Stacked tone and Volume on Jazz Bass?

I have done this on a bunch of basses and am considering building them up to sell on our website. I really like the versatility of controlling tone of each pickup independently, others don't seem to mind the three knob setup. I'm doing my MIJ re-issue as my next stack knob project.

I think the CTS stacked pots are fine but I disect the 250/500K and make them both 250K. You don't have to do this but the 250K tone pot with a .047uf capacitor gives a tone roll off I like.

The three hole plates are readily available but cost more than the standard four hole plates. I buy the Fender ones through Allparts because I am an Allparts dealer so that saves me some money.

I wire mine up like the original stack knobs with a couple extra resistors in place that eliminate the tone control cross talk between pickups. If you don't do this turning down one tone pot will affect both pickups which is a pain and the most common reason people don't like the stack knob setup. Done right though the tone controls act 100% independently.

 

Could you describe this in a bit more detail (specifically where you put the resistors, and what values)?

 

Yeah, I'd be very grateful for the info regarding the addition of resistors to eliminate tone control cross-talk between pups!!

Si.

 

What!! You guys want me to give up my secrets!!

Actually, it's quite simple. Just insert a 200K to 300K resistor inline between the output of each volume pot and the jack (2 resistors required). Don't ask me how this works, it just does. I got this from looking at the original wiring of an early sixties stack knob jazz. The downside to this mod is a very slight drop in output but it's not a big deal IMHO.

Somewhere I have a picture of the original wiring, I'll see if I can find it ...

 

subscribed for that pic

 

OK, I couldn't find the original 62 photo but here is a photo from one of my P+J stack pot basses. Notice that one end of the resistor is soldered to the pot lug that is normally wired to the jack. The other resistor in the background is coming from the same lug on the other volume pot. They both terminate at the white wire which in turn goes to the hot side of the jack. So, essentially the resistors are in series with the output side of the volume pot. Once again, I don't totally understand why this prevents tone control crosstalk but it does. Someday when I have time I'll figure it out.

 

Many thanks for the infos and pic

Si.

 

I think its because the signal now has to travel through TWO resistors to get to the other pickup's tone pot. It doesn't eliminate, but it reduces crosstalk twice as fast as it reduces output volume.

 

If you do send me a PM. I'm interested.

Thanks!
Rich

 

Beautiful soldering work, jwymore!! Wish my work looked that clean.

 

I just got the stack knob setup from fender, the kit has everything in it, only I cant make heads or tails of the diagram. It looks like wires soldered in 2 places. is there a better wiring diagram out there,

 

They are called "summing" resistors, and the way they work is that they provide isolation between the two stages. Essentially, it creates a two-channel passive mixer. Summing resistors are used in all passive and active analog mixers. As pointed out, you do lose some gain.

 

If you add 250K in series with each pickup, you will get a bit of roll off. Here's a quick simulation. Does this match the sound?

 

i cant see the pic from the inside of the bass!

 

yeesh, that looks like a lot of signal loss just for the "benefit" of two tone pots.

 

Hi,

I was very interested by the response curves posted by Dave, comparing stack knobs with/without resistors.

=> what tool did you use to get these curves ? Any hint on where I could possibly get it ?

Thanks for your help

Cheers,
Pablo

 

Yes, I would like to know more too. Mainly what are the units on the LH side of the graph? Db? Millivolts?

What one could summize from the graph is that the resonant peak of the pickup is affected by the resistor which I am not hearing when I A/B the two setups.

 

The circuit is what is called a "passive mixer". The resistors are in series with each pickup and keep the signals from each from talking to the other one. Note that when both volumes are in the full on position without the resistors both pickups are shorted to each other as are the two tone controls. The resistors make sure you've got a few hundred K ohms between them even full on.

There is a catch here. And that is that in a passive mixer there is always some loss. And the amount of loss depends on TWO things. The first is the size of the two series resistors and the other is input impedance of you amp. Note that amps tend to vary widely in that parameter which doesn't seem to be exactly standard. Some are as high as 1 meg and others as low as maybe 100k. My Fender BXR 200 is about 200K. So note that your series resistor forms a voltage divider with the input resistance of the amp. Note that so long as the series R is somewhat smaller than the input Z of your amp, the drop in output will only be a few dB and won't be significant. However if your amp has a lower input as some do, you may need to lower the series resistor values slightly to keep the losses lower. And note that too much series R will cause much greater roll-off of highs due to cable capacitance. It's best to keep the series R values as low as you can get away with to get rid of the cross-talk in the tone controls.