Pot load question

[garfo]

I've been using the Bourns mn taper Blend pots. These work really well and let the pickups ring at their most without feeding the treble into a blackhole.
Having said this, I've noticed that when using a 500k Blend pot there's a constant load of around 250k no matter where the pot is located (probably because it's two 500k pots in parallel).
Since my bass has humbuckers, what Volume pot value should go after the Blend pot, 250k(same load as the coming from the blend pot), or 500k?

[RobA]

I've forgotten why it does it, but you are right that it is a 250k load. If you trace the schematic for the wiring, you can see how it happens pretty easily.

I've got a blend pot on one guitar followed by a volume pot. I went with a 500k pot and it sounds fine this way. But, this guitar has the Duncan P-Rails in it and I tend to use them as either P-90's or as the rails. When I do use them as hum buckers, I do it in the parallel mode and not the series mode. I find the series hook up to be too dark and muddy. I think that's a problem with the pickup though. It was the same way when I had it in a guitar without the blend pot. But, it could be that I don't hear the loading of the 250k as much because I rarely use a pickup setting that isn't happy seeing 250k.

Another thing I did was used a switch pot for the volume. I use the switch to take the volume pot out of the circuit entirely so that it won't load the pickups further than the blend pot when I don't use the volume pot. I have it setup so that switch in is bypassed since that's how I normally use it.

I'm pretty happy with this setup. But, I just bought the parts to change it. I realized that I could use one of the MN blend pots to do pretty close to the standard Gibson two volume pot just by wiring each section as a normal volume pot. So, I'm going to put a 250k blend pot in that way and then go to a standard SG type switch. If, I used the standard humbucker mode of the pickups more, I'd use the 500k pot this way and it would then only load the pickups as a 500k pot would. I won't have a master volume with this setup, but that doesn't matter to me very much because I almost always have the current one switched out unless I'm jutting the guitar. The MN pot will act as a normal volume pot when the toggle switch is set to an individual pickup.

Anyway, ignoring all that, I'd try it with a 500k master volume pot first.   

[garfo]

Well, but if the load is already set, putting a 500k pot won't change the load to 500k, I believe it will still be 250k, and that's the tricky part. anyways, I'll try both and see what I get.

[RobA]

I think the load the pickups see is more complicated. I'm assuming there's nothing between the two pots, especially no active elements. The total load the pickups see is going to depend on how you have the blend pot setup. If it's floating (not grounded) then the total load the pickups see will be the portion of the blend pot they see added to the master volume's resistance. If the blend pot is in the grounded configuration, it's going to be a bit complicated. But, if you just look at two normal volumes one after the other, it'll look close to the same. In that case, the total load the pickups see is R_t + 1.0/(1.0/R_b + 1.0/R_vol), where R_t is the top portion of the first pot and R_b is the bottom portion of the first pot and R_vol is the resistance of the master volume. So, the total load varies from being the load of the first pot to the load of the two pots in parallel. If you used two 250k pots, it would vary from 250k to 125k. I think it's going to be weirder than that though because in the real blend pot situation, the loading is going to vary differently for each pickup in the sweep of the blend pot. With all of that going on, the 500k might be a bit better.

Were you doing this all as part of an active system? If so, then putting a simple voltage follower between the two pots will decouple them and remove any questions. In an active setting, I'd buffer between the two, use a 10k to 25k volume pot and then put a final buffer on the output to drive the cable. You could do the two buffers with single transistor stages and probably be good enough with minimal current use.   

[garfo]

No RobA, The pickups will first see the blend pot(ungrounded because it sound fuller to me)and then a master volume. Only after that I will have an on/on switch for passive/active circuit. Both pickups are humbuckers, one wired in series, and the other in parallel.

[RobA]

OK, so if I remember the no-grounded version of the blend pot right, when you are in the center position of the blend pot, then both pots see 0kΩ from the input of the pot to the output. So, the volume pot is going to look just like a standard volume pot. when you are all the way to one side or the other, one pickup sees 0k and the second sees half the pot value (125k for a 250k pot and 250k for a 500k pot). So, one pickup is going to see just the volume pot and one is going to see a modified voltage divider that has the 250k (as an example with the 500k MN pot) added to whatever is on top of the pot. So at full volume on the volume pot, the pickup that is turned off at the blend pot is going to see a voltage divider with 250k on top and 500k on the bottom (if you use a 500k pot for the volume). So, using the 250k volume pot would make the blend pot work better, but it would make the series wired humbucker darker sounding. Maybe it would be too dark with the series wound humbucker but OK with the parallel. It probably depends on what you prefer.

I guess I'd start by trying the 250k volume pot because that'll maximize the blend pot's ability to separate the two pickups, but if that's too dark for the series wired humbucker, I'd try the 500k pot.