idea for reducing volume ground reference on coil tap?

[luthierwnc]

Hi Guys,

I'm working on a new build and am getting serious about the wiring. I'm going to have two 4-conductor buckers and want to be able to tap the coils on either. My ears have never really liked the sound of single coils with 500k volume pots so I've sketched something up that might remedy it. The attached diagram shows a mini on-on-on switch as the pickup selector with two 500k push/pull pots for both volume and tone. One side of the switch is the standard coil shunt. On the other I've attached a 560k resistor (might be a 1 meg trimmer soldered to the back of one of the pots) to parallel the volume pot if either of the coils are tapped. With two knobs and a switch showing it is very economical for electronics cavity space. The plan is to put a Roland GK-3 pup just behind the lead pup but that doesn't need much room.

The only drawback I see now is that if I'm using a bucker/single combination, I still get a Fendery 270k to ground. Not a deal killer. Still, it is the things I don't see that usually bite me. Do you see anything that could doom this circuit?

Thanks, Skip

[cbarrow7625]

I always love seeing someone trying different things to address their own little pet peeves with the trade offs inherent in all of the "standard" way things are configured. Your idea should work fine when the volume is all the way up, but you may be disappointed with the results at lower volumes. The way I see it, there are two issues that will limit you ability to get the results I think you want. 1.) At full volume, the extra resistor will be in parallel with the pot & give you the 250K (or so) load you seek. But once you turn the knob down a little, you will still have the resistance of the 500k pot in series with the pickup signal, which will probably dull the tone more than you anticipate. Yes, it will still be better than not having the extra resistor (the resonant peak of the pickup will still be "tuned" where you want it), but without changing some other things you might not get what you want. that leads to #2.) Same problem really; dullness when you turn down the volume. To get that more "Fender-y" sound you should really put a bypass cap on the volume pot in single coil mode to help the highs come through.

The good news is that both things can be addressed with one simple modification to your plan; just add another 500k resistor in parallel with a 250pf-.001uf cap across terminals 1 & 2 of the volume pot when in single coil mode. Getting both things to switch in automatically when either pickup is put in single coil mode would be tricky, but not impossible; especially if you got hold of some 4PDT switches.

I know some folks just don't like the volume pot bypass cap; you could leave that out altogether. But the resistor bypass should still be implemented to help get a pot response more similar to an actual 250k pot.

Just another observation: You could just ground the floating end of the 500k tone pot to reduce the load to 250k instead of adding an extra resistor in the circuit (not that it is a big deal). That would also give you the additional "advantage" of essentially taking the tone cap completely out of the circuit when the tone knob is on "10" for that extra bright strat bridge pickup (with no tone control) sound.

[luthierwnc]

Thanks, I'd wondered about losing highs.

If I understand you correctly, this is a variation on the Tele bright cap. I've amended the graphic to show this with the new bits in purple. When the coil-taps are off, the bright cap has to go through the variable resistor so it will attenuate some highs but I'm guessing not much. When either tap is on, the pickup hots go through the cap-across-resistor circuit to the jack. In this case, I'll definitely use a trimmer for the parallel resistance to even out the loss through the 470k resistor.

Please let me know if this serves the case, sh

[cbarrow7625]

That's definitely a reasonable solution. Without going to a 3PDT or 4PDT switch you have to make some compromises to get the bright cap/resistor in the circuit. When the coil tap is off you will basically have about a 1 Meg resistor in parallel with the output jack, and you are correct that the 500pF cap will have negligible impact on the highs. The trimmer is a nice added touch regardless of why you added it; it will allow you to fine tune the sound in single coil mode. Heck, if you have two of them use one for the bypass resistor as well and you will be able to get the taper of the volume pot fine tuned as well.

Hopefully we haven't just improved your coil tap sound at the expense of the full humbucker tone. I guess you'll find out soon enough. Please report back how it works.

[luthierwnc]

I might start with something like this. Have a set of WCR Godwood (bridge) and Darkburst on order.

More sanding first. sh

[cbarrow7625]

Damn bro, you got a beautiful body and I just want to caress that neck........was that out loud?......uh, never mind....

No doubt that would reduce the resistive part of the compromise. If it was my Axe I would have that and the last version on a temporary toggle to figure out which was the better compromise (probably this one!).

Seriously, sweet looking wood.

[Steve A.]

Gibson used a treble boost control on some of their lap steels from the 40's and 50's which used a .001uF cap across a 500k pot. In the brightest position the circuit is essentially the same as your RC network, only it uses a .001uF paper in oil cap instead of a 500pF cap. I'd go with the larger value cap, polyester or mica (not ceramic).



Steve Ahola

[luthierwnc]

cbarrow, your forgot to mention how perfectly it sits in your lap! The jack/endpin combination will go at the end of the rounded back to make room so you can put the forward cut on your left leg or more like a classical straddling the two cutaways. Sort of a reverse bootay. Not sure the neck is so caressable. Until I round the neck it will be the ergo dobro.

Steve, thanks for the schematic. If I didn't need the push/pull switches on the pots, I suppose this could be set up as a dual volume pot with one of them wired to increase the treble bypass as the volume goes down. I still have a few .001 Sprague Vitamin Q caps in the bin along with some 0.02's that measure a little light -- .019ish. Since I almost never roll off all the tone, I don't need a big sweep.

In this configuration I figure I get eight different combinations:

bridge bucker
bridge single
both humbuckers
both singles
neck bucker
neck single
bridge bucker, neck single
neck bucker, bridge single

My tradition has been to only really like half or less so I'll find those first and tweak from there. Also, many thanks for your seminal work on Dumble circuits. This will probably be played through one of three variations on that series. I've got a 2nd gen hybrid, a 102 with the loop built in using the return as MV and one that Jeff Shook designed using a 5879 in the OD with a one-tube reverb. Last year I built a JTM45 variant that is cathode-biased with about 390VDC on the plates. I think the Godwood will like that too.

Thanks and cheers, sh

[luthierwnc]

A little further along.