Help appreciated

[john50]

Hi everyone - a newbie here needing help with quieting the beast problems. (i think I may have posted this already because I pressed return for a new paragraph and it vanished)

Anyway, did all the stuff - no knowledge of electronics but can solder and follow fender's wiring diagrams without problems. The issue is,
Everytime I screw down the scratch plate on the switch side or front of the jackplate, the guitar goes dead. This has happened on 2 guitars.

Can anyone suggest what may be wrong? Should the jack cavity be shielded (the photo on the website hasn't - but I did)?

I haven't a multimeter and haven't done the 400v cap mod because I can't locate a ring to go around the potshaft in the UK and we have a different mains voltage so the spec may not be correct but should I NOT use star washers against the scratchplate shielding on my pots and push the body of the pot right up against the scratchplate?

Thanks in advance for any help.
John

[zamzara]

john50 said:

Hi everyone - a newbie here needing help with quieting the beast problems. (i think I may have posted this already because I pressed return for a new paragraph and it vanished)

Anyway, did all the stuff - no knowledge of electronics but can solder and follow fender's wiring diagrams without problems. The issue is,
Everytime I screw down the scratch plate on the switch side or front of the jackplate, the guitar goes dead. This has happened on 2 guitars.

Can anyone suggest what may be wrong? Should the jack cavity be shielded (the photo on the website hasn't - but I did)?

I haven't a multimeter and haven't done the 400v cap mod because I can't locate a ring to go around the potshaft in the UK and we have a different mains voltage so the spec may not be correct but should I NOT use star washers against the scratchplate shielding on my pots and push the body of the pot right up against the scratchplate?

Thanks in advance for any help.
John

Hi, welcome to the site. It's almost certain that the sound is going dead because part of the circuit is being shorted against the shielding. I use sellotape to insulate parts of the shielding that will be close to switch terminals, and also between pot lugs and the shielded pickguard. If you used aluminium foil make sure it's smoothed down flat. You should also insulate any parts of the hot side on the jack socket itself that could contact shielding.

The 400v cap is suitable for any country's mains system as it will work for any voltage up to 400v. A cap rated 250v will also work in the US or UK. I'm not quite sure what you mean about star washers, but if you aren't using a washer as the ground then it's ok to solder all the grounds to the back of one of the pot shells, making sure that the pot shell is grounded against the pickguard.

[ChrisK]

The 400v cap is suitable for any country's mains system as it will work for any voltage up to 400v. A cap rated 250v will also work in the US or UK.

Uh, NO.

A 120 VAC mains will have nominal peak voltages of 170 VDC (120VAC [an R.M.S. rating] times 1.414) and 195+ VDC under high line conditions.

A 240 VAC mains will have nominal peak voltages of 340 VDC and 390+ VDC under high line conditions.

Per VDE specifications, for blocking peak mains voltage, for 120 VAC mains 400 VDC is stipulated and for 240 VAC mains 600 VDC is stipulated.

These ratings are needed for protection against these conditions as well as an additional 200 VDC of headroom for mains-borne transients and such.


This isn't low voltage, don't experiment, IT WILL KILL YOU!

There isn't much of a (repeat) market for discount parachutes (or condoms).

Safety first, safety last, it's your life!

[zamzara]

Well, I got that information from this page:



for what it's worth.

[sumgai]

zamzara,

Well, I got that information from this page:



for what it's worth.

Unfortunately, I see no reference on that page to mains voltages other than the US variety.

But the whole topic is muddied quite a bit, IMNSHO, by that page. Let's backtrack a bit here, shall we?

First, we need to remember that JohnA inserted that cap into the string-ground circuit to protect against a possible DC fault, not an AC fault. It's correct to assume that an amp's power supply will deliver the same B+ voltage regardless of the mains used in any given country. But at the same time, if a problem occurs by virtue of polarity (different outlet circuits), or a faulty mains circuit in the first place, then AC may also appear at the guitar's ground. It's important to remember that JohnA's capacitor is not designed to protect fully against this scenario.

In point of fact, John makes two mistakes about this cap. The level of seriousness is up to you, the reader, but I bring them to light for your safety, not to make anyone look bad.

First, caps are rated for a certain maximum voltage for a reason. But look here:

A 250V capacitor is also acceptable as it will probably stand up to 300V or so for at least several seconds.

(Towards the bottom, in the section "Why a 400v cap?")

To think that one could safely exceed a cap's maximum voltage rating is foolhardy, to be polite about it. Why John would suggest such a thing is beyond me. I don't recommend this practice to anyone, particularly not where safety of life and limb is concerned.


JohnA also says:

This will limit current on a 120V, 60hz system to a maximum of about 30ma – certainly enough to get your attention but less likely to be lethal for most people

(The last item in the Bill of Materials, about a third of the way down the page, and in smaller text.)

Again, I'm not gonna be the guinea pig to test that theory!!!

Listen, here's the short and sweet of it: As far as your body is concerned, there are three kinds of current: Perception, Let-Go, and Lethal. They break down like so:

Perception current - 0.5mA.

Let-Go current - 16mA or less of men, 10mA or less for women.

Lethal current - anything greater than the above.

Yeah, human bodies vary in their thresholds and senstivities, yadda yadda yadda. But the bottom line is, if you assume that you can't survive the tiniest current, and you take steps to avoid shocks like this, then you're more likely to live a long life. To do otherwise is like telling your buddies "Here, one y'all hold my beer, an' watch this."

I rest my case.


sumgai

[JohnH]

Good advice Sumgai. Although I haven't installed such caps, if I did, I would prefer the version in his S-tastic design, where it is not directly in the signal path, but purely acting as the string ground. He notes that that allows the cap value to be reduced from 0.33 to 0.1 and still be effective. Doing that would reduce the max shock and also probably make it easier to find one of a suitable voltage rating.

John

[ChrisK]

Something (or someone) is always the "fuse".

It ain't gonna be me!

While amps of the same model should generally have the same DC plate voltages independent of mains supply voltage (assuming that one IS using the proper version), the issue ISN'T only that of plate voltage appearing on the chassis ground due to an internal amp failure. (And some higher power tube amps have plate voltages well above 400 VDC.)

It's also a case of mis-wired venues (not that uncommon) where the chassis IS at mains potential. Actually, these are more likely that the prior fault condition, and generally out of the musician's control.

Especially in some "locals" where the concept of a safety ground wire is relatively new (uh, you know, them there 240 VAC 2 wire vintage plug places).

(You DO test each and every mains outlet that you plug into first with a wiring fault detector, DON'T YOU?)

Fuses are, and death IS.

[sumgai]

John,

Sometime ago we had a discussion on the value of this particular cap. As I recall, we ended up more or less agreeing that 0.33µf was a good compromise value. Too much smaller, and the protection against a full onslaught was reduced to unusable - the cap would charge too quickly, and you'd still get zapped. Too high a capacitance value, and you'd be inserting a too-large resistance (capacitive reactance) into the circuit between you and the ground - you'd never be able to fully quiet the buzz with your body.

~~~~~~~~~~~~~

Chris,

Yes, the improper mains problem is much more prevalent and problematic, that's a given. But this particular cap won't prevent anything of that nature from harming you. Whether we speak of RMS or peak values, the fact is, if you hit that cap with AC, it will just keep charging and discharging, and more than quickly enough to give you a good zapping, regardless of its value.

If I were to follow your school of thought about safety (and I surely would, if'n I could), I'd simply let RF stand as my best insulator against any problems at all! ;D (For those who are new to the program, I also use a Roland 13-pin setup (with Graphtech's Ghost system), and there's no chance in Hades that any manufacturer is gonna make a wireless unit with enough individual signal paths, all in one unit. Makes me weep, I tell ya, purely weep. )

HTH



sumgai

[JohnH]

Sumgai - I was refering to this, from the S-tastic page:



"Also note (from the accompanying photos) that I used this modification to test a slightly different approach to the problem of isolating the strings. Instead of using a very large capacitor between signal ground and all of the shielding, I used a fairly small (0.1uf) cap between the cavity shielding and the string ground. The smaller capacitor provides a little more protection from AC shock (see the article on shock hazards). The smaller capacitor is also physically smaller, making it easier to position in a tight body cavity, and more readily available. This approach does have a drawback, though – if metal knobs are used they may not be isolated. As far as noise rejection is concerned, this scheme worked at least as well as isolating the entire shield through an 0.33uf capacitor."

John

[sumgai]

John,

Note that, hidden within the prose, JohnA has committed a technical foul - he's violated the star ground principle he himself formulated. As I read it, the string ground goes to the cavity shield, and that goes to signal ground. In essence, the cavity shield is now the carrier for the string ground, and that's not a good thing, IMO. Unless JohnA were to refute the star ground methodology........

Short story: saving on capacitance is not the place to take a shortcut when protecting life and limb! Although I know you, you're gonna set up some kind of experiment that'll either prove or disprove the star ground business once and for all. ;D Good luck, compadre!


sumgai

[JohnH]

Nah!! being able to have a smaller capacitance between yourself and a good zap is safer than a larger one. Less ac gets through to you, or if its dc, the pulse is shorter until it gets blocked by the capacitiance.

John

[sumgai]

John,

Nah!! being able to have a smaller capacitance between yourself and a good zap is safer than a larger one. Less ac gets through to you, or if its dc, the pulse is shorter until it gets blocked by the capacitiance.

Technically speaking, that "less AC that gets through to you" is due to frequency response - the lower the cap value, the higher the frequency needs to be in order to pass through. But that also applies to the 50/60Hz hum that we're trying to knock out. Go too low in value, and the cap is worthless as far as hum/buzz/noise is concerned. Go too large, and the safety factor becomes non-existant. That's why we came back to the 0.33µf figure as good compromise.



sumgai

[stratik]

Hello:

The cap I'm installing is the one in the S-Tastic design and acting as a string ground. I must admit, I'm a bit confused now as to exactly what cap I should be using at all. In the QtB site, where we all started-out on this journey, we can read this:

"The .33uf designation is not critical. You could easily get away with a 0.47 or 0.22uf capacitor. The larger the value the better, up to a point (except that larger values will provide less protection from AC shock). If your control cavity is tighter than normal, you might want to go with a 0.1uf 400V because it will be physically smaller. I wouldn't go much lower than that though or you will start suffering noise"

In light of this I went out and bought a 0.1uF 500V cap. I mentioned this cap in an earlier thread(?) I posted and I was advised (I think) that I would've been better with a higher value. I then got hold of a 0.22uF 450V cap and I was happy 'cos I was about to finally finish my project. I then read the following:

"A 240 VAC mains will have nominal peak voltages of 340 VDC and 390+ VDC under high line conditions. Per VDE specifications, for blocking peak mains voltage, for 120 VAC mains 400 VDC is stipulated and for 240 VAC mains 600 VDC is stipulated."...

I'm from the UK and I'm sure there's a fair amount of us who read the forum who'll feel that we're now wasting our time unless we install a 600V cap. JohnH then goes on to say:

"Although I haven't installed such caps, if I did, I would prefer the version in his S-tastic design, where it is not directly in the signal path, but purely acting as the string ground. He notes that that allows the cap value to be reduced from 0.33 to 0.1 and still be effective."

That takes me back to my first line... Then Sumgai,

"Sometime ago we had a discussion on the value of this particular cap. As I recall, we ended up more or less agreeing that 0.33µf was a good compromise value. Too much smaller, and the protection against a full onslaught was reduced to unusable - the cap would charge too quickly, and you'd still get zapped. Too high a capacitance value, and you'd be inserting a too-large resistance (capacitive reactance) into the circuit between you and the ground - you'd never be able to fully quiet the buzz with your body."

I'm very eager to finish work on my guitar - I'm at the last hurdle - but unfortunately I don't know the first thing about electricity or 'capacitance reactance'. I rely heavily on what I read in the forum but I still see much to-ing and fro-ing and debate over this one item. Each time I go there I feel like I'm reading something different. There were recently 85 users online and I reckon as more and more folk come to GN2, more will be upgrading their guitars. I'd be delighted if all of the experts could agree about this cap, then, whenever the question is lifted, we're referred to a 'before you query' type board.

I know the site is originally aimed at the US but should we in the UK now buy a 600V cap?

This is the only forum I visit. Many thanks for any help.

[sumgai]

stratik,

Short answer: use anything from 0.1 to 0.47µf, it'll be good. If physical size is a consideration, then go smaller, you'll be safe enough - even in the UK.

Without re-hashing the entire thread, the synopsis is, this cap was never intended to prevent AC from getting through the faulty circuit, and into/through you - it's sole purpose is to block DC, should a faulty tube amp (with high voltages inside) go bad at just the wrong moment.

A side effect is the reduction of AC in some small amount, so all that's at question now is "how much is that small amount of reduction, and is the potential hazard reduced to a safe level?" I say the reduction is not enough, and that if you depend on the cap to make it safe, then your life insurance had best be paid up.

Others here may defend the cap's ability to reduce AC voltages by a significant margin, but I stand by Chris's First Rule Of Safety - someone is always the fuse, and it ain't gonna be me! For protection against AC problems, use what the engineers designed and approved for the job, a Ground Fault Circuit Interrupt device! (If I'm not mistaken, in the UK it's called a Residual Current Interrupt device.)

Bottom line for you: Chris is always right¹, but sometimes he's too right. We don't need, or want, to worry about AC when we're dealing with this cap. 'Nuff said.

HTH


sumgai

1. Except when he's wrong, in which case, see Rule #1.

[stratik]

Thank you, Sumgai for the additional information - that will be the 346,000th time y'all have covered this topic.

Now, 'The more I learn the less I know'..
I just read this:

"JohnA has committed a technical foul - he's violated the star ground principle he himself formulated. As I read it, the string ground goes to the cavity shield, and that goes to signal ground. In essence, the cavity shield is now the carrier for the string ground, and that's not a good thing, IMO"...

Q. Are you referring here to the S-tastic version, which is where I planned to locate my cap?.. meaning we shouldn't have the cap between the string ground and the cavity shield?
As I was going to put the cap there, should I re-think this and have my cap in the original location, i.e. between one of the pots and the Star ground?
So, let me put this final question, once and for all... In your opinion, should I locate a 0.22uF 400V between one of the pots and Star Ground, then just install the bridge wire to the bottom of the copper control cavity???

If you give me the definitive reply, I promise (kinda) never to darken your doorstep again with this.

Cheers!

stratik

[sumgai]

stratik,

Thank you, Sumgai for the additional information - that will be the 346,000th time y'all have covered this topic.

Now, 'The more I learn the less I know'..
I just read this:

"JohnA has committed a technical foul - he's violated the star ground principle he himself formulated. As I read it, the string ground goes to the cavity shield, and that goes to signal ground. In essence, the cavity shield is now the carrier for the string ground, and that's not a good thing, IMO"...

Q. Are you referring here to the S-tastic version, which is where I planned to locate my cap?

Yes, that was paraphased from JohnH, who was quoting from the S-tastic page.

So, let me put this final question, once and for all... In your opinion, should I locate a 0.22uF 400V between one of the pots and Star Ground, then just install the bridge wire to the bottom of the copper control cavity?

You can do that, and it'll work OK. But the idea behind the star ground in the first place was to reduce the number of possible paths for receiving interference, and to reduce the length of any necessary path to its shortest possible length.

Let's view this another way........ What you express above is to run the bridge wire through the copper shielding, to the cap, then to the star ground - that's serial, in my book. What I'm saying is to run the bridge wire and the shielding both to the cap, then to the star ground. That's putting the bridge wire and the shielding in parallel, no?

Is it possible that one offers better noise reduction over the other? I have no idea, but than again, the star ground principal is based on several parallel connections, and eschews any serial connections. That makes it your call, which way you go.


.................


Upon reflection, I see now that I have been dancing with one foot on each side of a fence. Above, it might look like I'm arguing that using the star grounding system is a point of safety. No, it's not, and I'm sorry if I gave that impression. Indeed, the star ground system is not for safety, it's for noise reduction. The cap is for safety, and so long as all user-accessible metal parts are protected by this cap, then I'm OK with whatever comes after that point in the wiring scheme, star ground or otherwise.

After all that blathering, let me summarize:

1. The cap's value should be between 0.1 and 0.47µf, and it's voltage rating must be at least 400 vDC to be of any good to you in that magic moment when things go south. (You may see "wvDC", that means 'working volts DC'. It's the same thing, just an older designation.)

2. The cap does not protect you in any appreciable way against an AC fault, whether it be directly from the mains supply or from inside the amp. Use a GFCI (RCI) device instead!

3. Star grounding is for reducing noise, not for safety. You can implement a partial star ground, or none at all, only the noise level will be at risk. Noise is not a safety hazard! (Unless you are easily irritated, and you start throwing things under these conditions. )

4. I am entirely too locquacious! ;D

HTH


sumgai

[stratik]

Hello again:

Rpt. Chorus... ;D

Thank you for your detailed reply, Sumgai - I see more clearly now the purpose of the cap and the Star Grounding, however, maybe I related badly where/how I plan to put the whole thing together.. Yes, I'm about to darken your doorstep again..

You said:

"What you express above is to run the bridge wire through the copper shielding, to the cap, then to the star ground"..

Let me know if this is the same thing:
I would take the bridge wire, solder one end to the 400V cap, solder the other end of the cap to a small ring terminal then fix the small ring terminal to the cavity floor with a wood screw, taping it up with insulating tape to prevent shorting.

... Now, this is what I've already completed, before the above:
I've fitted a large ring terminal over the vol pot then crimped and soldered a small piece of #20 wire to the end. At the other end of the #20 wire I've connected a smaller ring terminal - this smaller ring terminal is where all my ground wires are soldered onto. Will the whole thing work/work efficiently? Was Frankenstein's monster ever better organised? Would you prefer Frankenstein's monster to darken your doorstep next time?...

Please feel free to be loquacious... ;D

Stratik

[sumgai]

stratik,

Keeping it short, I've gotta run.......

What you are proposing will work, noise-wise. I don't think it's 100% safe, though.

Placing the cap between the bridge and the cavity shielding is protecting only the bridge/strings. There are other metal parts above the pickguard that the user/player might be touching when the amplifier goes ker-blooey^® (ChrisK, 2005). That's the only reason I'm on my high-horse about all this.

I suggest, gently , that you stop a moment, organize your thoughts, and determine where and how you use your instrument, what points of metal contact are possible, and try to make sure they are on the correct side of the capacitor. Doing so in your mind is good, doing so on paper is even better. Then transfer from the paper to your guitar, and all should be as safe as you can possibly make it.

There's another way to look at this......... if you draw everything out like I suggested, then you should notice that the only parts of the guitar wiring that are not protected by the cap are the pickup leads themselves. (!) You don't want the cap in the pickup signal path (not even the ground portion), that would be bad for the tone. You do want the cap protecting everything else that goes to ground. Overkill is, but safe is never enough. (Hope I don't get beaned for stretching the brevity mark on that one. )

HTH


sumgai

[stratik]

Sumgai:

Thank you, once more for taking the time to explain things.

I've been 'onstage' now for over 30 years and would like to think that I'll be doing it for another 30, incident-free. As the whole point of the QtB site is to do just that - improve the noise ratio/tone of my gtr, I'm thinking that I'll concentrate on getting that part as perfect as I possibly can and therefore omit the use of the 0.22uF inside the guts. As JohnA puts it: "On most guitars, the jack is a non-insulating type which means that the jack plate will be directly connected to the negative side of the jack even after this modification. Metal control knobs may also still be directly connected to the negative side of the jack even with the isolation capacitor in place".. So, I'm still in a quandary as I do have a metal jack plate and I may also have metal control knobs. My old gtr has been in bits for weeks now and I've spent a lot of time over this one area - I think it's time to hammer all the bits together again and press-on.
The way I see it, this forum has more than emphasised the need to be careful, sensible and diligent - discussion is good - but it seems that wherever I locate the 400V cap I'll never be completely risk-free. Aahh... I enjoy being pedantic and I think I've tried to understand, as well I can, what's going-on inside my Strat, but it appears, unless I can manufacture a device to sit unobtrusively between my jack socket and the connection from my amp, then I'm thinking, the 0.22uF 400V cap is a bit of extra 'foreign-body-something' inside my instrument for less than total-protection.
You said:

"Placing the cap between the bridge and the cavity shielding is protecting only the bridge/strings. There are other metal parts above the pickguard that the user/player might be touching when the amplifier goes ker-blooey® (ChrisK, 2005)"..
I'm aware of these facts. You then continue:

"You do want the cap protecting everything else that goes to ground"..

Where will it live?

As I previously mentioned, I know diddly-squat about electrickery.. however, unless I'm a complete numpty (?) I can't see where the cap could possibly go that would protect the jack plate (or metal control knobs or pickguard screws). In view of all of this, from now on I will always use an RCD - whereas previously I never did - and this will make me as safe as I possibly can make it.
Now, like you, I have to run (even at my age) but before I beat-it, I would say this: why don't you yourself compose a piece of 'logic' in a diagram/schematic/sketch/whatever that shows exactly the definitive and best way to protect against poss dangers? I'm not suggesting you usurp JohnA's original and good work, but, if you see a better and safer way of approaching the project, then you'd be doing everyone a favour by laying out another upgraded site - why on earth not? I don't want to have to screw-off the lid on my Strat for a long time again, but there'll be many, many others who'll just be starting-out on this road who'll benefit from another upgraded and/or more concise reference.

Thank you for all your help.
Now, where are my K-Swiss..

Stratik

[sumgai]

stratik,

You're correct, there's no easy to to protect the jack mounting plate of a Strat. However, in most cases, a player doesn't monkey around with that part very often. True, the chance is greater than zero that he will be in contact with it while the amp is making environmentally hostile noises, but the chances are much greater that the player will instead be contacting either his strings or one of his switches right when the magic spell is being cast.

I would install this part only if I knew that I was going to use the guitar with a tube amp (vintage or otherwise) at least once in awhile. If I was absolutely sure that solid-state was all I'd even encounter, then I'd leave it out.

And don't forget, for outdoor venues or for indoor venues that might be questionable in the "Quality of Electrical Wiring" department, the only thing that will protect you worth a damn is a GFCI (RCI) device, no matter what kind of amp you're plugged into.

Or as ChrisK says, it's kinda hard to get a shock from a wireless transmitter! ;D


sumgai