Why the Poor PU Mix w/Passive V/V/T Pots?

[Bongolation]

I once read a good explanation of this on another site, but I've forgotten it completely, and it seems like something I should know.

You know what I'm talking about: Two pickups, two volume controls, one tone control...the mix/volume adjustment is always goofy unless they're both full-on or one's full off.

What's that all about?

[JTE]

Define "goofy".

jte

[Jim Carr]

I recall reading about this, but seem to have forgotten all the details.

In any case, in any passive Jazz, the T pot is a low pass filter. The Stack Pot model allows two such T filters. Yes, the latter is harder to tweak on the fly, but is theoretically more flexible. You should be able to get a wider variety of tones from the Stack Pot model.

Yet there are trade offs with each. The problem with the stack pots, IMHO, is that the amount of energy in the upper partials is low anyway, so the effect of diddling a filter 10% on an partially "turned down" signal from (say) a "50% on" neck pickup is usually pretty marginal, IMHO.

Thus, the blunt filter of VVT and the more subtle Stack Pot have their differences. Even though Stack Pot is more flexible, it is only a little bit more flexible in reality, and a lot fussier to adjust on the fly, IMHO.

Some time ago, I turned to Fralin split coils, and though they sacrifice some highs and lows, they are more amenable to having either V at any setting. Now, my approach is to find a handful of settings that work on the instrument, and run with those, with as little setting "ideology" as possible.

One last contrarian thought: VVT often requires I move 3 knobs, each in a different location. Stack Pots let one move 4 pots from 2 locations. That seems like a possible advantage if you want to change everything during a performance (while playing). In reality, I find all I usually want to do is change one pickup and/or the tone pot, assuming I have gone to what I think is one of my "handful of settings." All IMHO, YMMV, etc.

[msiner]

Originally Posted by JTE

Define "goofy".

jte

The goofiness that I get is as follows. My bass is a P/J. When the P is set to 10 and the J to 0 you get what you would expect, but when you gradually start to increase the J you get almost no effect until you reach 10. This is the same behavior with the P at 0 and J at 10. However, if the P is instead at 9, the J will mix in much nicer as you increase the volume. I hope that makes sense. Basically, if either pickup is at 10, you can't effectively mix the outputs.

EDIT: I rewired everything with new pots and cap according to a wiring diagram from Seymour Duncan and I still get the same behavior. In the future, I will probably be going Vol/Blend/Tone.

[butchblack]

bump

[JTE]

OK, that goofiness is partially a result of the taper of the pots. Most guitar electonics parts are pretty low tolerance so the taper and the actual value of the pot can vary by a LOT (say ± 10% or 15%). And the PUPs interact with each other and the pots, so you have a lot of variables.

The stack-knob set up originally used on the production Jazz bass that Dr. Jim talks about would seem to give more flexibility, but they don't. The original circuit had bridging resistors in it to help keep the tone pots from interacting, but that cut the output. So when CBS started the Vintage Series, they left those resistors out and the tone knobs interact. So, if you have the neck PUP all the way on, and the bridge PUP anywhere except all the way off, the tone control for the bridge PUP will cut the treble on the neck PUP.

They all react in certain goofy ways no matter what you do. A V/Blend/tone set up has the effect of adding yet another volume control to the circuit along with the attendant loading of the PUPs. That's because a proper blend pot works just like the two volume controls on a typical Jazz bass, except they're on one shaft. So, if you start with your blend set to neck only, as you roll it up to the center, it does nothing to the bridge PUP, but turns up the neck PUP. At center both PUPs are on full. As you continue rolling it towards the neck PUP, it starts rolling off the bridge PUP. So the PUPs see the same two volume pots that a regular V/V/T rig has, but then we add a master volume too.

The only way to effectively eliminate this goofiness would be to have a true mixer amplifier built into the bass. But that's a whole 'nother set of issues.

Me, I like V/B/T. Probably because I played basses with only one PUP from 1976 until 1988 (Precisions and StingRays) so I'm used to one volume control. All my two PUP basses except one have blend pots. The only reason I haven't done that to the Guild is that I'm too lazy to mod it since I don't play it very much.

jte

[Bongolation]

Originally Posted by msiner

The goofiness that I get is as follows. My bass is a P/J. When the P is set to 10 and the J to 0 you get what you would expect, but when you gradually start to increase the J you get almost no effect until you reach 10. This is the same behavior with the P at 0 and J at 10. However, if the P is instead at 9, the J will mix in much nicer as you increase the volume.

Yep, that's pretty much it. They flat don't work right. 90% of the adjustment is in the last 5% of the pot, plus there's a wierd little tone artifact going on with the volume mix as well.