final questions on FrankenStrat rewire

[murrayatuptown]

Hi:

I'm putting this under nutzoid schemes because even if the 'design' doesn't qualify for such categorization, I usually do. My first guitar project, by the way..so please 'scuse the ignorance and any missed 'obviousisms'.

I took a Fender Starcaster (solid, not semi-hollow) neck, body & hardware (no electronics). Bought a Mighty-Mite pre-wired MM405TS HSS pickguard because it was cheaper than separate parts, and had my daughter choose a pickguard she wanted.

Phase one was to just wrestle everything together & figure out the setup. The floating tremelo had shifted prior to her first lesson which ended up a disaster - the action was too high and the teacher objected to such a setup for a beginner and began trying to adjust it, broke a string and it only got worse from there. I'm swapping the pickguards and rewiring right now, and will set up the tremelo/vibrato to be more 'mainstream' and rest against the body for downward pitch change only.

I added an SPST switch to allow all 3 PU's in parallel in addition to the normal Strat single & parallel pair combinations.

I changed the Volume & one Tone pot to push-pull to add phase reversal switching on middle and (dual-coil) bridge PU's.

I removed the 2nd tone pot and installed an Alps 2P4T rotary salvaged from a Harman-Kardon moving coil phono preamp capacitance trim pcb. It's got some screwy wiring and had polypropylene caps on its PCB. I had to find what model it was from & find the schematic to cut some traces for my needs. It is made for very small signals so I felt good about that choice vs. a toggle (I prefer wiping contacts to rocking I assume are in most toggle switches). I removed the 150 + 510 pF caps so I could add my own.

One pole (4-positions) selects output from the normal '5-way' common output, or adds a pickup in series with the neck when the 5P Strat switch is at Neck-only. The options are N, N+M, N+M+Bridge_Tap, N+M+Humbucker bridge. The Middle and Bridge remain phase-reversable (dual-coil).

The 2nd pole selects one of 4 tone caps - the normal one (0.033 uF with 500k pot) and ... see below for question #1

If you want to know more details, I'll share, but the quick questions are as follows.

1) I tried to calculate values of C to keep the LC resonant freq. the same the all four PU inductance combinations. I know it's an approximation, but wanted to start with some similarity...maybe each series combination's 'voice' will be more evident and not hidden behind a shifted tone cutoff freq.

Anyone know what the inductance is for the Mighty-Might Might-Bucker? I saw 18 or 22k ohms and 9.7 or 14 H I measure 16.15 K (P/U disconnected), so don't know what PU model this really is.

2) Re: John Atchley's safety grounding through a large capacitor -

I lined the body with copper. Do I simply ground the bridge/strings to the star ground through the capacitor? Otherwise I don't understand what it isolated....probably need more details.

Thanks

[newey]

Murrayuptown-

Sorry, but I had to move your thread. The schematics section is for posting of final designs, not for general questions, regardless of how nutzoid the issues, or person, may be.

As far as your questions, you can do the procedures outlined in ChrisK's Pickup Coil Response Tuning to determine best capacitor values.

We have recently had much discussion of the "isolation" cap as well, found in The Blocking Capacitor

[murrayatuptown]

No problem on the move - sorry for dropping it in the wrong place.

So, looks like I have to take a steel rule and xActo knife and cut a moat in the copper around the touchable parts to separate the signals.

Maybe I'm missing something, but if a 0.33 uF capacitor with Xc of 8 k at 60 Hz isn't a problem for hum/noise...why not a 0.1 uF to limit rms leakage current at 120 Vrms to 4.5 mA...more in the GFCI limit range?

I have the cavity copper lined. I may try the 0.1 and see what results I get.

Suggestion - look at 'X-CAP' AC line/mains-rated 'safety' capacitors, typically rated at 250-275 VAC for across-line use, with most international electrical safety agency ratings.

AC-durability and rating of a DC-rated is a compromise. An AC-rated capacitor will endure DC very well. Apps engr at Nissei Denki America says a DC rating for a true ac-rated cap can be derated 10% below the peak-to-peak value of the AC rating. (0.9*2.828*ac rating).

250 V rms = 707 v p-p x 0.9 = 636 VDC

AC rated caps are constructed with different physical factors to avoid corona and heating effects - essential for long life in high voltage or power electronics applications (inverters, ballasts, etc).

Just don't get one with an integral resistor (snubbers or larger values may require it for some category of safety rating).

Murray

[newey]

Maybe I'm missing something, but if a 0.33 uF capacitor with Xc of 8 k at 60 Hz isn't a problem for hum/noise...

The "blocking" capacitor should make no contribution to hum/noise as it's not in the signal chain. It's there strictly for safety.

JA's original article advised 400V, ChrisK advised to go to 600V- close to the 636V you suggested.

The point of ChrisK's article is that the addition of the safety capacitor provides only partial protection at best, and should not lull anyone into a false sense of security. Use of an outlet tester and a GFCI are still "best practices", and to truly eliminate the risk, a wireless system should be employed.

The types of failures at issue are thankfully rare to begin with. But even with the cap, the metal jack plate is not isolated, nor are metal knobs if the guitar has them. If one is touching those items when the failure occurs, the cap won't help.

I personally don't use the safety cap in my rewiring projects. I don't use a tube amp, so the DC plate failure issue is not a problem. I have tested the outlets I use, and I have a GFCI, so an AC mains fault is unlikely.

If I ever do get a tube amp, I'll get a wireless rig at the same time.

[murrayatuptown]

I agree with the low frequency (bad pun not intended) of occurrence...I haven't heard of it since, what, the Woodstock era? Maybe only celebrity guitar electrocutions get media coverage.

John A or Chris K probably have a better idea how much attention it needed.

You still have me thinking...I was given a Gibson BR-9 by someone who worked on my old archtop...I cut the power plug off recently so no one would plug it in, until I recap the power supply.

Plenty of people want tube amps; new product and vintage are still in demand.

I don't see anyone checking the outlet wiring at the local coffee house open mike night, I see borrowing an available instrument or amp and so on...and a wireless system might be both a luxury and/or impractical for some people and venues.

So perhaps including a blocking capacitor has minimal detriment in the case where the user is completely unaware, no more or less confident with the capacitor installed than without...no false sense of security.

I am more educated about this now than before, but I can see my warnings falling on deaf ears...I can present the outlet tester with the completed guitar...and assume it's one more layer of paranoia I'll be accused of exhibiting )

Maybe I need to research guitar electrocution history. If I am convincing enough, an acoustic guitar will be on a wish list next )

Thanks...I'm off to finish the guitar - I am running out of time before the next lesson.

[newey]

Maybe I need to research guitar electrocution history.

Keith Relf is the only one who comes to my mind, although I'm sure there are others.

An outlet tester is about $10. I don't play any gigs, coffeehouse or otherwise, but those around here who do advise stuffing one in the old guitar case and checking the stage outlets prior to setting up.

GFCI power strips are also fairly cheap. While the risk is low, so is the cost of ameliorating the risk. The potential hazard (electrocution of you!) is catastrophic. So, doing a mental risk/benefit analysis should result in one opting for the protections.

Of course, acoustic guitars, if amplified using a piezo system, do not present the same risk, since the only metal parts one is likely to touch are the strings, which are not grounded with a piezo.

[sumgai]

newey,

Technically speaking, K.R. didn't die on-stage, so his is not a case of a musician being electrocuted during a performance. So far, only one artist qualifies for that distinction, Les Harvey of 'Stone The Crows'.

The page I had long ago bookmarked has now disappeared from the web. Google turned up a new (to me) page, which is pretty much how I remember the "lost" one looking like.

elvispelvis.com/electrocuted.htm

Harvey is the second entry thereon.

HTH



sumgai

[sumgai]

Further, as to shock protection, let me refer one and all to this topic by our own ChrisK:

guitarnuts2.proboards.com/index.cgi?board=reference&action=display&thread=4184


Specifically, the GFCI outlet that he linked is single, and is meant to be used in a temporary setting - you take it with you, and you use it every time you hit the stage, even in those places where you play every week - you never know who's been at the circuit panel or the wiring, while you were out of the building. Mr. Murphy needs no help in this department, there are plenty of bozos looking to make your life a living danger, trust me on this!!!

As much to the point, I saw one of these contraptions in Lowes the other day, and it was priced at less than half of what Amazon wants. You now have no excuse to not keep one in your tool/parts box. Hell, take two, just in case someone else in the band is not as smart about these kinds of things as you are!

This goes for all of you! I don't wanna hear it - it's three latte's or your safety, which I don't call much a choice.


sumgai

[murrayatuptown]

Two more thoughts...

0) I get what you're saying, so don't take the following as looking for a way to avoid the recommended solutions...

1) If everything one can touch EXCEPT the strings and bridge (say a Strat-vintage type) are isolated through the recommended capacitor, and all the signal returns (or analog grounds) are separately 'star grounded', through what path would ungrounded strings/bridge introduce hum, noise, etc, and how significant a contributor would they tend to be in an otherwise 'quieted' instrument?

2) Given the 'all else connected to 'star ground', directly for analog returns and through a blocking capacitor for 'touchable' conductors' scenario, what failure modes or circumstances would make the unconnected strings/bridge a shock hazard?

Maybe inadvertent contact when string bending over a metal-cased humbucker?

I lied. One more question.

3) My Mighty-Mite Mighty Bucker has no metal topside (pole pieces appear to be isolated), but the brass base was connected to the stock aluminum foil 'safety' shield by one p/u mounting screw. I am wiring this pickup to a phase-reversal switch (ends of series-connected coils) with a separate tap for the 'floating' coil tap, and the 'low' end of the humbucker will be grounded or series connected depending on switch selection. I have separated the white and bare (drain or shield) wires which were soldered together when all three stock p/u's were normally grounded.

If the bare wire is connected to the brass humbucker p/u mounting plate/base, do I continue to connect the bare humbucker shield/drain wire to the capacitor-isolated 'relatively-safe-bus' and the the white or 'low' end of the humbucker to either the star ground or it's optional series wiring?

If the 'shield' of the 5-conductor humbucker cable is connected to the capacitor-isolated 'r-s-b', how effective is the shielding of the other 4 conductors, especially with series-wired configurations?

(Only about 7 wires left to solder, then re-string).

Ugh, another question (if you're still here). I have coppered the back cover. I realized there is no copper lining the rear cavity. Only a ground wire passes through to the fully shielded top-sdie cavity. I'm not sure what, if anything, I should ground the copper-shielded back plate to...it looks like a capacitive pickup to me right about now if I connect it....

Maybe I better put two screws in the pickguard and one string on
to see if I have the presumed 'normal' p/u phasing wrong...Murray's Law, you know?

Thanks

[ChrisK]

It's not an isolation capacitor (well only in a DC steady-state no transient occurring way), but a blocking capacitor. It's role is to introduce enough reactance (AC frequency-dependent resistance if you must) to limit the possible AC current from mains wiring defects from killing the operator, but to provide a low enough reactance to provide shielding coupling for noise issues. If you reduce the value, you will reduce the AC fault coupling and reduce the shielding effect. Atchley already had reduced the value from 1.0 uF to 0.33 uF to reduce operator electro-luminescence.


This might shed some light.

The "Blocking" Capacitor

The metal shells on pickups are part of the "Operator Touchable Circuit" since they are, uh................

[newey]

1) If everything one can touch EXCEPT the strings and bridge (say a Strat-vintage type) are isolated through the recommended capacitor, and all the signal returns (or analog grounds) are separately 'star grounded', through what path would ungrounded strings/bridge introduce hum, noise, etc, and how significant a contributor would they tend to be in an otherwise 'quieted' instrument?

Via magnetic induction through the pickup. How significant the noise is depends on several factors. Some guitars get by without grounding the strings.

2) Given the 'all else connected to 'star ground', directly for analog returns and through a blocking capacitor for 'touchable' conductors' scenario, what failure modes or circumstances would make the unconnected strings/bridge a shock hazard?

Maybe inadvertent contact when string bending over a metal-cased humbucker?

That might do it. But if bending strings is causing the strings to contact the pickups, you probably should lower the pickups.

[murrayatuptown]

Thank you.

My friend on the phone said I was taking too long on this & if it didn't work out he'd send me a spare Strat...I laughed...

[ChrisK]

Hmmm, opportunity strikes.

What are the conditions and time-span of said offer?

One could always have continuing problems..........................

[ChrisK]

My recommendation comes from;

120 VAC = 170 VDC peak

High line 130 VAC = 184 VDC peak

200 VDC peak = 141 VAC

However, this doesn't leave any (much) room for transients. MOV-based transient protection power-strips are fairly slow to respond (they utilize a material phase change for clamping). Transorbs (FAST, HARDY zener mechanism diodes) are much faster (nanoseconds). Regular zener diodes are neither fast nor hardy.

Ergo, I recommend 400 VDC for domestic (U.S.) use. Except for the tube amp plate voltage somehow traveling down the cable. Even if it's thru the signal conductor and not the shield, it's only a few thousand Ohms in the way (and guitar cables ARE NOT RATED nor Hi-Pot tested for 400 VDC).

So it's 600 VDC.

For those a'planet where the local illuminating company have negligently chosen up to 250 VAC for operator electro-luminescence, the numbers change to:

250 VAC = 354 VDC peak

High line 270 VAC = 383 VDC peak

Again, this doesn't leave any (much) room for transients.

400 VDC peak = 283 VAC

600 VDC peak = 424 VAC

[ashcatlt]

Jul 23, 2009 16:00:18 GMT -5 murrayatuptown said:

1) If everything one can touch EXCEPT the strings and bridge (say a Strat-vintage type) are isolated through the recommended capacitor, and all the signal returns (or analog grounds) are separately 'star grounded', through what path would ungrounded strings/bridge introduce hum, noise, etc, and how significant a contributor would they tend to be in an otherwise 'quieted' instrument?

The strings and bridge themselves do not contribute significantly to the noise. The body of the guitarist does. In normal playing situations, this is the closest source of EM radiation to the instrument. It also happens to be in near constant contact with either strings or bridge or both. Grounding the string/bridge structure helps to ground out this antenna - reducing its contribution to the noise signal.

A very well shielded guitar, especially one with functional hum-cancelling pickups (or combinations thereof) would tend to reject this noise anyway, and it may not make much difference whether the strings are grounded. I have owned guitars that for one reason or another sounded exactly the same with and without the bridge ground wire. Every little bit helps, though.