Ultimate No-load Ungrounded Blend Pot (Pics!)

Discussion in 'Pickups & Electronics [BG]' started by walterw, Aug 13, 2014.

  1. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    From your picture it appears that you cut the resistive element correctly so it should work. As vin97 says you should inspect the wiper arm for any (hopefully repairable) damage. Also it is possible that some debris from the cutting process managed to wedge itself under the wiper arm contact and insulated it from the trace. If that happened to be the case then the pot should work now that you have disassembled it and cleaned it up.
  2. screefer


    Nov 7, 2017
    The Island
    I did the mod with the pot intact. I then disassembled a second blend pot and cut the trace with an exacto knife, put the second wafer in the first pot.... Same results.... I've only got one more pot left so maybe let sleeping dogs lie, eh?
  3. screefer


    Nov 7, 2017
    The Island
    Thanks khutch, I think I'll disassemble my last good pot and swap out the lower wafer into the original pot with the good upper lugs. Then I'll wire the neck pup so it can be isolated/soloed completely and just blend in the bridge pup to taste...
    I appreciate the input from you and vin97
  4. Papaschtroumpf


    Dec 27, 2017
    Hello, use it possible to combine this mod with a push-pull to put both pickups in series?
  5. tallboybass

    tallboybass Supporting Member Supporting Member

    Feb 25, 2003
    Tulsa, Oklahoma
    I'm going to have a go at this mod tomorrow, I don't have an X-acto saw blade though. Any other suggestions on what to use on a 250k blend pot? Do some of them require disassembly?
  6. Papaschtroumpf


    Dec 27, 2017
    Thanks to vin97 for the link to 100k blend pots. Mine are on order.

    I am still confused about values and tapers for the T and V pots:
    My pickups are pretty low DC resistance (5.4k and 6.4k respectively). I think that means I should use 250k for both T and V?
    I think Audio taper for the Tone pot is the norm?

    I saw Walter's comment earlier that a linear volume is better, but I saw other posts on other forums stating that the volume would be more usable with audio taper because it's how "physics" work. Pots are cheap enough that maybe I can try both, but I'd love to get a better explanation of why I might prefer linear volume.

    I also found a diagram for mixing the VBT concept with a series/parallel option. Unfortunately I am not quite satisfied with the parallel/series switching options:
    - a push-pull doesn't have enough contacts to remover the blend pot out of the circuit when I parallel, so the blend pots will have some weird effect.
    - an S-1 switch would have enough contacts,, but they are expensive and I can't find one with jazz-bass style knobs (I could don Tele style)
    - I can find a 3PDT mini toggle switch but it seems a pain to fit in the control cavity.
    BlueTalon likes this.
  7. I am about to add this blend control to my BB415, and while wondering how it works I have thought of a way to add a "bonus" output via stereo jack.
    I added a picture of how I see it electrically, and describe it below.

    When pot is fully rotated into one or another direction, isolated pickup is essentially connected to remaining pin, via small resistance (if cut is done in position that allows outer viper to "land").
    I want to connect currently not-connected pins together, load them into 220k resistor to emulate usual volume pot wide open, and connect that to ring of TRS jack.
    When mono jack is inserted, pickup is grounded from both ends. When stereo jack is inserted "isolated" pickup is actually on ring.

    Since I already use no-load pot (made in same manner) for tone, I expect signal to be pretty consistent in stereo with tone wide open.

    Will try to do it, and post results.

    Attached Files:

  8. As promised, I am here to report on results of what I tried (as if someone want them).

    Output is indeed split when appropriate cable is used, and potentiometer is rotated to isolated position. On mono I noticed no hum/cracking/level drop when going over cut. So I call it a sucess. Will test it for running two amps when I get a chance.
    I added pics of my wiring to album.
    Yamaha BB415 wiring horror pics

    One last thing, warning to anyone who for some reason want to do the same. This will work only if you cut the traces to isolate pickups. If you use stock pot, you will get a mess.
    BlueTalon likes this.
  9. Papaschtroumpf


    Dec 27, 2017
    I didn't see an answer to the first question in that post and I had the same question (I read schematics just barely well enough to realize it's the "ungrounded" part in the title). I found this other post from Walter that explains why you want to leave them ungrounded: https://www.thegearpage.net/board/index.php?threads/add-blend-knob-or-volume-control.1680258/

    Basically grounded blend = tone suck, ungrounded blend = no tone suck but can't fully separate the two pickups unless you do the trace cut this thread is about.

    Hope this is helpful to someone to cross-reference the other thread to this one.
    PawleeP likes this.
  10. Papaschtroumpf


    Dec 27, 2017
    Somewhere I picked out this PDF with a zillion and a half bass diagrams, it's in German but not too hard to figure out.
    I wanted a blend but i also wanted series/parallel switching. There was a diagram in that book that used a 3PDT switch for "real" serial/parallel switching where the blend pot is taken out of the circuit when in parallel. I couldn't find a suitable 3PDT switch (the Fender S-1 almost seemed like a solution but seems fragile and very expensive)

    They also offer this diagram using a single DPDT (push-pull) that still gets you series/parallel but the way I understand it, in parallel the blend knob is a little screwy and controls how much of the bridge pickup is added to the parallel mix. I think you would only care if you want to quickly switch between series/parallel without messing with your blend. It's a compromise I'm willing to make for the novelty of having a parallel option.

    I reproduced the diagram below include my proposed changes in red (my pickups are around 5K so I picked 250K for V & T, pretending I actually understand those things, and picked a 100K MN based on info in this post)

    OK, on to my actual question: if I remove the ground lug from the blend pot (red X indicates no connection), will the series/parallel function still work?
    I think yes but based on my limited diagram reading skills, I would love it if someone would confirm.

    Last edited: May 4, 2018
    Jazzygreg, Arnel M. and 5StringPocket like this.
  11. Papaschtroumpf


    Dec 27, 2017
    Another question:

    I just want to verify I understand what the no-load pot described above. I think you are talking about cutting the carbon trace at one end of the track like the red line here?


    I added the red line to the pic, it is from another post on making no-load pots by opening the pot and using nail polish to disconnect the sweeper from the carbon trace at the end of the run. Your method simply avoid opening the pot, which with an MN pot is probably a good thing?

    My MN pot is a mini pot and i am quite worried about messing it up, fine mechanical skills are not my forte.

    If I don't do the zero load mod on my pot, will the fact that it's a 100K MN vs 500K MN make the blend less effective? I think yes because the no load mod is in essence to jump from an almost 500K (or 100K) to an infinite resistance so the value matters, so I will have 5x more of the unwanted pickup in the blend with my 500K? How bad is that? will I go from "you almost can't hear it" to "you hear it a little?"
  12. Yes, look at the "schematic" picture I made. You cut the trace where resistance is high, essentially making high resistance into infinite resistance. Just be careful, or better use multimeter in resistance measurement mode, because that high resistance may be on different side (one circle upper leg, another - lower, or both upper, or both lower) depending on how it is done mechanically by manufacturer. Measure three times, cut once.
    When I did this I was unable to do the cut without opening, so I both opened the pot and did the cut. It was fiddly, yes, but I wanted to take away the conductive strip dust left after the cutting any way. With nail polish be sure to do a "covered stop" so that viper is standing on nail polish in maxed position. Otherwise circuits are electrically the same, if you don't want to use those "dead" pins.
    Resistor value is more about the speed of blending. With 100k you have a long "knob arc" where blend is happening, because of comparetively small values of added resistance. With 500k you go through that useful values much faster, having short "knob arc" of usable values and the rest is practically the same (99.5 or 99.95 of some pickup).
    When values are low (50k, 25k or even 10k) you will not get full blend, because even on full turn pickup will still get through (without cut), or blend will have a sudden jump (e.g. 75/25 to 100/0).

    I will think on your schematic some time, maybe I can think of something. What helped me when I was doing mine was to paint it all in as mechanically as possible, like a pipe system.
  13. I think this will not work. Think of parallel mode (switch in lower position). Top pickup will have both terminals connected to blend circuit, it's ground will not go to the ground of jack at all.

    This sounds like a best solution for this. But you description is confusing. When in parallel blend should be in circuit, blending the amount of signal from each pickup. When in series pickups become "unified two-coil pickup", there is nothing to blend, just control volume of it with master volume. I can think of a circuit where each pickup has it's own viper of blend knob as volume, and that is in series, but it feels like an imagination stretch, with tone issues and other issues etc etc.
  14. Papaschtroumpf


    Dec 27, 2017
    Yes typo, I meant blend taken out when in series.
  15. Papaschtroumpf


    Dec 27, 2017
    I see that now. I cut the ground connection in the wrong place. I still need the P/P center pin to be grounded, but no connected to the blend (not at a computer now so I can't fix the diagram). The X should be between the P/P center pin and the blend and I think it would work as intended.

    Edit: in fact that's what I did when modifying the physical wiring diagram above.
    Last edited: May 5, 2018
  16. That might do the trick, yes.
    In this configuration, in series mode blend will act as a strange volume control, adding resistance in series from top pickup hot to jack hot - you will have to plan it in such way that top (picture-wise) potentiometer is zero resistance from middle to top pins in neutral position, possibly in "blending" position(s) too. And watch out for the cut, it might be that cut will cut hot completely.
    In parallel, I see how "zero resistance to 100k" in series is added from PU to jack hot. Maybe even remove connection between resistors in blend pot? This will be ungrounded blend, not sure for no-load though. Again, watch out for cut, it might add a break somewhere, and think twice to have zero resistance for series.

    Maybe we need 3rd opinion for this surgery.
    Or, if you don't have to drill more holes (I assume push-pull in volume makes this not necessary) assemble it on cardboard, wire together with alligator clipped wires to save on soldering, and test it.

    Edit: added schematic
    Edit 2:
    Even the cut might work... If cuts are done on both potentiometers closer to X marks, wiper is connected only to NC pin, it should not affect series mode and allow no (cross)-load between pickups in stop positions. Only downside that in "bottom only" pickup switching to series will disconnect whole thing. Otherwise - really good idea.
    Last edited: May 5, 2018
  17. vin97


    Mar 7, 2016
    Last edited: May 5, 2018
    ELynx, rufus.K and Papaschtroumpf like this.
  18. Papaschtroumpf


    Dec 27, 2017
    Thanks Vin97. I indeed missed this diagram if itiwas posted earlier. I need to compare with what I did, may end up being the same but nice to have a "known good" schematic.
  19. Papaschtroumpf


    Dec 27, 2017
    Wired it up and it seems to work, at least by tapping the pickups lightly with a metal screwdriver, I havent assembled the whole bass yet.
    I do have one issue: I didn't check the push-pull up vs down behavior. As diagramed it has series by default and parallel when the push-pull is up. I wanted it the other way around, with series the more "niverlty setting". I think I just need to swap the upper right connection with the lower right, and same on the left side to fix it?
  20. Papaschtroumpf


    Dec 27, 2017
    Switched the push-pull function around and assembled the bass. I like it, thanks all
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