SB-2 and blend pot

[jglunt2112]

A while back Ken was kind enough to share with me his wiring diagram for a blend pot on an SB-2. I tried that. In fact, I tried it using blend pots with audio taper, linear taper, and MN taper, and I didn't care for any of them. (No fault of Ken's.) Eventually I restored it to stock--just two volume controls. To make a long story short, I had a bad experience last weekend with our sound person and have decided I'd like to try either: (1) running a concentric pot with separate pickup volumes into a single pot as master volume, or (2) try the blend pot experiment again. Since I have a host of blend pots laying around, maybe I try that first. I found this odd diagram that shows an unusual way to wire an MN taper blend pot. Perhaps the most unusual thing is using a linear taper volume pot (and supposedly that is not a typo). The second most unusual might be that the blend pot uses no ground at all. Others claim they tried it and it worked well, but it seems weird to me. Could I please get some feedback about this?

jazz bass ungrounded blend.jpg (23.22 KiB) Viewed 4047 times

[Ken Baker]

Other than to post the original SB-2 diagrams, I really have nothing to add. I've never seen an ungrounded blend.

https://www.bassesbyleo.com/paul/g&l_bb ... iagram.pdf

https://www.bassesbyleo.com/paul/g&l_bb ... iagram.jpg

Ken...

[jglunt2112]

Thanks, Ken. I have never heard of any pot being intentionally not grounded either. This came from a 7 year old post from Talk Bass, and some strange things can get posted there, but some good things as well. The whole purpose of this was to get a smoother blend with full volume from both pickups at center detente. Word is that when the pot is ungrounded, even when the blend is moved all the way to one side, the opposing pickup is still faintly on. I can see how that would be true. The solution he recommended for that was cutting the traces on the ground lugs of the blend pot. I'm too ignorant to know exactly what the traces are, and I don't know enough about the actual workings of a pot to know what good that would do. This guy (Walter W., from alpha-music.com) swore it worked to silence the opposing pickup. He also said it wasn't a necessity if you didn't normally play specifically with one pickup or the other but blended the two. I found it odd as well that he recommended a linear taper volume pot. In my limited experience of being my own guitar tech I've never seen any instrument maker recommend a linear volume pot. I emailed him a couple days ago and asked why a linear volume pot, but I've received no answer so far.

Any ideas on using a concentric stacked pot for individual pickup volumes and running that signal through a master volume? TDR1138 actually suggested that to me a while back, but I haven't tried it.

[Ken Baker]

Any ideas on using a concentric stacked pot for individual pickup volumes and running that signal through a master volume? TDR1138 actually suggested that to me a while back, but I haven't tried it.

A 250KA dual-concentric pot for stacked volumes works well; really no different than the pair of single pots. I've not done this with a master volume, but with a tone pot (250KA pot and .047µF). A volume pot should work, though you may see some mild weirdness due to cumulative resistance. Doing it with a volume and a tone pot would be worthy of an experiment, but may have you deciding which to keep - volume or tone.

Ken...

[jglunt2112]

A volume pot should work, though you may see some mild weirdness due to cumulative resistance.

I'm more than mildly weird, so maybe the mild weirdness you refer to wouldn't be readily noticeable. But just in case, can you describe "mild weirdness?" If I move ahead with this, should I look for a 50k or 100k concentric pot? Or go with a 250k concentric and maybe a 50 or 100k volume? Or just keep one of the 250k volumes with a 250k concentric and see how it goes? And all audio taper, I assume.

[Ken Baker]

Mild weirdness. Dang! I got myself into another one, didn't I?

As I understand such things, You'd be adding the resistance of one of the pots to the "bone stock" total resistance of the control circuit (coils don't count here). What you may hear is a slight treble bump.

As to pot value, the guy who designed the bass and its pickups used 250KA.

Ken...

[jglunt2112]

Yeah, that feller sure knew a lot. Those of us in the music business owe him so much! That sounds wonderful, Ken. I will go accordingly. A slight treble bump shouldn't be a problem. The SB-2 has a lot of umph. (That's a technical term used only by us more-than-mildly weird folks). With this setup, I can always use the concentric pot to find the sweet spot for pickup balance, then tweak my overall volume as needed with the master volume. Thanks! This will be a fun experiment. I'll let you know how things come out.

[bigtone23]

I have a bass I wired with a 250K M/N ungrounded blend. It works perfectly. You can also drop the value of the blend down to 100K, as it will have a wider, more usable sweep. The effect of the lower value on loading is pretty minimal.
My 2 cents on linear vs audio taper volumes: linear is, by a long shot, my preference for VVT or basses played with a cleaner tone. You also get a wider, more usable sweep. Also, in a VVT, you can get more detailed moves with blending.
Audio taper is my choice for instrument played with distortion and you need to roll back the volume to clean up the signal.

[jglunt2112]

I used audio taper, linear taper, and MN taper blend pots, but all in 250k. All of the blend took place in about 10% of the movement of the pot. The only thing the different tapers did for me was move that 10% spot around to different places. I read about running the MN taper ungrounded, but I wasn't exactly sure what that meant and how it would affect hum, so I don't remember trying it. (All this was a number of months ago.) My blend pot experiments have frustrated me. I had a Steinberger Synapse that had master volume/blend/treble/bass on it from the factory, and the blend pot worked great. But that bass had active pickups and a preamp, and the blend pot is 25k. I recently picked up a concentric A250k pot to wire the individual pickup volumes to, then go from there to a master volume. Just to hedge my bet, I also ordered an A100k pot to use as the master volume if one of the A250k pots seems wonky. If this experiment fails, I might see about picking up a 100k MN taper and wire it in ungrounded. Thanks for your recommendations!

[jglunt2112]

I finally got around to experimenting with a concentric volume/volume control on my SB-2, running into a master volume. It actually works very well, much better in my opinion than the various blend pot experiments I had tried. And it keeps the integrity of the SB-2 being a bass without a tone control. With this setup, I can use the concentric pot to adjust individual pickup volumes, adjusting the tone, then use the master volume to adjust the volume without any drastic tone change. I used a 250kA concentric combination and just used one of the original 250kA pots as a master. The only unusual thing I noticed was that the volume of the bass seems to have increased. I don't see how that could happen, so it was probably "just my imagination runnin' away with me." For me, a master volume on a bass is a real plus. This experiment worked well for me.