Where do I connect the Bare wire in the QTB mod?

[whocares]

Hi, I'm new from the broad.

I have already done the QTB mod to my MIM Fender Standard Stratocaster half year ago.
Everything is going fine, and I'm quite satisfied with the mod.

Recently, I have bought a single size mini-humbuncker from Dimarzio.
It's a four conductors pickup. (Red, Green , Black and White with a Bare wire shielded with the green wire)

I have read the QTP, it says :
"For pickups which have a separate non-signal-carrying shield braid (hint, there must be at least two wires besides the braid), move the negative lead to the signal return ring but leave the non-signal-carrying shield soldered to the pot shell or other shield-ground point."

I have figured it out how to connect the wires except the bare wire. (Red to the hot, Black and White solder together and taped, Green to the signal return ring)

The question is:
Should I connect the bare wire to the pot shell instead of the signal return ring?
I have no idea if this bare wire carring signal or not.....all I know is bare and the green wires needed to be ground.

Thanks in advance.

[pete12345]

If you have four conductors PLUS the bare wire (i.e five in total) connect the bare wire to a pot shell. If you have four in total, the bare wire carries a signal and should go to the signal return ring.

Pete

P.S Welcome to the Nutzhouse!!

[whocares]

thanks Pete for your prompt reply.

Your reply is really helpful, thanks.

[jb12string]

At the risk of hijacking the thread with my first post, can anyone tell me where I can find the .33uf 400V metal film Capacitor to put on the ground? I found a polyester film one at allied electronics but its 2.20 plus 5.00 handling plus shipping, seems kinda silly to pay 6 times the price of the product to get it too me.

[D2o]

Jul 3, 2008 15:43:06 GMT -5 jb12string said:

At the risk of hijacking the thread with my first post, can anyone tell me where I can find the .33uf 400V metal film Capacitor to put on the ground? I found a polyester film one at allied electronics but its 2.20 plus 5.00 handling plus shipping, seems kinda silly to pay 6 times the price of the product to get it too me.

Hi jb, and Welcome to GN2!

Before I let you hijack this, I want to hijack you (I am assuming your username means you have a 12 string guitar?):

Any thoughts on tuning 12 strings? Should they be standard or tuned down an octave or so?

How do you like them apples?! - helping us out before we've even helped you out ...

Now, on to your question: you can try our links page to see what you can find ( guitarnuts2.proboards45.com/index.cgi?board=reference&action=display&thread=3185 ) ... but actually finding one of those caps has been a persistent problem. Kinda sucks to pay way more for shipping a cap than the cap, but what can you do sometimes? ...

Do you live in a metropolitan area where you may have an electronic surplus store? I don't know where you are located, but here is a place in Toronto you could look into: www.justradios.com/

Cheshirecat used to have some, but he hasn't been around in a while.

I think you can wire two .67uf 200V caps in series and get the equivalent of a .335uf 400V cap.

Sorry I can't be of any more help than that.

[ChrisK]

I think you can wire two .67uf 200V caps in series and get the equivalent of a .335uf 400V cap.

The capacitance will be roughly halved compared to just one capacitor (Ceff = C1 * C2/[C1 + C2]).

The voltage will not be equally divided in half.

The ratio of capacitance will partially determine the voltage division as will the ratio of leakages thru each capacitor. The only completely safe method is to use a 400 Volt rating for each capacitor. When capacitors with a less than full applied voltage rating are put in series, voltage balancing resistors are put in parallel with each. This tends to eliminate the isolation (and the point).

......but its 2.20 plus 5.00 handling plus shipping, seems kinda....

That's why there are minimum order quantities. It costs the vendor a certain minimum to process an order regardless of the actual value of the order.

If you're looking at Allied, look at mouser.com or digikey.com. If they have a charge for an order under their minimum, order enough stuff to meet the minimum. This is usually $25 or so. I find the latter two vendors to be less expensive.

Digikey has an excellent online search engine to find parts within their system.

While not labeled or named as guitar parts, digikey has a number of parts that are identical or considerable better that parts thus targeted. Most guitar industry parts are the cheapest available (I mean this in the most demeaning way). Most often, the vendor won't mention the manufacturer's name since the parts are of cheap quality or expensive price (and they certainly don't want you to find the actual cost for said parts).

Here is one cap for $0.57. Order 10 of them and be set for a while.
search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=495-1314-ND

Do a search in "capacitors" and "poly film" for 0.33 uF and 400 VDC, you'll see all these valued caps in their arsenal.
search.digikey.com/scripts/DkSearch/dksus.dll

They list over 20, of which 13 types are carried in stock.


I use them for many projects, including start-up technology companies. Warning: this web site contains (GASP!) actual data.

If you look for toggle switches, you'll find high quality ones.
If you look for rotary switches, you'll find high quality ones.

They do carry a few items.........although their SMT chip resistor selection is somewhat limited (only 182,000 items).

[D2o]

The voltage will not be equally divided in half.

I agree, but I was saying it will be doubled (which I think you are also alluding to), but at the bottom of sumgai's post, reply #30, here in ground loops ( guitarnuts2.proboards45.com/index.cgi?board=wiring&action=display&thread=1474&page=3 )

I asked:

P.S. Out of curiosity from a previous question, can two .67uF 200v caps be combined in series to effectively have one .33uF 400v cap ... i.e. do you get a 400v working load, or a 200v working load?

and received this reply:

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

Was he just being nice to me, or is there something to it?

[ChrisK]

No,

I AM NOT stating, alluding, or even inferring that the voltage rating will be doubled.

IT WILL NOT BE.

Most film capacitors are rated with a +/- 20% tolerance on the capacitance.

Assuming a 0.33 uF cap, one can be as little as 0.275 uF and the other can be as much as 0.396 uF.


Let's look at the voltage divider rule;

Vlower = Zlower/(Zlower + Zupper)

At 60 Hz, 0.275 uF has a reactance of 9,646 Ohms.

At 60 Hz, 0.396 uF has a reactance of 6,698 Ohms.

Per the voltage divider rule: 9,646 / (9,646 + 6,698) = 0.59

Therefore, the lower value of capacitance will have a voltage across it that is 0.59 times the applied voltage. In this case, the maximum value that can safely be assumed for this circuit is 200 VDC (the rating for each cap) divided by this value, or 200/0.59 = 339 VDC. This is solely for capacitance value variations.


Now, there will be a DC leakage current thru each capacitor that is generally related to the capacitance value. The larger the capacitance, the greater the value, which can be represented as a parallel resistance across each capacitor. The extent and effect of this is variable. In the case of quality capacitors, this effect will be dominated by the AC voltage divider effect as shown above.

[jb12string]

On a 12-string, the E,A,D,and G strings are all tuned to octaves, the B and E are unison.
Hijack#2, is there a way to recieve email notification of replies to threads?

[andywelsh]

Re hijack #2

Normal tuning has the higher octave doubles on the bass side whereas Rickenbackers traditionally put them on the treble side. Try both because there is a subtle difference.

Furthermore, I find it useful to tune down a tone or so (ie 2 frets) to help with the tension. Open tunings also sound fantastic.

HTH

Andy

[D2o]

Thanks, Chris

I think I understand the effect of tolerances now.

I have not used capacitors in series, for the simple reason that it would take up valuable cavity space. However, I didn't realize that there were tolerance and DC leakage issues ... although, now that you mention it, I wonder if the 400 VDC cap, on it's own, should not be closer to 500 VDC in order to ensure 400+ VDC in the case of a -20% tolerance?

Nevertheless, if I did need a less than ideal last resort like two caps in series, it would actually require the use of two 0.67 uF 339+ VDC caps (say 400 VDC on account of leakage?) to effectively replace one 0.33uF 400 VDC cap, correct?

[D2o]

Jul 4, 2008 8:30:54 GMT -5 andywelsh said:

Re hijack #2

Normal tuning

No, that was hijack #1.

Re: hijack #2 ... I don't believe there is a way to get email notification about new posts. I remember reading something about it, but I don't recall there being a solution.

Oh well, you'll just have to check in often and hang out with us then

[ashcatlt]

Actually, that's hijack #3.

#1 is the question of the capacitor. I think they'll both fail if neither one is rated high enough to handle the unwanted voltage.

#2 was trying to recruit some discussion for this thread.

#3 is the email thing.

[D2o]

Doh!

;D

That'll teach us ... maybe.

I remember that we debated whether the caps in series would or wouldn't work in the same link I gave before. Come to think of it "we", you and I, debated that ...

It would seem like caps, in series, would blow in series as each one is overwhelmed by voltage, but I don't think that is actually the case.

To use an analogy, how I understand caps in series to work, in terms of voltage load, is not so much as a "chain", where it is as strong as the weakest link, but more of a "braid", where each additional "strand" (cap) adds to the overall ability to handle a load. And I haven't even been drinking yet.

Anyway, I haven't tried it, nor do I really know how? However it would be tested, I would want to be doing it with smaller VDC ratings, otherwise, well ...

[glow=red,2,300]y'know.[/glow]

I wouldn't want to make like Cheech and Chong and go up in smoke, after all.

[sumgai]

Hey, hey, let's not be talking like I'm no longer here or anything, K? And while I'm at it, just for the record, I not only make nice, I truly am a nice guy - just ask my cat!

DD, what Chris gave you was the Engineer's (and therefore correct) viewpoint. I kept it simple for two reasons: 1) no need to get all high-falutin, there ain't no profit in that picture; and 2) this particular application (the isolation against a very short duration surge in current) does allow a series connection of caps because they are intended to fail.

If the cap opens up, due to a surge, then it can't carry current, right? Afterwards, there's no longer any "grounding" effect of touching the strings to quiet the buzzing, but at least, you're safe, and that's the bottom line.

Yes, a capacitor does sometimes short, and that would be bad, but usually, that occurs under a heavy load, not so often just a short surge. So isn't a short circuit a heavy load? Yes, but the real question is, how long has it been in existance? A heavy load is normally defined as a steady drain, not a quick impulse. A short circuit is "heavy" only for a very short time, about 1/60th of a second (in a power-supply circuit) - just long enough for the dialectric material to melt away, but not long enough for the burnt material to fuse itself back together.

Of course, exceptions always aboond, that's why we don't say "hey, this is the perfect solution!". Or IOW, YM just MV.

HTH




sumgai

[ChrisK]

I think I understand the effect of tolerances now.

Not at all.

However, I didn't realize that there were tolerance and DC leakage issues ... although, now that you mention it, I wonder if the 400 VDC cap, on it's own, should not be closer to 500 VDC in order to ensure 400+ VDC in the case of a -20% tolerance?

Uh, no, you've missed the point.

A voltage divider is a voltage divider. It requires at least two components. One component is not a divider. A 400 VDC capacitor, used by itself is a 400 VDC capacitor regardless of its value and tolerance thereof.

The tolerance in capacitance value, which goes directly to the impedance (reactance) of each capacitor, effects an asymmetrical voltage divider with the cap with the lowest capacitance (highest reactance) having the larger voltage drop across it.

The only safe course of action is to use capacitors with a voltage rating equal to the maximum applied across the entire circuit.

Nevertheless, if I did need a less than ideal last resort like two caps in series, it would actually require the use of two 0.67 uF 339+ VDC caps (say 400 VDC on account of leakage?) to effectively replace one 0.33uF 400 VDC cap, correct?

Yes, one could replace a single 0.33 uF 400 VDC cap with two 0.66 uF 400 VDC caps in series.

It would seem like caps, in series, would blow in series as each one is overwhelmed by voltage, but I don't think that is actually the case.

Although the use of two safety caps in series with less than the full applied voltage for each does blow, caps tend to short. If they opened, they would just evaporate and no longer be of concern (or coupling). If one shorts, and a higher than rated voltage is applied across the others, they will then likely short also.

Eventually (or quickly depending on one's perception and tolerance for high potential rudely applied to one's body), the applied voltage will be fully applied to the operator (guitarist), causing a very realistic Pete Townsend-like impression, with arms flailing, eyes glowing, and effervescent leaping into the lower atmosphere.

Depending on the response time of one's band-mates (generally related to the ability of folk to stop laughing), said chimeric display will possibly (depending on the severity and intensity of bassist/drummer jokes previously dispensed) be fortunately truncated.

To use an analogy, how I understand caps in series to work, in terms of voltage load, is not so much as a "chain", where it is as strong as the weakest link, but more of a "braid", where each additional "strand" (cap) adds to the overall ability to handle a load. And I haven't even been drinking yet.

No, not at all. It's a chain as weak as its weakest link. Just don't ever use safety capacitors in series.

Anyway, I haven't tried it, nor do I really know how?

Whew!


This is a safety capacitor. It is used to help save your life.

You may have noticed that there isn't a market for discount parachutes nor prophylactics either (just tie them together, you'll be fine).


Putting two inadequate capacitors in series is the same as stacking a step stool onto a swivel chair for lack of a properly sized step ladder. Gravity sucks.


JUST SAY NO.

[jb12string]

Can the star grounding be done temporarily w/o the capacitor? What effect will not having the capacitor have on hum?

[jb12string]

Ok, didn't see the second page so let me explain something, I've used this guitar and another guitar into the same amp, on the same outlet in the same building w/o incident several times in the past, will omitting the cap until it arrives hamper me in some way?

[ashcatlt]

Leaving out the safety cap should actually improve the noise-reduction capacity of the shielding, in that more of the frequency content of the noise signal will conduct to ground.

I don't think there has ever been a documented case of anyone dying from the particular rare instance which this cap is meant to avert. I'd think it's safe to say that, in the history of amplified guitar music, the number of guitars actually fitted with this safety cap mod are a minority so small as to be insignificant.

Course, you don't want to be the first...

[D2o]

Thanks, sumgai

I didn't mean to speak of you in the third person kind of way, it's just that it was in a different thread is all. "We didont mean to rrrough you up, squeaky" (line from Aristocats)

So, I still have the basic impression that, under the right circumstances, there is some matematical merit to this otherwise meritless method ... but it's best left as a last resort, and a temporary one at that.

But, you are right, that Chris is right (who would have known, eh?) so I won't mention caps in series again.

Chris,

Jul 4, 2008 18:33:16 GMT -5 ChrisK said:

I think I understand the effect of tolerances now.

Not at all.

My statement that I think I understand was in relation to the need for two .67 uF 400 VDC caps is series ... but you would certainly never know it from the layout of my previous post.

What "happened" was as I was logging out a passing thought about the -20 500 VDC thing kinda pooped into my head and I threw it in there without thinking about it.

I just didn't realize at the time how much I didn't think about it.

Thanks for catching it and setting me straight.

[ChrisK]

I actually detest these types of posts where something is less than well understood (not everyone a'planet is an EE) and I have to correct things that MUST be well understood in order to be done, uh, well.

I tend to wax strenuous because I'm concerned that someone will advise or do something that is dangerous, until I realize that safety is THE only real issue and that no one should further "help" things in the particular situation or application.


That being said, if one looks at the power supply in this schematic (region A4 - rotate it right),
www.fender.com/support/amp_schematics/pdfs/65_Deluxe_Reverb_Schematic.pdf
one sees that C31 (a 220 uF 100 VDC cap) and C32 (a 47 uF 500 VDC cap) are in series. Ignoring the tolerance issue for the moment, for the 396 VDC B+ rail (dang, that's precise), one would expect that there will be 70 VDC across the 220 uF cap and 326 VDC across the 47 Uf cap. So far, so good since both are fairly well within their ratings.

But, since these are electrolytic caps, with the usual schlocky tolerance and significant leakage, balancing resistors are added to help keep things civil. The resistors will result in there being 69 and 327 VDC across the caps. Dang, this is pretty much the same as what the capacitive reactance will cause.

Don't get too smug, the caps have that tolerance issue and so do the resistors as well.

But, it keeps things civil.


BTW, the effective nominal capacitance of this series circuit is 38.7 uF.

And, we haven't touched on the question as to why just a single 500 VDC cap isn't used. It might have to do with the peak volatages seen here, and the higher excursions that occur since this is a "saggy" tube rectifier circuit supplying a class AB output stage.

Oh yeah, and all of that combined tolerance stuff everywhere.