Sheilded wire capacitance and other guitar wiring questions

[JWK]

FWIW in the various low-end 2013, 2014 & 2016 Gibsons that I've examined they have used shielded multi-conductor cable to connect the selector switch to the control compartment. These guitars are all noisier than they would be if the cavities were shielded and better wiring techniques used (like individual cables as Terry recommended.)

IMO it is definitely worth it to shield the control cavities, even in guitars with humbuckers. I usually use copper foil tape with the seams soldered so that they don't lift up. Heck, when I'm done you usually can't see any tape at all, just 60/40 solder.

While the humbucker design usually eliminates the 60/120hz hum very well I think that shielding the control compartment is necessary to eliminate the other "gremlins"- those higher pitched static-y noises that I get at home living in a cheaply built, poorly wired 4-plex from 1971.



Steve Ahola

However, being a semi-hollow, there is no control cavity to shield. So I'm trying to find the best shielded wire layout.

[big_teee]

I have some small insulated shielded I use.

[Steve A.]

However, being a semi-hollow, there is no control cavity to shield. So I'm trying to find the best shielded wire layout.

I was rambling on about shielding in general, as with Les Paul-style guitars. On an ES-335 style guitar I guess you *could* solder a lead to a piece of copper foil tape which you would stick on the bottom directly under the pots.

I did have a question for you: are you aware of the challenges of working on semi-hollow guitars? You need to pull the pots out through the F-hole, making sure that you can reinstall them exactly as they were (some people tie dental floss around the shaft.) The local luthier showed me the gynocological pliers that he uses to get everything aligned properly.

BTW I googled your guitar and see that they are regarded very highly. I would suggest that you do nothing more than change the pickups and clean the pots with DeOxit. Being a semi-hollow it's probably not going to be played extremely loud with insane amounts of gain so I would not worry about the multiconductor cable going to the selector switch. And be sure to save the original pickups in case you decide to sell it someday.

Just my opinions here as always...

Steve Ahola

[big_teee]

I do a modified version of this, except I use all insulated shielded cable.
electronicscloseup.jpg Photo by freddarl82 | Photobucket

[JWK]

I did have a question for you: are you aware of the challenges of working on semi-hollow guitars? You need to pull the pots out through the F-hole, making sure that you can reinstall them exactly as they were (some people tie dental floss around the shaft.) The local luthier showed me the gynocological pliers that he uses to get everything aligned properly.

BTW I googled your guitar and see that they are regarded very highly. I would suggest that you do nothing more than change the pickups and clean the pots with DeOxit. Being a semi-hollow it's probably not going to be played extremely loud with insane amounts of gain so I would not worry about the multiconductor cable going to the selector switch. And be sure to save the original pickups in case you decide to sell it someday.

Just my opinions here as always...

Steve Ahola

There is an access plate on the back of the guitar. It's pretty much the same shape as a tele control plate, cuz it only needs to access two pots. No need to pull all the acrobatics of the usual semi-hollow. Yes, I plan on saving all the parts to put it back to stock in case I sell someday. I have a sort of love/hate relationship with this guitar, so it's entirely possible.

[Steve A.]

There is an access plate on the back of the guitar. It's pretty much the same shape as a tele control plate, cuz it only needs to access two pots. No need to pull all the acrobatics of the usual semi-hollow. Yes, I plan on saving all the parts to put it back to stock in case I sell someday. I have a sort of love/hate relationship with this guitar, so it's entirely possible.

Yikes! Now that you mention it I remember that you told us about the rear access cover. My bad!

After reading about these guitars I think that shielding the control compartment might lower the resale value.

Steve A.

[dai h.]

Steve (or anyone), are there any negative effects on the wood when applying the soldering iron to the copper foil?

(replying to my own post here... lol )

was thinking about how a piece of wood would cook and burn, and I'm thinking as long as I solder in a normal manner (i.e. quick) the body will be okay(?). Additionally, I'd be soldering to a bit heat sink, so (besides being quick) seems another factor to help minimize any negative effects of the soldering iron heat. Still not completely sure about any paint that would be heated and the best way to deal with edges (to try to form a "box" with the foil that is a low impedance connection all the way around it).

re: ground loops (another thing or two): an easy one to think of is when you have a stereo unbalanced out of two shielded wires from one device to another input. The grounds (shields/signal return) of the two wires form a loop. If hypothetically(not something likely to happen in practice because of the way cables are typically made) the hot and ground on each cable were just two separate wires, and you grabbed the ground wires, pulled them apart and made a really big hoop, this would be bad (as in you'd be making a big antenna for noise to be induced in via this hoop). So to avoid this, you'd run the cables tightly together side by side.

Also, just because you have a ground loop doesn't necessarily mean it will automatically result in a drastic hum, buzz, instability, etc. problem. Some redundant connections (ground loop connection) don't do much. (Something I vaguely recall from the Dave Funk book is) lots of ground loops making a ground plane can be good. I've also traced out the mixer section on an old cassette Tascam 488mkII Portastudio cheap-ish mixer section and I noticed there were a bunch of loops, but they don't seem to result in a hummy buzzy mess.

(Unless you get lucky or something) it's a good idea to try to think about what is going on in the grounds. Shielding, "doing a pure star ground", or making a ground plane are not panaceas, so I'd be very careful if you catch that notion.

[Fluoroscope 5000]

“ScottA” of Zexcoil did a bunch of tests with internal wiring which showed that single unshielded wires with separate grounds measure significantly less capacitance than shielded wires. You might reduce total capacitance by 50pF or so using twisted pairs instead. Twisting pairs probably won’t make a difference inside a shielded cavity, unless maybe there are close parallel + wire runs. Parallel - runs shouldn’t matter, init? It all goes to ground along with the shielding.

I like the high end of my GFS neck AlNiCo PAF with my ~160pF/8’ cable, but it had more “cut” than I liked with a ~240pF/12’ cable. I don’t really know what the wiring is like inside my sealed ‘Aurora Stealth’ guitar, but it does have an LP-style upper-horn toggle. I think it’s shielded multistrand - - as were the OEM pickups. Considering the average inductance for a ~7.8k DCR/42 AWG AlNiCo PAF being ~4.5H, I’d think the internal pickup C pretty low to deliver the bell-like high end detail with 500k pots, but who knows? ~50pF or so has made a significant difference to me in that and other cases.

Regarding hum in passive hum-free pickup guitars: I know a guy that supposedly got hum from an ungrounded metal (Brass?) pickup mounting bracket I think the pickup screw mounts were plastic. Pickups themselves are quite long loops of wire, and certainly do pick up transformer hum if it’s not somehow canceled. My understanding is that the total wire surface area connected to + sends any uncanceled EMI to the preamp. It obviously won’t go to the preamp if it’s just connected to ground, like the pickup chassis and hardware. It’s not possible to isolate the hardware and signal grounds in a passive guitar circuit, and I don't think ground loops are possible without at least separated circuits, but eddy-currents from metal parts near pickups can increase hum. That may have actually been the case with the Brass mounting bracket. Maybe grounding it drew the eddy-currents to ground?