Ultimate No-load Ungrounded Blend Pot (Pics!)

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


  1. walterw

    walterw Supportive Fender Commercial User

    Feb 20, 2009
    alpha-music.com
    after talking about the "ungrounded blend" thing for a while now, i finally got around to snapping some pics of one while i was installing it!

    this is a 250k M/N pot that i've also modded to be "no-load" by cutting traces, so that at each extreme the "off" pickup is entirely disconnected. (i figured out that even with no parallel ground load path, at the ends of the travel each pickup was still being loaded by the other pickup's path to ground, so the 250kΩ of the pot plus the 8kΩ or whatever of the "off" pickup.)

    instead, with the traces cut this pot really is just like a 3-way switch in the middle and at each end of its travel, no signal loss at all.

    the sweep is smoother than a typical 500k grounded blend, too; i'd love to try this with say, a 100k M/N for even smoother shading between pickups, but i don't know if that's really a thing.

    start with a little x-acto saw like this to cut the trace on the one side:
    IMG_0678.jpg

    imagejpg1-1.jpg

    flip the pot over and do the same thing:
    IMG_0682.jpg

    you know you've got it when you can turn it all the way up or down and the resistance from the center to the output side goes from short to almost 250k to "open" on the wafer of the pickup you want "off".

    here's the jumper across the output lugs of the two wafers:
    IMG_0684.jpg

    (i ran it around the back to get it out of the way.)

    and finally, here are the outside connections, two pickups in, one output wire, and done!
    imagejpg1.jpg
     
    Last edited: Aug 13, 2014
  2. JustForSport

    JustForSport

    Nov 17, 2011
    Thanks- gotta try it!
    Clean job!
    (coupla px of the cut traces would've been nice, but every pot's diff, so...)
     
  3. walterw

    walterw Supportive Fender Commercial User

    Feb 20, 2009
    alpha-music.com
    edited to highlight the little cut on the pot.
     
    Last edited: Aug 13, 2014
  4. JustForSport

    JustForSport

    Nov 17, 2011
    Ah, Yes-
    didn't realize you meant that trace (of course you said nothing about the carbon tracks inside):
    My oversight...
     
  5. Thanks for posting this. I had installed a 500K MN blend a few weeks ago (grounded) and found that all of the blend was taking place in a narrow range in the middle. So I had ordered a 250K MN and was just waiting for some free time to see if that was an improvement. I saw your post tonight and used a needle file to cut the traces just like in your picture (and ohm meter to verify when I had them cut) and it works a real treat. Very, very smooth blending, you can hear the change of the second pickup coming in pretty soon as you start to rotate away from full lock. Awesome tip!
     
    Last edited: Aug 14, 2014
  6. I don't wanna steal Walter's thunder, but this mod won't really change the way the blend happens. (Compared to a regular MN, wired without grounds that is.) I believe the mod is more about giving you true solo'd pickups at each end of rotation. I would guess the improvement in your blending action is more to do with changing to a 250K pot than anything else. But I'm glad you now have a working blend control!

    You could try making one. If you disassemble a dual gang 200/220/250K linear pot, you could try painting half the track in conductive paint. Not the sort you do shielding with, I mean something harder than this. (Years ago I bought a tiny jar of this silver paint you use to repair car window demisters to experiment with this exact thing. It worked but was a bit crackly.) Or perhaps you could cut a tiny crescent of copper tape to stick over half the track. Obviously this is for testing only.

    You can also join dual gang 16mm pots back to back pretty easily as well. I found a hex circuit board stand-off that fits exactly into the bottom of the wiper in these pots. Cut to the right length and epoxy'd in place, you can then solder the casings together or just super glue them together. Then you cut off one of the shafts so it fits in a guitar route. With two 250K MNs joined like this, you could wire them in parallel for a 125K MN.
     
  7. walterw

    walterw Supportive Fender Commercial User

    Feb 20, 2009
    alpha-music.com
    Right, the blending "speed" is the same with the traces cut, that's indeed just about getting the solo'ed pickup truly "solo".

    using the lower-value blend pot ungrounded is what slows down the panning, with the price being that the "off" pickup is less "off". cutting the traces solves that problem.
     
    Last edited: Aug 14, 2014
    PawleeP and MattZilla like this.
  8. CrashAlpha

    CrashAlpha

    May 16, 2012
    Montreal
    Great thread... can someone tell me: I was looking to install 1MB vol pots on my bass for my VVT P/J SD setup, I am looking to recover highs. If I go for a VBT setup, should I go for a lower rated blend pot, like 100K?

    Also, do I understand correctly, that a grounded blend is going to sound more "modern"? I'm looking for NOT-vintage tone.
     
  9. walterw

    walterw Supportive Fender Commercial User

    Feb 20, 2009
    alpha-music.com
    if you can find a 100k M/N blend pot, tell me where! left stock and ungrounded, it probably wouldn't be enough to make your "off" pickup go away fully, but if that was fixed by cutting it for no-load operation it might be just right.
    if by "modern" you mean brighter, then no, just the opposite. grounding the blend is what causes it to darken the sound. if you ground the blend, even a 500k blend pot will affect the brightness a little.
     
  10. CrashAlpha

    CrashAlpha

    May 16, 2012
    Montreal
    Wow - you're right, I can't find a 100K M/N - just AC, which I understand has a signal loss problem at center.
     
    Last edited: Aug 28, 2014
  11. back in the 70's or 80's I would buy (at Radio Shack) an alps dual audio taper 100K and then get a reverse audio taper and swap the back of the dual. It wasn't quite as good as the MN you can buy now, but it was pretty close (I think center position was something like 85K and 15K)

    It was a long time ago ... now I think about it seems like it only came in linear taper and I had to dismantle it (tricky) and replace both wafers with one audio taper and one reverse audio taper. I do remember needing to carefully grind the end of the shaft so I could slide the wiper off to replace the inside wafer. It was a project but there was no such thing as an MN taper back then (at least not that I knew of - I figured I was inventing this thing - but almost every time I ever invent something I find out it already exists somewhere).
     
    Last edited: Aug 28, 2014
  12. walterw

    walterw Supportive Fender Commercial User

    Feb 20, 2009
    alpha-music.com
    yep; as far as i can tell you built yourself an "A/C" blend pot, which indeed leaves some resistance in the center.

    it's been pointed out (i think by robbieK) that the A/C and its small load in the center can be a good thing if you want the tonal effect of isolating the pickups from each other somewhat, and don't mind the signal loss.
     
  13. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    Wow, I missed this when it was first posted but this is just what I have been looking for! I am trying to get a "no load" position for the P pickup in my P/J bass and while I knew you could modify a blend pot to do this I had no idea it was possible to do this without opening up the pot, which many have done successfully but it could potentially lead to a busted up pot. So I was about to replace the blend control in my P/J with a five position, two pole rotary that would give me no load soloed pickups, plus both, plus two positions where the blend was set with external resistors but if this mod is this easy on the blend pot in my bass then I am going to have to try this. I'm not sure if this can be done with every blend pot and the switches aren't terribly expensive but if this works it is free....
     
  14. walterw

    walterw Supportive Fender Commercial User

    Feb 20, 2009
    alpha-music.com
    i tried that a while back (well, a variation anyway, this exact thing was my suggestion but the customer didn't want solo'ed pickups so we went with a 3-way rotary with the middle three choices), and what i discovered was that the resistor value wasn't a fixed thing!

    depending on what i plugged the bass into, sometimes it would have that perfect "one pickup slightly loaded by the other one" sound and sometimes it would just sound like one pickup by itself.

    i suppose the input impedance of the various things i plugged it into was affecting that balance; if so, the answer would of course have been an onboard preamp, so the load the pickups saw never changed.
     
  15. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    Yeah, that could be. The Fender preamps I have measure about 100k input impedance so they definitely load the pickups more than the supposed industry standard 1 Meg amplifier input will. Most external devices claim to be 1 Meg but how many people verify that? Truth be told I find the blend settings fairly subtle compared to the one/the other/and both positions of a standard blend or VVT setup. A pure switched setup would do me pretty well, that is all I have on my Squier VI and I can't say that I miss the ability to finesse the blends on that instrument. At the same time I hate to give up a capability that my other basses came with. I guess the fear is that some day I might REALLY want to have an intermediate blend, even though that seems rather unlikely. Oh well, not everything we do has to make sense!

    I have gotten as far as modifying the blend pot on a spare preamp I have. It was as easy as you say and I don't have an Xacto (or do you say exact-zero?) saw so I just used the tip of an Xaxto knife with a number 11 blade. I'm not sure if my pot was different than the one you photographed or if you were just unable to get a photo of the exact place where you had to make the cut but the resistance element in the pot I modified was buried deeper inside the pot housing than the photo you posted implies. It was easy to reach inside with the tip of the knife blade and scrape the element enough to sever the connection to the ground lug of the pot. It really did not take much scraping. I was able to use a stereo microscope at work to do the surgery but I suspect that you could do it without optical aid if you were careful.

    I have not had time to try it yet. This is my month to play at church so I can't be modifying my main bass right now. I am going to convert an old fretless Jazz I have to a "deluxe active jazz bass fretless" using that spare preamp so that will probably be my first bass to use the modification to the blend pot. Right now I am waiting on Digikey to deliver some other parts I need for the work and I also need to get a "Reggie Hamilton" volume pot for it since it will have an active/passive switch and it would be nice to have a volume control in active mode. I'm also going to replace the single coil pickups with Fender Super 55s so it could be a while before I get all this done. I think the results will be good. It is a rather beat up old bass that I bought used on TB but it plays nicely so I have a warm spot in my heart for it and I think the modifications will make it even better.

    You know you can't get 100k blend pots easily but you can get 25k blend pots meant to be used with some active pickups. You might try setting a blend pot you have modified to the point where you just start to hear the other pickup starting to become present in the blend as you turn it up and then measure the resistance at that setting. Some tests and simulations I have run suggest that 25k is more than enough to cut a pickup to effectively zero in the blend but other ears may disagree. If it works for your ears a 25k blend pot would give you a lot more useful rotational range as you are trying to get that blend just right.
     
  16. walterw

    walterw Supportive Fender Commercial User

    Feb 20, 2009
    alpha-music.com
    i'd already tried one of those with high hopes, but i found that it mostly sounded like "both pickups on" until it hit one end of the travel, where it abruptly switched to one pickup when it hit the cut trace.
     
  17. walterw

    walterw Supportive Fender Commercial User

    Feb 20, 2009
    alpha-music.com
    OK, after some prompting i finally made a diagram of the whole thing, done in my apparently now traditional goofy MS paint style :)

    jazz bass ungrounded blend.jpg
    it also works just fine without cutting the traces, you still get the smooth blending between pickups and you still avoid extra loading loss from the blend pot; the only drawback is that when you're cranked over to one pickup you still get a slight bit of the other, "off" pickup in the mix. (you don't really hear it though)

    if you're a player who mostly shades between the two pickups rather than running either one solo, then this will work nicely without cutting the traces on the blend pot.
     
    Last edited: Oct 10, 2017
  18. Growlmonkee

    Growlmonkee

    Jan 30, 2013
    Florida, U.S.
    Walter, you're a gem!, I think you solved exactly what I do not like about 2 pickup blend pots, I'm gonna try it on one of my own basses this week, the one with the no-load tone pot, that I often play with tone on 10.
     
  19. Callused Finger

    Callused Finger

    Feb 22, 2007
    New York
    Thanks for posting this. I was looking for info on this just last week.
     
  20. vin97

    vin97

    Mar 7, 2016
    Germany
    After a lot of help from walter via PM, I completed the VBT conversion on my fretless jazz today and I am very happy with the results.
    I used 250k pots (linear volume, audio no-load tone, M/N unground no-load blend) as suggested by walter.

    The volume has no more effect on tone and the solo pickup sound is the same as before (except even louder harmonics on the bridge PU).
    Obviously mixing the two pickups works much better now and in my opinion, this is how all passive basses should be built.

    ....Next on my list is shielding, although it looks to be a lot of work.
     
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    Primary TB Assistant

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