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which pots for 2 x J pups ?

Discussion in 'Luthier's Corner' started by MAGUS®, Apr 2, 2005.


  1. MAGUS®

    MAGUS®

    Dec 23, 2004
    UK
    Hi - i bought an Ag OBP-3 off a friend, and it came with the pre, bass/mid/treb/ pots and mid mini switch, but No blend/pan or vol pot

    i've been told i need to add a dual 250k log pan ( blend ? ) pot and a 250k log vol pot.

    1st - why does the pan/blend need to be dual ? surely a single will do ? Doesnt the knob just move the sound from one pup to the other ?

    2nd - 250k/250k log pots are hard to find - only seen "mini " pots.

    3rd - wouldn't it be better to have a single pan/blend, but dual vol instead ?

    4th - are pan pots and vol pots the same item, values wise ?
     
  2. tjclem

    tjclem Supporting Member Commercial User

    Jun 6, 2004
    Central Florida
    Owner and builder Clementbass
  3. Trevorus

    Trevorus

    Oct 18, 2002
    Urbana, IL
    1: It does need to be dual. You will still bleed through a bit if you use a single pot. On a normal volume control, when you turn the pot down, it's shorting more of the signal to ground, not just adding resistence to the pickup's signal. Also, the blend needs to be activated by one shaft, hence doubling. In effect, it is two volumes, but it's wired and controlled a bit differently.

    2: You may have a tough time finding the proper pots in the UK. Some preamps need a dual 25K pot for the blend because that circuit is somewhere inside the preamp circuitry. But a lot of preamps can also use a regular 250/250 before it goes into the preamp. I'm not really sure about the OBP-3.

    3: This depends on your preference and the availability of spots to install controls. You could wire up two volumes, or you could use a blend. Whichever you like better.

    4: Not all pots are created equal. You can get pots in a number of values, though blend pots tend to be more limited. But, as far as how it functions, they are the same.
     
  4. MAGUS®

    MAGUS®

    Dec 23, 2004
    UK
    Hey !

    Its much easier to get single 250k vol pots in the UK - CTS pots are available here, so myabe i'll just go for 2 of those instead, and forget the blend.

    thx !
     
  5. FBB Custom

    FBB Custom TalkBass Pro Commercial User

    Jan 26, 2002
    Maryland
    Owner: FBB Bass Works
    Magus,

    I think you are misunderstanding the use of the term "dual". A blend pot has to be dual taper, that's how they work with two sources. This is different than dual concentric. A blend pot is more or less two volume pots, with reversed tapers, and attached to the same stem. So as you turn the knob, one source is being bled increasingly to ground and the opposite for the other source. When a blend pot is fully open on both tapers (center dentente), there is 0 resistance between the pickups and hot and 250kOhm from pickups to ground.

    You could use two 250k volumes instead of the volume/blend configuration. The resistance rating of a pot is the load that the pickup "sees" and needs to be matched to the impedance of the pickup. Most passive bass pickups want to see a 250k load.
     
  6. MAGUS®

    MAGUS®

    Dec 23, 2004
    UK
    FBB - i have done away with the idea of both blend AND volume - i have ordered myself 2 x 250k CTS audio taper pots. I will then have a Vol for each pup. The Ag also has a treb / mid / bass pots + mid mini switch. I think thats enough options/pots for now !