1960s Jazz Bass - Isolated Dual Stacked Vol/Tones - How does it affect tone?

Discussion in 'Pickups & Electronics [BG]' started by Antisyzygy, Jan 1, 2016.


  1. Antisyzygy

    Antisyzygy

    Dec 8, 2014
    Washington
    So the 1960s jazz bass had a pair of stacked/concentric knobs where each one controlled the volume and tone for a separate, individual pickup.

    The two pickups and their tone/volume circuit were electronically isolated by using some resistors. This was done to prevent the tone knobs from interacting, as for example without the resistors if you rolled down your bridge tone knob it would also bleed treble off the neck pickup's signal.

    This will get rid of that mid-scoop that occurs when both Jazz pickups are on 100%. I like the mid-scoop, so in my book it's kinda a trade-off to get the separate tone circuits to not interact with each other.

    To that end I was curious, how does the electronically-isolated pickups change the sound? Meaning if both pickups are on 100%, what does a dual-concentric Jazz bass sound like in comparison to a modern Jazz bass (with the mid-scoop) with both pickups on 100%? Even educated guesses based on how the signal should be affected theoretically is fine with me.

    I've found some reviews for 1960s Jazz basses on Youtube but NOBODY tells you what settings they're using so I have no idea what I'm listening to.

    FenderJazzBassConcentricPot.jpg


    Thanks so much!!!!
     
    Last edited: Jan 2, 2016
  2. Wish I had a $ for each time I've read this. But nah, the resistors in the original jazz circuit are there to isolate the volume controls. Before opamps came along, this is more or less how old mixing desks worked, and also how Leo summed the channels in his twin amps. When Leo realized you could wire the pots "backwards" (IOW, with the pickups to the wipers), he went with a single tone pot. Maybe because he knew it was pointless having two tone pots now, and/or it was cheaper!

    If you really want isolated tone pots with a passive circuit, instead of a couple of 220K (or was it 270k?) resistors summing the wipers to the jack, switch the pickups to the wipers (like the normal VVT circuit) and sum the clockwise volume pot lugs with much smaller resitors. Around 15K is usually about right.

    There's no need to guess. It's very simple to wire up a test rig with a pot and a switch to find the sweet spot for isolation resistors. That's exactly what I did many years ago when I was trying to understand blending and pickup interactions better. I can show you if you like, but yeah something around 15K (from memory) seems to be a nice value, and the difference is barely audible.

    Be aware that the re-issued VTVT stacked-knob jazzes didn't have the original 60's circuit. (And for a good reason - it sucks!) It is just a variation on Leo's clever reversed pot VVT, but with the tone pots on the pickup side of the circuit. I'm not sure why the reissues don't have a couple of small resistors to isolate the tone pots. Perhaps they just didn't think people would care...
     
    Antisyzygy likes this.
  3. Antisyzygy

    Antisyzygy

    Dec 8, 2014
    Washington
    I guess that's why spreading misinformation is so nefarious! I was just told and/or read that enough times that that's what I thought the resistors were intended for.

    So pickup hot to center lug, then resistors to the clockwise part from there? I guess what I'm not understanding is where the resistors go to from there. I'm not that great with electronics, I took a course one time. I've wired up basic stuff is all, like a series/parallel switch or a new tone pot.

    I don't have all the parts to wire up a test VVT setup at the moment but I'm interested in learning more. I do have a P-bass harness I ripped out of a bass so I think I'd just need a second volume pot.

    Thanks!

    Long story short, I was thinking about doing this to my G&L SB-2 which only has two volume pots. It kinda sounds like a bad idea the more I learn about it from generous folks like you.
     
    Last edited: Jan 3, 2016
  4. +1. The way that Leo did it was pretty bad, and you can get much better results using lower resistances.
     
  5. They were intended to isolate the volume pots, but with the added bonus of giving you isolated tone pots. The 220k resistors that Leo used in the original scheme sort of killed two birds with one stone.

    The resistors sum the signals to the jack.
     
  6. OK, here's the 1960 circuit.

    1960 VTVT.jpg

    Without the two 220k resistors, if you turned one vol down, the output is short to earth. IOW, either vol acts as a master vol. The losses, both in treble, and overall output are pretty high. As I mentioned, it's entirely understandible that Leo designed this circuit like this, because early mixers used this method to isolate the channel faders. And the Fender twins summed their channels like this as well, to isolate the each channel's vol pot.

    Here's the now standard VVT circuit:

    Standard VVT.jpg

    I'm not sure if it was Leo who had this idea, but it was pretty clever really. The pickups are now connected to the vol pot wipers, so neither acts as a master volume. It's certainly not a perfect circuit, but probably the best compromise for passive VVT.

    Here's how I'd wire VTVT:

    RobbieK VTVT.jpg

    It's just like the 1960 diagram, but the volume pots are wired like the standard VVT. This means that you can use much smaller summing resistors to isolate the tone pots.

    Like any component choice in a high impedence passive circuit, choosing those summing resistors is a compromise. If they are zero ohms, IOW a short, or simply not there, you get the fender reissue circuit, and the tone pots will interact when the volume controls are both maxed. If they are too large, you start getting the problems of the 1960 circuit. I haven't wired this for years, but I seem to remember that 15K seemed to be the best choice for the summing resistors.

    A jig like this will help you decide:

    VTVT test jig.jpg

    A dual gang pot is used as the two summing resistors, and the switch simply shorts the two resistances so you can switch back and forth. With the switch closed, the pot is instantly wound all the way down to a short. IOW, you have the reissue circuit. This way you can wind up till you get a nice amount of tone pot isolation, then short the resistors to see how much volume you have lost. (You'll have to tap the pickups with a screwdriver to hear/test the tone pot isolation.) When you find your sweet spot, disconnect it from the circuit, measure the resistance (with the switch open of course), then choose a pair of fixed resistors that are closest to this.

    If you add an opamp to the 1960 circuit, it works fine:

    active mixer.jpg

    This is an adaptation of "active mixing". This is how most modern mixing desks sum their channels. It looks like the 1960 circuit, but the summing resistors feed an inverting opamp with the same sized resistor in its feed back loop. (The pots, summing resistors, and feedback resistor should all be the same size. 240K is the closest common resistor size to a 250k pot.) The gain on this opamp configuration is set by the resistance to the Vref in conjunction with that feedback resistor. IOW, the gain changes with movements of either vol pot, automatically making up for the losses. Clever huh?

    This is only one way to bias this circuit. It shouldn't be too hard to wire one of the pickup wires and the tone caps to the Vref (instead of earth as normal), but if its too much of a hassle then a coupling cap could be used for each channel. The volume pots have to be connected to Vref. You could also bias the battery to +/- 4.5 and have earth as your Vref, but you'd need to find a switching jack instead of a stereo one as the battery negative won't be switching to earth anymore.

    FWIW, in practise, you'll never find a mixing desk circuit with such huge value faders, summing and feedback resistors. Typically these are much smaller (around 10k) to keep the noise floor low. But with only two channels (pickups!), this circuit is fine.
     
    Last edited: Jan 4, 2016
  7. Antisyzygy

    Antisyzygy

    Dec 8, 2014
    Washington
    Thanks so much!
     
  8. tortburst

    tortburst

    Apr 1, 2007
    New Jersey
    Le sigh
    I wish I saw this post before buying a stack knob harness built to the 60s spec with the 220k resistors.

    I did that because I've read things on several boards that state "the bridging resistors are required to isolate the interaction of the tone controls" but this reduces the output.

    As soon as I put it in I started wondering if there was a better way.

    Unfortunately I'm not sure toy smart enough to understand all that was said here.

    Would I be able to re wire my harness with a little knowledge and the right parts?
     
  9. Antisyzygy

    Antisyzygy

    Dec 8, 2014
    Washington
    I'm by no means an expert but I've put together a few simple harnesses before. Most of the time I order a premade custom harness if it's anything real complicated. The main thing to do is learn to read schematics and learn to solder. Hackerspaces or community college circuits courses might help.

    I took circuits 101 and a logic circuits course in college and I can't design things worth a crap since I didn't take it far enough, but it taught me to solder things together and how to read basic schematics. Really LRC circuits are most of what you need to know, and that's introduced early on. It's all I remember. I can't remember anything about opamps which are in active circuits.

    You might consider finding a harness maker on Ebay or something and sending them the schematic you want. They're around. Sometimes they can make solderless harnesses so you only need to use a wire stripper and a screwdriver to install the harness.
     
    tortburst likes this.
  10. yep, you should be able to rewire the pots and caps you have. but you'll need a couple of 15k resistors. these will be just afew cents from an electronics shop. buy 1/4 or 1/8th watt metal film. get your tech to follow my third diagram.
     
  11. tortburst

    tortburst

    Apr 1, 2007
    New Jersey
    Ah I thought I was to follow the first


    My goal is to have the stack knob jazz harness with each volume and tone working independently, with as little loss as possible.
     
  12. Antisyzygy

    Antisyzygy

    Dec 8, 2014
    Washington

    This is the one he shared that loses the least output (was my understanding) :

    RobbieK VTVT.jpg
     
  13. tortburst

    tortburst

    Apr 1, 2007
    New Jersey
    Ok good to know.
    What's the 3rd one for?

    This is all very fascinating and I want to wire up a couple of these to see for myself how they sound and behave.

    What I would need is a diagram that translates these circuits into a picture of the actual harness(s).

    I have enough skill to solder stuff together and get in trouble, so I can copy what I see. I managed to add coil tapping to a g&l l2000. This is cool.
     
  14. smeet

    smeet Supporting Member

    Nov 27, 2006
    Woodland Hills, CA
    Interesting. But I don't see how this would lose the mid scoop when both pickups are up. The phase relationships/cancellation of the various harmonics between pickups is still the same, and that is where the scoop comes from...?
     
  15. tortburst

    tortburst

    Apr 1, 2007
    New Jersey
    noob questions here

    Ok I'm getting better at seeing how this is the 1960 circuit diagram above..

    proxy.php?image=http%3A%2F%2Fi591.photobucket.com%2Falbums%2Fss360%2FGrissle209%2Fimg002copy.png



    And I see how this is the standard parallel VVT setup as we know it


    What I don't see is how this VTVT diagram is like the 1960 diagram with the volume pots wired like the standard VVT

    It looks the same to my untrained eye. It runs through each tone and volume independently before passing through the resistors then joining at the output.

    Im probably missing some subtlety about where the resistors are joined.

    And does this achieve the goal of each volume and tone working completely independently with minimal output loss?

    Thanks
     
  16. Notice the resistors in the original are connected at one end to the vol pot wipers (middle lug). At the other end they are joined and this junction is the output signal (that goes to the jack).

    The original VVT has the pickups connected to the wipers, and the output signal is summed, without resistors, from the clockwise lugs of the vol pots. (Note the tone control is on this "master" output.) This is not the way pots are typically wired as volume controls in any other audio electronics. But in this application, it's cool because it (more or less) isolates the action of the volume pots. With two tone pots, you have to wire them directly to the pickups, but with the volume pots maxed, the pickups, and tone pots of course, are simply summed in parallel. So the tone pots are not isolated. IOW, either will roll down the tone for both pickups. This is exactly how the reissue VTVT basses were/are wired.

    For whatever reason, it's very common around here for people to ask about which VTVT, the original or the reissue, was better. The original just happens to have isolated tone controls due to the way the volume pots are wired and therefore summed with 220K resistors. The reissue has that same bright tone and healthy output that the VVT has, but the extra tone control is redundant when the volume pots are both maxed.

    My circuit is simply the reissue circuit, but instead of summing the clockwise vol pot lugs directly, they are summed with 15k resistors. This is (depending on the pickups) enough to isolate the tone controls, but with virtually no volume or tonal loss.

    One problem is that most guitar techs can't read electronics schematics. This is potentially as limiting to them as a musician who only reads tabs not musical notation.

    My scratchy hand-drawn diagrams are based on standard electronics schematics symbols. I'm afraid this is how I think about and visualise circuits, so you'll just have to translate! :help:

    [​IMG]
     
    Last edited: Jan 25, 2016
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  17. tortburst

    tortburst

    Apr 1, 2007
    New Jersey
    Q: with your circuit, would the user still experience VVT syndrome if one volume pot was at 100% and the other less?

    Q: how do you know which lug to use based on the diagram? Is it the position of arrow?

    The VVT setup is interesting b/c it looks like it uses the wiper/output lug as the input, and the input lug as the output, passing it down the chain from pot to pot to output.

    i appreciate the reply

    i'm going to have to digest this a little at a time

    first things first

    this is what a wiper is
    potentiometer-construction-1317165052_500_344.png

    I started reading up on this topic here
    Guitar & Bass Wiring Techniques

    diagram from that article
    [​IMG]

    The wiper is the output terminal, or middle lug.

    I think the reason lots of people ask about the jazz bass stack knob harness is b/c it's confusing. You look at the knobs and think "oh cool, each pickup has it's own isolated tone control"......BUT NOOOOOOoooooo....
    • Which circuit utilizes both volumes and tones independently and has the least output loss?
    • Is there a circuit that utilizes both volumes and tones independently, or do you have to choose btwn crosstalk btwn volumes or crosstalk btwn tones b/c you can't stop both at the same time?
    • What is cross talk?
    • Why does VVT syndrome happen and how can you prevent it?
    • Does VVT syndrome come into play with the tone pots in certain dual concentric circuits?
    • Why am I doing this to myself?
    I bought a 61 stack knob harness and was surprised that I got a hum when both tone knobs weren't in the same position which led me down this path of trying to learn why, and see if there's a better way.

    I have a foggy idea of what you've said here but I'm going to have to do more research and stare at the harnesses a bit more.
     
    Antisyzygy likes this.
  18. what is VVT syndrome?

    The arrow is the wiper. In these schematics, the anti-clockwise lug is the one connected to earth. On published schematics you often see some extra annotation like lug numbers, or an arrow, or just a little statement to indicate which way the pot will travel, when it's not otherwise clear.

    Please don't take this the wrong way, I mean no disrespect, but if you have a background in electronics, the passive circuits in guitars are very simple. (Although the maths going on is surprisingly complex.) Even the circuits in active guitars are very simple. Although speaking personally, something I love is pure simplicity in design - using nothing more than what is needed. But that's getting OT for sure.

    If you have a re-issue VTVT bass, and you want isolated tone controls, simply back both volume controls off ever-so-slightly. This means the signal is summed through a couple of small resistances and is essentially the same as what is going on in my circuit.

    The silly thing about all this is that isolated tone controls, especially in a jazz bass where both pickups are very similar, is almost purely academic. With both pickups maxed, if you roll down one tone control, I bet you can't tell by ear which pickup is having its treble reduced. The only time I think it is useful is if you like quick changes of tone and balance say, mid-song, and this could potentially reduce the number of knobs you have to touch.

    My circuit. (The 3rd schematic from post #6.)

    There are many. Mine, the original VTVT, VBTT with a log-antilog blend pot, VBTT with an MN blend pot plus summing resistors, the fifth schematic from post #6 (theoretical active mixing), and any number of other active circuits/buffering etc could do this.

    I'm not sure. As an electronics term, I only know it as one used in a different context. Crosstalk between stereo channels or channels in a mixer. I'm guessing you are simply refering to "interactions" that happen with the controls of a passive guitar circuit. Without the extra electrons supplied by a 9v battery, passive pickups and their controls (plus for that matter, your cable and the input components of your amp/pedals/di etc) are all going to "interact". Thinking of these components individually is hopeless. They are an electronic system. (Circuit, really).

    Yeah I've never heard this expression. What do you mean exactly?

    That's not the harness. It's more to do with single coil pickups.

    If by research, you mean reading internet forums, then this and staring are both wastes of time. You need to get an iron, a meter, and a cheap two pickup bass to mess around with. Where I live I have the advantage of three or four electronics shops close by, and pots, caps, jacks etc etc are very cheap. You really need to get your hands dirty.
     
  19. ChristoMephisto

    ChristoMephisto

    Sep 4, 2011
    Saw on another post from maybe this forum or another, but they suggested an inline capacitor between the bridge pup and the volume pot to help isolate it from the other pup and prevent a volume drop when at full. They were referencing a Rickenbacker with the .0047, and suggested a .01 for a jazz bass due to the pickup location. Maybe larger imho to make sure there is no bass loss.
    The tone pot would be before the inline cap also. Not sure if this would work or not, perhaps one of the gurus here would know more on this.
     
  20. tortburst

    tortburst

    Apr 1, 2007
    New Jersey
    Thanks for all of that.


    VVT syndrome is layman slang for the inability to blend the two pickups if one is 100% and the other less then 100%.
    Ex: front pickup at 100%, rear pickup at 90%, I only hear the front pickup. The rear pickup is gone.
    "Why does that happen?" , and my purchase of a stack knob harness, has led me to this thread.

    You touched on that in your response about independent tone controls in the reissue circuit, and the need to back off the two volumes a bit to creat resistance.

    Ok I'll give this one a try. I need smaller resistors though. I only have the 220K ones that are in my 1960 style harness.
    I guess I could try reconfiguring it your way with the larger resistors for now.

    I was probably using crosstalk incorrectly to describe how I perceive the various components interacting.

    No offense taken, this is a new subject for me entirely.

    Only thing I lack is the meter. I'll need to get one of those.

    Now this I don't understand.
    If both pickups are at equal volume and there's no single coil hum, then why would I get single coil hum from adjusting the tone knobs to unequal setting? Is it b/c the output changed?
     
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