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Capacitor values that work well for PJ Bass with Dual Vol/Tones?

Discussion in 'Pickups & Electronics [BG]' started by Antisyzygy, Dec 29, 2015.


  1. Antisyzygy

    Antisyzygy

    Dec 8, 2014
    Washington
    I am considering trying out a mod on my G&L SB-2. I can always revert that mod easily enough. I'm not an expert but I'm good enough with a soldering iron.

    I think I might benefit from having a tone knob available, so I figured why not make the controls looked balanced and go with a dual stacked V/T configuration like on a vintage Jazz bass.

    To that end, I had a question, and also was wondering what value of capacitance might work well for each pickup.

    For the neck pickup I was thinking a 0.022uf with a resistor in series to keep the tone pot from having the resonant bump occur when rolled off to 0. This is basically exactly like a SB-1s tone pot. I have a SB-1 and I like the way it works so I'm copying that configuration for my neck pickup. It also can serve as a backup then, as I use the SB-1 and it's tone pot a lot.

    However the bridge pickup I am not sure what would work the best. Maybe I should keep the resonant bump for that one? Maybe I should use a low-valued cap to bleed off the hum and give a honkier tone when rolled off? I'm not sure. What do you guys think?

    Also, does adding dual Vol/Tones cause the pickup interactions to change? I would like to preserve the scoop when both pickups are full on.
     
  2. It's far better to experiment than to try to philosophize about what capacitance to choose. Try a bunch of different values and settle on the ones that work best.
     
  3. Antisyzygy

    Antisyzygy

    Dec 8, 2014
    Washington
    Hey thanks.

    I just figured there might be some preset cut-off freq. that works well with bridge pickups. I'm not sure.

    I know for sure what I want on the neck since I have the SB-1 I tried out myself.

    Do you know if a dual V/T setup would ruin that mid-scoop that occurs when both pickups are full on?
     
  4. It depends how you wire it. You can wire the standard way, but then the tone controls won't be independent of each other. To isolate the tone controls from each other, you need to put a resistor in series with each signal path, but this changes the way that the pickups will load against each other when mixed.
     
    iiipopes and Antisyzygy like this.
  5. iiipopes

    iiipopes

    May 4, 2009
    When the Jazz bass was a new instrument in 1960, and had the stacked knobs, the factory put a .047 on the neck pickup as its tone cap (well, .05 in the older nomenclature) and a .033 on the bridge pickup as its tone cap (likewise, .03 on the older nomenclature), along with dual stacked 250 kohm volume/500 kohm tone pots. A little known fact is that there are a couple of resistors in the original circuit that are there to try to isolate each tone cap. If anyone is going with dual V-T, it's hard to beat the original. Here is a link that talks about it more close up (the picture of the controls is about 2/3 to 3/4 the way down the scroll:
    Stack pots for a 60's Road Worn Jazz Bass?
     
    Leo Smith and Antisyzygy like this.
  6. Antisyzygy

    Antisyzygy

    Dec 8, 2014
    Washington
    I don't know why I didn't think of the interacting part. It makes sense having two tone knobs in parallel to ground that they would interact. Capacitance sums in parallel if I remember correctly.

    The other problem is if I don't want the tone knobs interacting then I lose the scoop, which is one thing I didn't want to lose. I didn't want to completely change the way the SB-2 gets it's tones by balancing the pickups, I just wanted to add a tone knob on top. I thought it would be cool to have one for each pickup. That doesn't seem doable now that you guys cleared that up for me.

    Though to be honest, I'm not sure if that original Jazz design would be better than allowing the volumes to interact. I guess I'm not sure how decoupling the stacked pots affects the tones you can achieve.

    I dig this suggestion from your link :

    3) The best trick I came up with was to make one of the tone controls the master tone control and wire the other to be a bass cut control for the bridge pickup. This sticks a 4.7 nF cap in series with the bridge pickup and removes some lows. You lose the scooped tone of a standard jazz bass though.
    I'm curious though. Why would you lose the scooped tone? I mean couldn't you get rid of the resistors that decouple the volumes and still do this?

    Thanks!
     
    Last edited: Dec 29, 2015
  7. iiipopes

    iiipopes

    May 4, 2009
    To keep scoop, keep the now-standard V-V-T wiring. The scoop comes from the inductance of both pickups interacting, and the tone pot dimed. To have flexibility on the tone rolloff, has the OP considered having a push-pull tone control, with, say, a .033 in one position and the option to add (with DPDT push-pulls, it can be either on the push or the pull) to have a .01 or .015 as a secondary capacitor in parallel to switch the rolloff frequency? So one position emulates a traditional .047 tone cap on the pot, and the other position has the lesser value so that more mids come through the mix when the tone is rolled off a little bit?
     
  8. Antisyzygy

    Antisyzygy

    Dec 8, 2014
    Washington
    Yeah, I think I need to keep the mid scoop somehow. I didn't want to lose the base functionality of this bass. It's configured Vol/Vol for a reason I presume, i.e. intended to let you exploit that scoop as needed.

    Also I just like having it. I do tend to keep the bridge pickup at about 90% when it's on but I like having the mid-scoop available sometimes.

    Some other options I thought of are :

    1. Master Vol / Master Tone
    2. Neck Vol / Bridge Vol

    ==================

    1. Master Vol / Master Tone
    2. Bridge Volume

    So the P pickup is always at 100% relative to the J

    ==================

    1. Neck Volume / Bridge volume
    2. Master Tone / Master Passive Bass-Cut

    Some G&Ls have a passive bass cut, so maybe it will work OK.

    ==================

    1. Neck Volume / Bridge Volume
    2. Master Tone / Cap Selector

    The cap selector I was thinking could have two caps that are active at either extreme of the knob setting. I'm actually not sure if that works, I'd have to write out a diagram. I'm really an amateur when it comes to electrical stuff, I only understand the basics from a circuits 101 class in college.

    I have a tone-pot that is a push pull. When down it's a standard 0.047 uF. When pulled up and at 10 it's a mid-boost (actually I think it's a treble cut, but with a low-valued cap that has a higher resonant freq. so it sounds like it's boosting something). At 0 it's a mid-scoop so I think it's feeding the signal through an inductor. I can see there is a transformer on the pot. I attached some pics.

    Anyway, I'd like to stick to dual stacked knobs just for visual consistency. I'm just trying to think of something that doesn't remove the both-pickups-on-mid-scoop but gives me some more EQ control. For treble-cutting I'd prefer to stick with at least one 0.022uF tone control setting just to be able to mimic an SB-1.
     

    Attached Files:

    Last edited: Dec 29, 2015
  9. iiipopes

    iiipopes

    May 4, 2009
    I forgot to add that the picture I referred to in the link to the basschat.co.uk thread is what line6man talked about in his post above.
     
  10. Why not just use a blend pot? It is electrically identical, but much more practical to adjust. With the right modifications, you might be able to make a concentric knob set fit over a single shaft, to keep with the aesthetic.

    This works for some people. It's practical if you never favor the bridge pickup, but impractical otherwise.

    This would be difficult to pull off, because you want a 1M Ohms pot for the bass cut, but that value is not appropriate for a standard passive tone control. You would need to buy two different concentric pots, and then swap their wafers.

    This is not possible unless you want to settle for a push/pull switch to choose between two capacitances. I'm not aware of any company that offers a stacked pot with a pot and a rotary switch.
     
    Antisyzygy likes this.
  11. Antisyzygy

    Antisyzygy

    Dec 8, 2014
    Washington

    Good points. Thanks! I didn't think about the 1MOhm pots thing. I actually copied a G&L schematic one time to add a bass roll off to some project I was working on, and I still plum forgot that's how it's done.

    In that post from IIlpopes, I think they mentioned something about a bass roll off just for the bridge pickup? Would that change anything? Here's the quote :

    3) The best trick I came up with was to make one of the tone controls the master tone control and wire the other to be a bass cut control for the bridge pickup. This sticks a 4.7 nF cap in series with the bridge pickup and removes some lows. You lose the scooped tone of a standard jazz bass though. SOURCE
    I don't understand why you lose the scooped tone, I'd intuitively think it would make an even more scooped tone but intuition can be wrong. However that sounds like it might be neat to be able to control the bass on the bridge pickup. The SB-2 really seems designed to be favoring the neck position. I've never been able to get a pleasing sound out of the bridge by itself.


    I have this weird tone pot I mentioned further up (there's pics too). It's a push/pull and when pulled up it seems to change between two different tone caps at either end of the rotation. I think on one end it's an inductor and a capacitor, then on the other it's a capacitor. The two capacitors and inductor must be interacting all the time, but I think the pot diverts most of the signal through one side or the other depending on where it's at in the rotation. I might be totally wrong, that's just what I thought at first. I haven't really tried to trace the wires.

    Anyway, I bet you get tired of folks like me having all these crazy ideas you've probably seen go wrong 100 times before! Take care, and thanks.
     
    Last edited: Dec 29, 2015

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