Strat wiring project: Star ground and blender pot

[bluesman13]

Hi there,

I've got a lefty strat, and did star grounding (quieting the beast) several years ago. However, I'm just putting this wiring harness back in the guitar after removing a different one. But before I do that, I want to clean up my soldering and also add a blender pot so I can have a master volume, master tone, and a "blender" pot for blending in the bridge pickup when in neck pickup mode, or blend in the neck pickup when in bridge pickup mode.

I'm using the stewmac wiring diagram located on their website at: www.stewmac.com/How-To/Online_Resources/Learn_About_Guitar_Pickups_and_Electronics_and_Wiring/Emerson_Pro_CTS_Blender_No-load_Pot_Instructions.html

In addition, I plan on adding a treble bleed circuit to the volume pot from then as well: www.stewmac.com/Pickups_and_Electronics/Components_and_Parts/Capacitors_and_Resistors/Golden_Age_Treble_Bleed_Circuit.html

I bought some white 20-2 gauge solid (not stranded wire) insulated wire from Home Depot and was wondering if that would work for the wiring between the pots and selector switch?
A long time ago I installed reverse taper pots (since I'm lefty).

The other question I had was regarding the location of the star grounded to the pot. Previously, I attached it to the shaft of the vol pot, but I saw a recent diagram that it was now attached to the tone pot? Thoughts on that?

thanks for the help, guys!

Steve

[newey]

You can attach the star ground to any pot, so long as it is electrically continuous with your shielding. I tend to not use the pot shafts for the star ground, I screw it into the side of the cavity, more secure that way.

20 gauge wire is a bit thick but you can probably make it work just fine.

As for the treble bleed, you can probably buy the components cheaper than the "kit" from StewMac. Read JohnH's thread "A better treble bleed" for the skinny on all things TB.

[bluesman13]

thx newey. does it matter if the wire is stranded or solid? I checked out that link to the other thread - that's way above my head (I'm sure it's amazing!) - I can follow a wiring diagram like the one on the stew mac website.

[newey]

You can use either stranded or solid wire, the differences between the two types aren't relevant at the small voltages in a guitar circuit.

The reason I posted the link to JohnH's thread is that he gives recommended values for the cap and resistor, which you can then buy and twist together yourself. You don't need to understand the research that got him to the recommended values. StewMac charging $6-$7 for a cap and resistor is obscene, it's probably less than 50¢s; worth of parts. And don't believe all that stuff about how you need this brand or that brand of capacitor- if it's the right value, the type of cap doesn't matter.

The wiring diagram will be the same whether you decide to line StewMac's pockets or not.

[bluesman13]

thx newey,

I dont mind ordering the parts from a place like mouser (and save some monry - thx), but there's like thousands of choices for each item. Do these look ok? 

I've picked the two parts for the treble bleed here:
resistor: www.mouser.com/Search/ProductDetail.aspx?R=PR02000201203JR500virtualkey59420000virtualkey594-5083NW120K0J
capacitor: www.mouser.com/Search/ProductDetail.aspx?R=D102K25Y5PL63L6Rvirtualkey59420000virtualkey594-D102K25Y5PL63L6R

and then since that adds up to all of $.66, I was going to replace the star ground capacitor as noted in shielding the beast with something smaller (since the one I have in there now is the size of a large cherry bomb with this:
www.mouser.com/Search/ProductDetail.aspx?R=DME4P33K-Fvirtualkey59850000virtualkey5989-400V.33-F

If those look right, I'm going to find a couple of other things since I imagine the shipping is going to exceed the parts...

SH

[newey]

Those are all OK, although JohnH found the treble bleed optimal with a 150K resistor; you show a 120K. Also, although the original Guitarnuts shielding thread recommended a 400 volt, 0.33µf cap, ChrisK convincingly argued that a 600V "blocking cap" was preferable, as tube amp failure could theoretically expose one to over 400V. However, the difference isn't major and the 600V ones are harder to find.

[bluesman13]

thx newey. I'll adjust that.

I'm using the attached diagram as a guide. As you can see in the diagram, the blender pot is not grounded. If it needs to be grounded, can the right side open lug of the blender pot be attached to the tone pot ground that has a .022 cap attached to it (and then attach to the star ground before the capacitor?

Also this diagram shows two wires from ground to the body of the guitar. Is that necessary?

thx for all your assistance, I appreciate everyione's help.
Steve

Attachments:

[sumgai]

Steve,

Technically speaking, those two wires are going to different places on the body.  Observe:

One is going to string ground, which on a Strat is usually a solder job to the vibrato's spring 'claw'.

The other is going to the shielding inside of the cavity, often a screw through the shielding and into the wood.  Tightened up against the shielding, this usually works well enough, but some of us actually solder the wire to the copper, that's not hard to do.

Usually, these two locations are electrically separated - it would take an unusually anal overly cautious modder to connect the two via some sort of wiring, or even more detailed shielding (and which I would consider to be highly unnecessary).  But yes, both wires are necessary in order to achieve the best possible hum reduction.

HTH

sumgai

[bluesman13]

SG,
thx for the response! 

any thoughts on whether the blender pot needs grounded and whether using the open right lug of the blender pot to the left lug on the master tone pot that has a .22 cap attached to it would work?

[newey]

any thoughts on whether the blender pot needs grounded and whether using the open right lug of the blender pot to the left lug on the master tone pot that has a .22 cap attached to it would work?

The answers to your two questions are "maybe" and "Hell No!".

When you speak about "grounding a pot", that's a shorthand way of saying "grounding the shell of the pot" (also sometimes called the "frame" or "chassis" of the pot), usually by soldering a wire from the back of the pot to a grounding point. Often, a single ground wire is used to "daisy-chain" all the pot backs together, and then to ground.

The "maybe" comes about because it's not at all clear that an ungrounded pot can actually add any noise. I've built guitars both ways and never noticed any real difference. But most guitar manufacturers do ground the pot shells, and wire costs money, so they would presumable omit it if they thought they could get away without it. I'm sure an EE would tell you that best wiring practices would call for the grounds to be run.

However, the original Guitarnuts shielding exercise calls for these shell ground wires to be omitted, since the pot shells are grounded through contact with the shielding, and John Atchley thought that leaving those wires resulted in undesirable "ground loops". We have had years of discussion on the topic of ground loops and the consensus seems to be that the small loops that result from wiring the pot backs together are meaningless in the context of passive guitar circuitry. (other ground loops, however, do matter- like in an amp chassis or pedal)

But connecting anything to the lugs of a pot doesn't ground the shell- the lug connects to the internal "guts" of the pot. Your blender pot is wired with only those two wires for a reason, and it won't work right if the third lug is attached to ground. (Hence, the "Hell no" part).

Without getting too technical here, pots can be wired in different ways to do different things. A volume control, for example, uses all 3 lugs of a pot. This is called wiring it as a "potentiometer". Tone controls (and your blend pot) are wired with only two lugs used. This is called a "rheostat". The difference between the two ways gets technical, but for your purposes just know that the two wirings are not interchangeable. Wire the pots as shown in your diagram.