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Blend Pot Woes

Discussion in 'Pickups & Electronics [BG]' started by portaflexer, Aug 29, 2007.


  1. portaflexer

    portaflexer

    Jan 31, 2007
    Dallas, TX
    Hey guys. I decided to install a blend pot in my Fender Jazz bass. It's setup so there is a master volume, master tone, and a bridge-to-neck blend control. I used all 250K pots and downloaded the wiring diagram off Seymour Duncan's website.

    I wired everything up like the diagram, but something isn't quite right. The blend works. It clearly fades from bridge to neck, but the volume drops significantly when you get towards the middle. I'm not talking about a little bit... I'm talking about a significant drop in volume. I traced all the wires again and it appears to be wired correctly.

    Is this a sign of a bad pot? or did I just screw up somewhere in the wiring. If a blend pot is going to make the bass sound like this, I'll just change it back.
     
  2. Scott in Dallas

    Scott in Dallas Commercial User

    Aug 16, 2005
    Dallas, north Texas
    Builder and Owner: DJ Ash Guitars
    Per the other thread, it sounds like the pickups are out of phase.
     
  3. portaflexer

    portaflexer

    Jan 31, 2007
    Dallas, TX
    I'll have to look again. Maybe I mixed them up.

    BTW... I did do a search before posting this, but then again... I may be searching for the wrong criteria (or it could just be buried). If you a direct link to a thread with more information... I'd appreciate it.
     
  4. Scott in Dallas

    Scott in Dallas Commercial User

    Aug 16, 2005
    Dallas, north Texas
    Builder and Owner: DJ Ash Guitars
  5. portaflexer

    portaflexer

    Jan 31, 2007
    Dallas, TX
    I traced all the wires again and I definitely wired it right according to the diagram (both whites go to the center leads on the blend pot, both blacks go to ground). I'm wondering if the Pot itself is wired differently and I should be running the corner leads together instead of diagonally across the pot. Could that cause an out of phase issue?
     
  6. If it's a phase issue just invert the two wires of one pickup.:)
     
  7. portaflexer

    portaflexer

    Jan 31, 2007
    Dallas, TX
    I thought of that too, and I can try that to prove the theory, but white is white and black is black. It's pretty black and white (no pun intended) so I'm guessing it's the pot that's wired wrong, and not the pickups.
     
  8. Scott in Dallas

    Scott in Dallas Commercial User

    Aug 16, 2005
    Dallas, north Texas
    Builder and Owner: DJ Ash Guitars
    The Stewmac blend pots are supposed to be 100 percent at the center detent. If you have one that's straight linear, it might put both pickups at 50 percent at the center. Could you have one of those? That might be the cause.
     
  9. portaflexer

    portaflexer

    Jan 31, 2007
    Dallas, TX
    Funny you mention StewMac. That's the first place I went for the pot... they are currently out of stock so I went to eBay and bought it from one of the guitar parts reatailers there. I emailed him last night to ask if this was normal and he thinks its a defective pot. Long story short... he's sending me a replacement pot. It should be here early next week. I suppose it's entirely possible that this pot is designed to do that, but it sure sounds like a phase issue to me.
     
  10. DavidRavenMoon

    DavidRavenMoon Banned

    Oct 20, 2004
    They aren't. Stew-Mac blend pots are audio (log) taper. The problem with the StewMac pots are the top and bottom section have the taper going in the same direction!

    So think about that for a moment. On a linear taper pot, half way is half way. So if it's a 250K pot, at center detent you have 125k from the wiper to either outside lug.

    On an audio taper pot, when the pot is half way, you get about 225k between the wiper and the left lug, and 25k on between the wiper and the right lug. Now if the 225k is between the pickup and ground, you are getting pretty much the full signal.

    Now here's where the problem comes in. To do this properly, one half of the ganged pot has to be reversed (anti-log). On the StewMac pot, it is not reversed. It's really a dual ganged audio taper volume control with center detent.

    So when you have the pot in the center, one pickup is seeing 225k to ground, while the other is seeing 25k!

    Now having it log/anti-log should work, but I was using one of these the other day, and while it sounded fine in the middle, or either extreme, it was hard to get any kind of blend. It worked more like a switch. That wasn't much better.

    Now on the other hand, I have a 25k EMG blend pot here, and it reads 9.5k from the wiper to either outside lug, so you can see its linear taper.

    There has to be a good source of blend pots out there somewhere. :meh:
     
  11. portaflexer

    portaflexer

    Jan 31, 2007
    Dallas, TX
    I'm not an electronics guru, but I think I actually followed what you said. Basically... blend pots really don't do what they're supposed to do. So... is what I'm seeing normal? If so... maybe I'll just go back to the old setup.

    My whole idea behind the blend pot was that I could easily cut the volume on stage with just one knob. The old way... I have to turn two knobs and I generally like to fade a little towards the neck. I thought this would be the answer, but apparently not.
     
  12. portaflexer

    portaflexer

    Jan 31, 2007
    Dallas, TX
    Well I tried messing with the phase and that didn't help. I think it's just a defective pot. I'm going to try another one and if that doesn't work any better... I'm going back to vol/vol/tone.
     
  13. Scott in Dallas

    Scott in Dallas Commercial User

    Aug 16, 2005
    Dallas, north Texas
    Builder and Owner: DJ Ash Guitars
    I'm glad you told me this before I bought one. I just double checked and Stewmac advertises them as having "both signals full at the center detent." If they aren't capable of doing that then it's a big reason folks are unable to make them work.

    After looking at the wiring diagrams, it looks like theirs is supposed to be wired the same as any other blend pot, so it's not a wiring mistake, either.
     
  14. portaflexer

    portaflexer

    Jan 31, 2007
    Dallas, TX
    I bought another blend pot from a different retailer. I'm going to try that one and see how it performs. If it does the same thing... I'm going back to the original setup. BTW... go to eBay and search for "jazz bass plate prewired". You'll find a couple of people selling prewired Jazz Bass control plates for under $30. You just hook up your ground and your pickup and you're off. I might just pick one of these up so I can experiment. Keep one setup normal and one for playing around.
     
  15. DavidRavenMoon

    DavidRavenMoon Banned

    Oct 20, 2004
    I have a customer that regularly has me put them in basses for him. They seem to work OK, but I don't think they are giving the full signal of both pickups in the center. I don't see how it could.

    I've been meaning to talk to them about this.
     
  16. I can't see buying cheap Chinese pots from StewMac when you can buy good quality pots online for nearly the same price.
     
  17. DavidRavenMoon

    DavidRavenMoon Banned

    Oct 20, 2004
    Well they are Alpha pots... they do make good quality stuff, but StewMac carries the cheap parts.

    I haven't found too many sources for blend pots. I'd like to get the parts EMG uses.
     
  18. Anybody find an awesome pot yet? I have this issue as of now.
     
  19. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    I have since found the bland pots from Allparts work as they should. In the center you get both pickups at full volume.

    The only thing I don't care for is the taper, which seems to make it hard to mix the two pickups, you get a small window of blending before you are soloing that pickup.

    But I'm working on adding some resistors to flatten out the taper a bit, and I'll share what I find when I get there.
     

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