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Post by sumgai on Apr 4, 2007 18:10:03 GMT -5
Dan, Well, the confusion arises because the term 'ground' is used so often. In this scenario, it's more like saying "the common connection point". Several circuits can indeed share a common conductor that is holding the same voltage, relative to each of the separate circuits. In most cases, we are referring to DC voltages, and of course, 0 volts is the most easily attained common voltage. When it comes to AC, we also usually speak of 0 volts, but sometimes not. Witness the case of a bypass capacitor on the cathode of a preamp tube........ well, maybe another time, eh? The important part here is, the pickup circuit, carrying the signal we want, is separate from the radio circuit, which carries the signal we don't want. In both cases, we are striving for 0 volts AC and DC. However, in the case of a catastrophic failure in a tube amp, it's possible to momentarily apply a high DC voltage to the ground circuit. That's what the cap in series with the strings is trying to prevent, conduction of that DC into you. Bear in mind, this is all predicated on your being in physical contact with some other piece of equipment that is also well grounded. The chances of that are slim, but the insurance policy is cheap, ain't it? Yes, your arithmetic is correct. The downsides are cost and low density use of space within the cavity. But electrically, you'd be just as safe as a single unit of the specified value. HTH sumgai
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Post by ashcatlt on Apr 4, 2007 23:19:08 GMT -5
I've taken this thread way off into weird world. I am a genius, and thanks for noticing. I haven't much formal training or experience with this kind of thing, though. I also tend to only post on this forum after I've been drinking. I like my world.
The AC/DC stuff (I like them too, everything before that whole "Thunderstruck," "Money Talks" debacle) was kind of a brain fart. It is worth noting, however, that I don't think the cap in the QTB mod would have saved my friend from his near death experience.
The Chattanooga thing made sense to me, and was me answering my own question which nobody else took any notice of.
I think I'm digging what's going on here. It's generally easier for the guitar signal to go down the cable to the amp than it is to go through the bridge > your body > your shoes > earth. So the capacitor between the "signal ground" and the "bridge ground" can almost be considered to be unconnected on the bridge side. It's not a meaningful part of the audio circuit and won't affect the tone.
If you put the capacitor into the guitar cable, it is part of of the audio circuit and will affect the tone.
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Post by dd842 on Apr 5, 2007 8:52:43 GMT -5
ash, If you have taken this off on a tangent, it's a good thing you did ... at least as far as I am concerned. If you are digging this, and assuming (I am using that risky word deliberately) that I am also - then what you have done is provoked thought and therefore we get an opportunity to learn. Electronics is new to me. I find it interesting and I hope I begin to learn enough to have at least some rudementary understanding of it someday. By the way, if I did not address one of your questions I am sorry - I thought I did. It could be the way it was written .. ya know, wit' da booze and everything? ;D sumgai, In the meantime, having thought about your explanations overnight, (I think) I get it. It needs to muddle around my head a bit more before I can see if I can ‘splain it to myself, but it has made me think of another question. Most strats (...and, I believe, most electric guitars.) have a control cavity housing the pots and selector switch, and a separate cavity housing the output jack. However, I used to have a single humbucker guitar in which the tone pot, volume pot, and output jack were all housed together in the same cavity. When I shielded the guitar, I allowed the pots and pickups to contact the shield as instructed in QTB. I also allowed the output jack to contact the shield (note: in this case, the output jack was attached just like you would attach a pot by tightening a nut on a post that protrudes through a hole in the cavity ... kinda like this: www.epiphone.com/press/P_Jack.jpg), but now that I think about it, seeing as we are trying to keep the pickup side of the circuit and the ground side of the circuit isolated, technically, I am not so sure I should have done that. There was no ill effect or anything as far as I could tell - in fact it was stunningly silent after the shielding and the capacitor, in this case all I could find was a .47uf 400v capacitor, was installed - but still ... in order to avoid possibly affecting tone by introducing the second cap into the tone equation, shouldn't the output jack have been electrically isolated from the shield in such a setup (for example, slip a thin plastic collar or something between the jack and the shield)? Or does it no make’a no deefference? ... 'cause I am not sure how I avoided 1) a ground loop (if that is what would happen if the string ground and the shield ground co-mingle ), and 2) affecting the tone ... unless I did affect tone ... a .47uF cap + .22uF cap, in series, just happens to = .345uF? (Edit: While this may be true, I just realized that my "Early Onset Alzheimers" has caused me to forget a decimal point in the .022uF tone cap ... so much for that theory.) If nothing else, I am getting good at confusing myself ... Thanks, Dan
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Post by sumgai on Apr 5, 2007 13:09:30 GMT -5
Dan, You're not the only one! I just downed the last of my 222's, I got such a headache trying to decipher your message. It's the middle of the day right now (for me), let me digest this over the afternoon work break (that's where I look like I'm accomplishing things on the Honey-Do list, or IOW, I'm taking a break from where I'd rather be! ;D), and I'll be back this evening. Hopefully, with a cogent response. sumgai
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Post by dd842 on Apr 5, 2007 13:47:50 GMT -5
Thanks, sumgai
...and if you need clarification, just don't ask me ... as, having read my post, I'll be darned if I know what I am talking about either! Maybe I should become a politician?
Dan
P.S. Basically, is the output jack coming into contact with the shield going to cause either a ground loop or affect the tone.
Geez, that was easy! ... why didn't I just ask it like that before?
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Post by ashcatlt on Apr 5, 2007 23:14:47 GMT -5
i'm thinking it's actually better (or at least, not worse) from a shielding perspective to have the output jack touch the shield. that is, for noise and ground loop issues you should be better off this way. it would eliminate the need for the shielded wire to the output jack, among other things.
unfortunately, i think it eliminates the effectiveness of the DC blocking cap. that's the 400V cap that keeps you from dying. the whole point of that cap is to isolate the ground wire coming from your amp from the metal parts of your guitar - the pots, switches, bridge, strings, etc. from what i'm seeing, this does not include the jack itself. with the metal part of the jack contacting the shield, which then contacts all those other metal parts, you might as well not even have the cap.
edit - sorry, no i think it will cause a ground loop. i think (?) you'd be better off from the perspective of ground loops to put the ground lug of the pots connecting to the shield and cut the wires that take them to the ground lug on the jack. let the shield carry that side of the signal.
that said, it's my understanding that the ground loop issue inside a guitar's control cavity is a minor (almost non-) issue. it's really more of a "best electrical practice" thing than any real practical concern.
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Post by sumgai on Apr 6, 2007 2:36:29 GMT -5
ash, This is the correct answer, and a good analysis. Too bad you changed your mind with:
Bogus. Contradicts your above statements, which are spot on for both safety and best electrical practices.
Also a good analysis. The amount of problems relating to noise and/or signal quality is very small indeed, at these voltage levels. But once you get used to doing something the way that everyone else considers "a good habit to get into", it just makes sense to continue doing things that way, even if it's not strictly necessary.
sumgai
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Post by sumgai on Apr 6, 2007 3:16:08 GMT -5
Dan, Depends on what the wiring looks like. Ground loops are constructed, they don't appear out of thin air. If you pay attention to what you're doing, you won't create one, will you? That said, it's easy to "continue to march", and mount an output jack just like it's always been mounted, right on the metal control panel (in the case of Tele), or securely to the pickguard - which you just freshly covered with shielding - right down to cutting out the exact hole dimensions. Makes ya wanna sigh, don't it? OK, now that you've realized, after the fact, what you've done, you wanna correct it, right? Not so easy. If the shielding under a pickguard is carrying the ground portion of the pickup signal, you'll need to take the jack out, strip away enough shielding foil to completely miss the jack, and then reassemble. If you have a guitar where the output jack is mounted directly to a metal control plate, you can't do that, so the solution would be to use an insulated jack. These can be found cheaply enough, but they are usually plastic. Try to get your hands on a metal one, such as those made by Switchcraft - they come with special washers to do the insulation trick. The higher price will pay off in the long run, trust me on this. ;D In both cases, you'll need to run a length of wire from the ground lug of the jack to the central "star" ground lug that you installed for QtB. For the jack-metal-plate guitars, you'll also need to run a wire from the metal plate (a lug under the pot would be fine) over to the same side of the 0.33µf cap as you used for the bridge/strings. The wiring itself on these controls can't be grounded to the plate in any way, they must be connected directly to the star ground lug. That should fix any potential loop problems. sumgai Edit - corrected several references to the Tele style output jack, making it more generic.
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Post by dd842 on Apr 6, 2007 8:18:21 GMT -5
Thanks, guys
I figured the output jack contacts the shield it must create a connection electrically. I just wasn't sure why it still worked out so well.
By the way, the guitar I am talking about in this example, my first electric guitar, is a story in itself.
It was a cruddy little "Cruise" single humbucker model from the 80's that I bought on a total whim in an antique shop ... even after the guy demonstrating it said "I am not sure why it's humming like that - it must be something in the store - but I checked it all out and it's fine."
Yeah, right.
But the good news is I discovered this forum while fixing that fine instrument.
So I have two theories on why it still worked out:
1) I screwed up the shield where the output jack was attached in such a way that there was an interruption in electrical flow (for example, if the edge of aluminum tape is not folded over and it is just adhesive that is connecting one piece of aluminum to the other),
or
2) I got away with some "filtered bleeding" ... which I will now theorize and solicit correction if need be:
- The stock wiring had a ground from the vibrato to the volume, with no cap, so noise transmitted through me entered into the equation.
- The QTB mod took the noise from me and filtered it through the cap and should have dumped all of the filtered noise (aka "quiet") directly into the ground sleeve of the output jack.
- But, touching the shield, it allowed the noise from me to have another kick at the can because it could bleed back in via the shield.
- However, you guys say a loop is insignificant - so this bled amount of noise is less than when the vibrato was wired directly to the volume pot. And the noise that is being bled is now filtered noise (aka "quiet").
- Which is why, even after my screwup, the guitar was still fantastically quiet.
Do either of those theories hold any water?
I don't have that guitar anymore ... it was a little too girly lookin' for my taste. Like I said, on a whim.
But I know who I sold it to.
Should I track her down and fix the problem? I mean, is she in any danger, or is it a screwup that is fine if I got away with it?
Dan
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Post by sumgai on Apr 7, 2007 1:51:17 GMT -5
Dan,
We've got to have a talk here, buddy. Do you realize that ashcatlt makes more sense in his posts than you do, and you don't have the bottle to rely on as an excuse?! ;D
I don't pretend to know all of what you said, but the fact is, you are now much better equipped to properly wire up a QtB mod than you were way back then.
Find her, offer to "check it out" for free (of course), and double check your work. If you're in doubt, take pictures, post 'em, and ask for confirmation/suggestions.
Like the rest of us, she's in no more danger than we are. Unless she's performing outdoors, in swampy conditions, with a road crew that can't even spell electricity, let alone hook it up. Then yeah, she might in deep kimchee!
What the hey, post a picture of her too, with the guitar! ;D
sumgai
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Post by dd842 on Apr 7, 2007 10:50:00 GMT -5
Dan, We've got to have a talk here, buddy. Do you realize that ashcatlt makes more sense in his posts than you do, and you don't have the bottle to rely on as an excuse?! ;D LOL ... uhhh, yup! I've clearly seen that much, at least ... ;D I let myself fall into a catch 22, or, in your case, a 222, by attempting to discuss this. I'll put an ad on Craigslist and see if I can't track her down. Dan
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Post by sumgai on Apr 8, 2007 0:34:00 GMT -5
My name is NOT Major Major Major!
;D
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Post by dd842 on Apr 8, 2007 9:00:32 GMT -5
My name is NOT Major Major Major! Yeah, but the problem is that once a person finds out the real first and middle name, they die from the shock ;D
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Post by sumgai on Apr 8, 2007 10:51:06 GMT -5
Yes, it is a rather electrifying experience, I'll grant you that!
;D
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