

A large grounded surface anywhere near a circuit does help to reduce electrostatic interference, whether it is inside or outside. When it's on the inside, the circuit part becomes the middle node of what is in effect, a capacitive voltage divider between the grounded part and the electrostatic source. The majority of the electrons collect on the grounded part, vs. the circuit, so that the interfering voltage signals to the circuit are greatly reduced. It's because the grounded part is so much larger, and also close to the circuit (compared with the electrostatic source).

A good demonstration of that is the V5 pickup tester circuit bench setup. Even though the wiring is open on the workbench for testing, the recommended sheet of conductive material under the apparatus works effectively to control hum. It's not as perfect as a shielded box, but it works amazingly well. I use a layer of tinfoil underneath a thick paper work surface.

For the same reasons, a well designed ground plane on a PCB can by itself, dramatically reduce interference. In some cases, so well that a shielded box is not necessary and plastic can be used - even though the conductors are not sandwiched between grounded layers, but are exposed on one or both sides.

So it's a plus to ground the blades, but does not grounding the blades cause the noise to become worse?

[stratotarts]

One or other end of the coil is usually grounded. If the outer windings of the coil are grounded, it may help shield an uncovered pickup. However, in that case, if the blade is ungrounded it might capacitively couple noise into the inner windings since they are in close proximity and connected to the signal output. But it also might be a shotgun approach - just ground everything that you possibly can.

[antigua]

Jul 7, 2018 22:41:22 GMT -5 stratotarts said:

One or other end of the coil is usually grounded. If the outer windings of the coil are grounded, it may help shield an uncovered pickup. However, in that case, if the blade is ungrounded it might capacitively couple noise into the inner windings since they are in close proximity and connected to the signal output. But it also might be a shotgun approach - just ground everything that you possibly can.

So a stray piece of conductive metal that is not connected to anything can receive noise, and capacitively deliver it to the circuit, but if you ground the conductive metal, it shorts out that voltage potential between the circuit and the piece of metal, thereby eliminating it as a noise source? Is that correct?

If so, does this mean that if you ground the blades, you want the coil's "negative" lead to be at the start of the coils, on the inside, so that it's closest to the blade, but if you have a shield around the coil, then you would want the opposite, for the negative end of the coil to be on the outside, closes to the grounded shield? That would be an interesting implication, as some coils have grounded pole pieces, while others have non-grounded poles, but they do have outside shielding, so it would mean you want the positive and negative ends of the coil to be differently oriented, depending on the circumstances. 

[stratotarts]

It's kind of a finesse, or fine detail as to whether the inside or outside of the coil is ground, because of the same proximity effect that I mentioned. The fact that any part of the coil is grounded, assures a good shielding effect except from very close external conductors carrying interference. The finesse is that when it is on the outside, the inside windings are physically inaccessible to proximate conductors (such as a players finger). This may make it seem like a clever idea. However they are proximate to the poles or blade and if those come in contact with an external voltage, it will be capacitively coupled into the coil circuit. In that case, grounding the poles will help by bypassing the interference. However, it will also increase the pickups overall capacitance slightly because it places a ground surface near the windings with the greatest signal voltage on them.  If the inside windings are grounded, this effect is largely reduced because the interference will go mostly to ground, and that could be seen as an advantage.

A good way to summarize it would be, it is good to place the grounded part of the coil in the place where you expect the most electrostatic interference. If the coil has an added outer shield or copper wrap, they would protect the outer windings and so it would make sense to ground the inner windings instead and the poles and blade may not need grounding. If on the other hand, the poles or blade is grounded by choice, the outer windings could be connected to ground and provide the "no-cost outer shield".

If capacitance is a concern, the best scenario would be with inner windings grounded, no external shield or a shield that is physically much bigger than the coil. In that case it makes no difference whether the poles/blade is grounded or not. If interference is the primary concern, grounded pole/blade with ground on the outside of the coil would be the optimum configuration.

It is because the differences are fairly subtle, that there isn't acquiescence to these principles in many designs.

[antigua]

Jul 4, 2018 13:23:05 GMT -5 straylight said:

Try touching the poles of a regular strat pickup. Shouldn't cause much if any buzz as even though they're probably not grounded they're closer to the ground side of the coil. Ususally. A Reverse Wound Reverse (magnetic) Polarity coil will behave similarly.You can spot if someone has cheated with a Reverse Connected Reverse Polarity middle pickup becasue it's quite noisy when you touch it. Humbuckers with a conductive baseplate tend to have the poles contact grounded but not always (airbuckers).

Thanks for this explanation, I forgot to reply back when you originally posted it. 

I saw someone on another forum suggest that if the outer windings of the coil, which is to say the end of the coil, is ground, then the outer layer of winds would double as shielding. One the one hand it seems plausible, because even though the outer layer is a part of the coil, those windings are very close to the ground side of the circuit. On the other hand, I've had/have guitars with phase switching ability, and switching this flips the side of the coil that ground versus hot, and yet I've never noticed a real difference in the noise level one way or another, but it might just be that I'm in low noise environments. Do you think it's plausible for the outer layer to serve as shielding, if the coil end is to ground?

The other issue is, as you note, in a Fender single coil, the ground is at start, as can be seen in the picture below, where the coil end joins up with the white wire and the coil start connects to the black wire. If it were possible for a grounded outer layer to serve as shielding, then this benefit would be lost on these types of Strat pickups. 

BTW, do you believe that Fender has the coil ground at the start specifically so that touching the pole pieces won't cause noise, because if that's the case, maybe they could have just painted over the pole pieces, as with the rails, so that the guitarist's hand wouldn't make electrical contact with them.

[straylight]

Dec 4, 2018 2:54:23 GMT -5 antigua said:

BTW, do you believe that Fender has the coil ground at the start specifically so that touching the pole pieces won't cause noise, because if that's the case, maybe they could have just painted over the pole pieces, as with the rails, so that the guitarist's hand wouldn't make electrical contact with them.

Yes, but touching lacqured polepieces casues some noise if the ground is to the outside. The core of a pickup appears to work as a better aerial than the outside of the winds. This has got me thinking though, as touching some humbucker bobbins causes a bit of noise at high gain.

Experiments I shall have to try:

1. earthed copper tape with conductive adhesive on underside of pickup
   
2. threading a copper wire into the base of pickup before inserting magnets
   
3. wiring two identical uncovered humbuckers centre start and centre finish on both coils rather than both clockwise as is most conventient on my current setup.