Advise on using Linear Pots & Resistors for Active EMGs?

[OzziePete]

Hi all,

I have had a project that's been stalled for ages. I am relatively newish to soldering and electronics, have some understanding of guitar electronics, but I have jumped into the deep end with this project. Quite frankly, the project has been the cause of headaches for a few years now, and as I approach the New Year, I want to make 2008 the year that the JazzMonster is born!

I am trying to basically do the guitar equivalent of shoving a V8 motor into a Mini!

I have two EMG 89s (dual sound) along with an SPC and an EXG pot EQs.

I am trying to shove this all (along with the 18v double battery box) into a Jazzmaster body. The body has routs for 3 Humbuckers, and a Wilkinson trem, but the other routed features for the typical JM body are there. I've gone through about 3 pickguards trying various incarnations of ideas, and will be needing another to complete this project. I just need some time to work on it, and a clear direction of what I want to achieve in control settings for the instrument.

I am trying to keep a retro theme with this guitar and have bought a stack of Jag slide switches and Mustang switshes to use instead of toggles or DPDT mini toggles. I also have the JM/ Jag upper bout control harness that holds the two roller mini pots. I have also bought the two mini pots and noticed that one of these is a 50k Linear Pot.

The EMGs require 25k pots for the volume. But the EMG site recommends 25k audio taper (log.) pots.

With routed space a problem, and the wish to keep things in style with the JMs and Hags of days past, I was wondering if I could use the 50k Linear mini pots (have 1, need to buy another?) in the roller pot harness, but place a resistor of 25k before the 50k pot, so the pickups "see" 25k? (any diagrams on how that could be wired would be also appreciated, please.)That way, I have a use for the harness and can utilise the space that's there.

I can then use the two standard control spot for the SPC and EXG EQ pots.

With Linear Pots I believe that the effective range only comes into play for the pot's use in the last third of the turn, and if I also use a 25k solid resistor with it, does this mean I will have even less effective roll on the pot? In other words, would the pot have little effect until maybe the last third or so of the sweep of the pot  -then some dramatic increase in volume in the last bit of the sweep?

Some input from some wiser heads on this forum will be greatly appreciated.

Cheers
Pete

P.S. The battery box will probably end up routed at the rear of the body behind the trem. It is going to be wired in series so I can get 18v across the active components of the wiring and have a touch more headroom.

 

[ildar]

It would work, but don't put the resistor before the pot as that would actually be 75k in series. Rather, solder the resistor across lugs 1 and 3, which would be parallel and give you 25k.

It won't affect the useable range, just the resistance.

 

[OzziePete]

Thanx Ildar.

That sounds like the way that folks put the resistor for the treble 'bleed' circuit on single coils.[link below]

http://www.seymourduncan.com/support/wiring-diagrams/schematics.php?schematic=treble_bleed

I noticed a US Fat Tele uses 500k pots, but has a combo of a humbucker and a single coil bridge pickup.

Checked the Fender wiring diag. from their website, and a resistor is used to cut the pot down to 250k or so for the single coil, but it is wired between the switch contact and the ground of the pot. I'm guessing that when the switch is engaged, the resistor is also placed into parallel, and then grounded so it works the same time as the pot is engaged (?). [link below]

http://www.fender.com/support/diagrams/pdf_temp1/telecaster/0108000_02B/SD0108000_02BPg2.pdf

 

[ildar]

If I read the diagram right, it seems to actually be a pop protector, or additional ground protection. The key there is that it's going from the switch to ground, and not to any hot connection on the pot.
The method I indicated joins both hot lugs.
The treble bleed method I usually use is a smallish cap (250pf-.001uf) from lug one to ground.

 

[OzziePete]

If I read the diagram right, it seems to actually be a pop protector, or additional ground protection. The key there is that it's going from the switch to ground, and not to any hot connection on the pot.
The method I indicated joins both hot lugs.
The treble bleed method I usually use is a smallish cap (250pf-.001uf) from lug one to ground.

Thanx for clearing that up. :toothy10:

 

[ildar]

I'm being stupid.
If the resistor is across the outer lugs, then it does go to ground as lug 3 gets grounded. So, that is exactly the same way to use a treble bleed cap.

 

[OzziePete]

I did some checking on the net (ain't Google great!) and found a formula or two that helps work out resistances when you have series or parallel wiring.

One formula worked on reciprocals, and is quite complex. Another is more simple.

The simple formula is shown as: a x b / a + b = c

a is resistor 1, b is resistor 2 and c is the resultant Ohms.

Using that formula, I can use 2 x 50Kohm pots in parallel to get 25Kohms (which the EMGs need)

50 x 50 / 50 + 50 = 25

So the use of the 50K Roller Pots looks a goer!  :icon_thumright: