Roscoe Beck V + OBP-3 + rewire + questions

Discussion in 'Pickups & Electronics [BG]' started by hamder, Oct 31, 2005.

  1. Disclaimer, love the bass as is : just want to improve with onboard tone controls which I
    grew used to during my Stingray years.

    Quick Roscoe Beck overview.
    Fender Jazz bass V with dual pickpus, has 1 volume, 1 tone with push-pull which reduces output of neck
    pickup with a certain % (think its 15) but only when neck and bridge are both in, 3 way switch to
    choose between neck and bridge PU's, dual 3-way switches to run PU's in serial, parallel or single coil.

    What I would like to do is to rewire the bass so that it is totally untouched tonewise
    when pre-amp is bypassed, to do that I was planning to do the following (on a new pick-guard):

    - Add the pre-amp to the loop after existing mix, volume and tone.

    - Move the volume & tone to a dual pot, 250-taper stacked with 250k-linear.
    (assuming taper on volume, linear on tone...?)

    - Replace the 3 way switch with a 5 way, is currently "Bridge", "Bridge + Neck" and "Neck", with a 5 way
    it will be "Bridge", "Bridge + Neck-15%", "Bridge + Neck", "Bridge-15% + Neck" and "Neck".
    This is to preserve the unique tone push-pull switch the Roscoe beck has, yet saving one swtich.

    - Add OBP-3 bypass switch.

    - Add OBP-3 Bass and treble in one stacked pot.

    - Add OBP-3 Middle control in one 50K linear, center detended, DPDT on-on push-pull potentiometer.

    Have some questions though :

    Can I just add the OBP-3 after existing circuit and tone control (wiring the bypass as described by aguilar)
    or will the old passive tone control mess things up ?

    If I cannot get hold of a 250k linear/taper stacked pot, what would be my best option to get close,
    take two linear or two taper ?

    Wonder if anyone has actually ever found a 50K linear, center detended, DPDT push-pull pot for the OBP-3 ?

  2. mahrous


    Aug 13, 2005
    the Aggie actually likes to have 25k pots not 50k and they are linears.
    Aggie comes with 3 25k linear pots for bass, treble and mid and a DPDT toggle switch.

    this is how i am gonna have my Aggie wired into a bass i am building (7stringer)
    Volume - Blend - Bass - Treble - Mid - Mid Variable Freq Selector - Active/Passive Switch - Parallel/Series/SC for each pup.
    i dont see the absolute need to find something as rare as linear/log stacked pot ... infact, i thought the RBV has humbuckers - function better with 500k pots.
  3. Ok, I have the pre-amp in shipping, haven't actually seen it yet, all their wiring diagrams shows 50K linear,
    so this is a small surprise.

    Somewhat similar project you got, except no tone. My goal is to have as few buttons as possible, the RB
    has plenty already :smug:

    It has 250K originally, do you know what a change to 500k would do, soundwise ?
    (its my understanding that the RBv has very low output PU's)

    Is it ok to use "log" on the tone control as well, I guess it will just lose treble at a slower rate
    at the beginning and more at the end of rotation?
    (rarely uses tone but I would like it to be still kind of useful)

  4. mahrous


    Aug 13, 2005
    my bad!
    humbuckers dont necessarily function better with 500k pots. its pretty much personal preference there. in general less resistence pots attenuate more treble and high frequencies. so 500k will give u more open sounding pups there. i dont think it directly relates to the output of the pups, but i will let any expert correct us at this.

    log or taper audio pots just change the tone at a steady rate rather than Linear. Which has a steep curve at the beginning and at the end of the rotation.
    stacked pots are easily found on and other similar websites like etc etc. i believe most of them are only Log/Taper Audios which i personally prefer. it gives me more focused control on every slight turn on my tone/frequency knobs.
  5. In regards to log/linear, the end results should be the same, 0 resistance at one end, and whatever
    value resistence in the other end, its just how you get to the end points, right ?
    (think I'll just go with a dual log then, for passive volume/tone)

    Never quite understood why the value of a volume pot could possibly affect the tone...
    In my mind the volume pot when dialed all the way up is applying 0 resistance, and when dialed all the way
    down applies all of its resistance 250k or 500k, if a 250 is replaced with a 500, that should just mean that
    silence is reached sooner....?
  6. mahrous


    Aug 13, 2005
    yes 0 resistence or full resistence are reached on their ends.

    but the curve is steep at the beginning and at the end. meaning as you near the end or the beginning of the dial, ur tone changes more aggressively compared to Log pots which have stable change as you turn the knob.

    no the silence isnt reached sooner. i dont have a 100% correct answer, but all i know is that it affects the tone and the frequencies that are passed to your output jack.
  7. Finally done, ended up really well, the preamp is very transparant until you start turning the knobs,
    then it really comes to life; in particular I enjoy the treble center freq (6000 I think)...sparkling.

    Pic :


    Hard to see, but theres 2 * stacked knobs for vol/tone, bass/treble and a black bypass switch
    next to the 5-way switch.

    Options on this bass is endless now....:)

    Mahrous, thx for sparring. On the subject of passive tone control, it consists of a pot and a capacitor,
    is this how they work ? :

    Pot controls how much signal is run through capacitor, capacitor
    acts as treble filter, when pot value is increased
    less signal is run through capacitor and treble increases, when pot value is
    decreased aproaching 0 more signal is routed through capacitor and is
    filtered thus treble is turned down.....?

    That would explain why a 250 / 500 k tone pot change would make a difference .
    (I think there is definately more high end treble in the passive setting now after changing the vol/tone pot)
  8. jongor

    jongor Supporting Member

    Jan 11, 2003
    How's is sound?

    Sound file?
  9. bowthing


    Feb 16, 2007
    Where did you get the extra pick guard?