Quick question: wiring/shielding using QTB mod

[zedsnotdead]

Hello again everyone!

Following my previous "ask-for-help" post (guitarnuts2.proboards.com/index.cgi?board=wiring&action=display&thread=4930&page=1) about out-of-phase pickups, i thought: Hell, im going to shield the damn thing and re-wire all of it!

So, just a couple of questions:

1- Bridge HB pickup has a bare conductor. Also the braid coming from the jack cable. Where are those going? To the star-ground or Shield-ground? Sorry i couldn't figure it out, i'm confused.. And if i don't use the protection cap (.33 400v), can i solder it to any of the grounds?

2- Is it ok to use a FTP cable conductor to make the wirings? I have lots of it here, and they are pretty thin and rigid. And they are all coloured, so it's easy not to mess with the wiring. Is that ok?

Thanks a lot!

PS: think i'm going regret this... Murphy's law never leaves me, is always present on a day-to-day basis....

[newey]

Zed said;

think i'm going regret this... Murphy's law never leaves me, is always present on a day-to-day basis....

Yeah, I foreclosed on Murphy but the SOB still won't leave. ;D

1- Bridge HB pickup has a bare conductor. Also the braid coming from the jack cable. Where are those going? To the star-ground or Shield-ground?

Do you mean it has two conductors, one bare, plus a braided shield? Or just a bare conductor, and the braided shield is coming from elsewhere.

Before we can say where things go, we must know how this is being wired- I suspect you said how in the other thread, but I'll have to go back and look.

[zedsnotdead]

Hi!

Its a Dimarzio HB, it has 4 wires + 1 bare (naked i mean, no isolation, not braid)

[newey]

Ok, the bare wire will always go to ground. If you are using star grounding, it goes to the star grounding point; if not, it goes to the back of one of the pots, whichever one is used to collect the grounds.

As far as the FTP cable, as long as it's big enough to solder to without difficulty, and thin enough to be routed easily, it should be fine. I use 22 or 24 AWG wire. Anything larger than 22 becomes tough to work with.

If you omit the protection cap, yes, you would just solder to the grounding point instead.

[zedsnotdead]

Well, the FTP cable is something like this:



It's very thin and rigid. im hoping to use the conductors to connect things up. The colors will help a bit...

[ashcatlt]

I started to type a reply at work today, but then I actually had to work.

I use the wires out of recycled MIDI cables, which are probably about the same size. Mine are all close to 20 yrs old to begin with. They will break if messed with too much in the soldering/desoldering stage, but I haven't had any fail yet once installed.

If we're star-grounding then we're star-grounding. All "grounds" should collect at one point. Shield grounds, "signal returns" they all end up at one place ultimately, and that's the amp chassis via the sleeve lug of the output jack. This is not always the most convenient place to run all those wires to, though. Some folks chose to stop off at the back off the Volume pot along the way. In QTB, we all meet at an isolated ring terminal before jumping on the bus to the chassis.

This is true even when we intend to install the "safety cap". The only difference here is that we want all the "ground ends" of the touchable stuff (strings, pots, switches, but we can't include the jack/jackplate usually) to have to pass through a large capacitor along the way. The cap makes it so that touching the strings is not the same as touching the amp's faceplate which might be carrying a whole lot of DC voltage*.

We don't want any signal returns to run through the cap, because we might end up losing some of the extreme low frequencies coming out of the guitar.


The bare wire on the HB is some attempt at a shield ground. More likely than not, it has nothing to do with the signal path. It is also quite likely connected (conductively) to something you can touch. It wants to be on the touchable side of the cap.

If you're using the 2 conductor + shield cable as specified in QTB, the shield braid must only connect at one end.

If connected at the jack end, it is connected to the amplifier, and wants to be insulated to avoid contact with touchable stuff. Otherwise, it's not connected at the jack end, and can go to star-ground on either side of the cap. Technically, I think you get better noise rejection if the shield doesn't go through the cap. I'll buy you a drink if you can hear the difference on this little run, though.


If there's no safety cap, they all go the same place. Try to avoid redundancy along the way.


*If a bunch of stuff goes wrong that day, and you spill a beer or two, that faceplate might end up carrying a whole lot of DC voltage. There's a sticky at the top of this forum where we've discussed this, and what we really think about the pros/cons of this capacitor.

[zedsnotdead]

Just bought a cap, but it's a bit different than the one in QTB instructions.

It's a 47uF/430V and it seems like a "cilinder". Like this one:



Is this cap ok?

[ashcatlt]

47uF is ~150 times the prescribed capacitance. It'll block DC just as well as the other, but might not do so well in "protection" verses a 60 Hz AC fault. It will (theoretically) provide better noise response in that more lower frequencies will be "dumped" to ground.

[zedsnotdead]

150x ? i thought it was just a bit higher than the 33uF described in QTB...
This one's 47uF...

[ashcatlt]

I think you're missing a decimal point.

33 uF = 100 x 0.33 uF
47 uF =~ 1.5 x 33uF
47 uF =~ 150 x .33uF

[zedsnotdead]

sh*t....

I really missed that!

But there's no problem at all using this cap, is there?

[newey]

As Ash said, it

might not do so well in "protection" verses a 60 Hz AC fault.

Since the sole purpose of using this cap is the (theoretical) safety advantage, and since, as we discussed in the link about the use of this cap, those benefits may not in fact make much difference to your particular case, I don't think it makes a whole lot of sense to do it without using the specified cap.

I mean, if you're going to the trouble to install it, why take the chance that the higher-value cap may not give as much protection?

Now, know too that the value quoted in the original QTB mod was a logical "guesstimate" at .33µf and 400V. ChrisK later recommended going to 600V, in light of the actual voltages which could theoretically be produced in a ground fault situation. But none of this is written in stone, because to our knowledge, this "safety" cap feature has never been empirically tested. We know of no real-world case where someone avoided a nasty shock due to having the cap.

So, yours might work just as well, or not, no one can really say. As Ash noted, it ought to block DC just as well, but there's also a time factor involved. The cap is supposed to prevent shock until the mains breaker trips, which should happen fairly quickly- but there is a time lag, and the cap has to be able to cover that delay.

Understand that the cap has nothing to do with the efficacy of your QTB shielding, it's strictly there for the added safety factor, which may well be a marginal improvement anyway.

[ashcatlt]

Understand that the cap has nothing to do with the efficacy of your QTB shielding, it's strictly there for the added safety factor, which may well be a marginal improvement anyway.

Hate to quibble, but this is not technically true. As I sort of mentioned above the cap acts as a sort of high-pass filter on the noise which is shunted to ground. The shielding is not effective for those frequencies which do not pass this cap.

This was part of the problem with implementing the safety cap into QTB to begin with. If we want to protect against the (far more likely, but less likely to be deadly) AC fault, we need the cap to block (or reduce) 60 Hz to some significant extent. Unfortunately, this is where we have some of the biggest trouble when it comes to noise in our guitars.

[JohnH]

There seems no point in putting in that large cap, even the 0.33 only provides marginal protection. No production guitars have these caps, and safety is only really ensured with properly maintained amps and earthed mains supply. If you omit the cap, the QTB shielding is still valid, the cap would be replaced by a wire link.

[zedsnotdead]

Just finished wiring the guitar.

i did the QTB without the safety cap, shielding it with aluminium foil (not copper).
Also corrected the "out-of-phase" problem (that i posted here guitarnuts2.proboards.com/index.cgi?board=wiring&action=display&thread=4930&page=1)

And, guys... it has a NEW LIFE of it's own!!!!!!!!!!!!!!!

So quiet, shuggy sounding, the harmonics and sustain goes and goes...god damn!!!!

Again, THANK YOU SO MUCH for your help!