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Looking for Help with Tobias - Bartolini TCT Wiring

Discussion in 'Pickups & Electronics [BG]' started by bucephylus, Dec 15, 2015.


  1. evilgus

    evilgus Supporting Member

    Feb 27, 2006
    Sydney, Australia
    Very interesting! I've had a few goes at fixing old Bart circuits in Tobiases and seen a fair bit of variety, but not that much! :) Also, I remember Mike or maybe Bob Lee saying that the early, un-marked MCT modules were 250 Hz. Now that they are available again, I've got spares in each value, but not the switchable 250/375/500 unit.

    EG
     
    bucephylus likes this.
  2. bucephylus

    bucephylus Supporting Member Commercial User

    Aug 18, 2002
    General Manager TecPadz LLC
    Here is an update, because I have now looked at and evaluated two issues that I had mentioned.

    First, the biasing resistors on the Mid pot. I just installed those yesterday, and the difference was instantly apparent. Really cleaned up the sound. All pan settings now produce much more useful tones. Why the original buyer or the shop decided to leave those out is beyond me. Huge improvement.

    Second, the A/P switch, Bartolini shows two different wiring configurations for the A/P switch option. The first is where the A/P switch is a Push/Pull on the volume knob:
    Bartolini A-P 1.jpg

    The second is where the A/P is a discrete switch (as installed by Mike & Co):
    Bartolini TCT 2.jpg

    If you study these, you will see that the difference is that the discrete switch calls for 150k ohm and 1.2M ohm grounding resistors on the active loop, whereas the switch on the Vol pot does not call for any grounding resistors on the active loop. I am guessing that the role of the grounding resistors is to eliminate spikes when switching?

    For reference please refer to my diagram in Post #9 above. You will see that I had wired the active loop directly through the center switch terminals and had left the strange RC network in place just as a fall back in case I was missing something. The way the switch worked with that scheme was active and active with the strange RC network.

    But, looking at the TCT diagram in the last image, I noticed that, in fact, the resistors in my strange RC network are actually the recommended resistors for the discrete switch. I still don't know whose idea the big capacitor was, but switching back and forth between active and active w/RC, I can say the capacitor was eating a lot of signal. So, I simply cut that capacitor out of the switching circuit. Here, I was thinking to test the active circuit against the active loop with those grounding resistors. And, the result is that the grounding resistors definitely suck tone and volume out of the signal. Not as much as the capacitor; but enough so that the dynamics of the bass are noticeably compromised.

    Couple points here, and there are reasons I am going into so much detail. First, I suspect there are a lot of Bartolini circuits out there with the discrete A/P switch. I know it was common at one time. Anyone with that switch might be wise to check if the grounding resistors are there, because my finding is that they degrade the tone of the instrument.

    Second point (or question) is whether or not those grounding resistors are really needed? I mean, if one only uses that switch in the event of battery failure (which I have never, not once experienced), who really cares if there is a chance for a signal spike? So, I could see keeping the switch and stripping out the grounding resistors; and I may do that yet. Why Bartolini recommends those resistors is beyond me.

    Anyhow, lots of improvements in the tone of the bass. I think I am almost done, and am confident the instrument now behaves the way Mike intended it.
     
  3. That parallel r/c is a weird one. What value is the r and the c?

    Actually many bassists use the active-passive switch a lot. A bad spike can damage speakers. Also a popping switch will freak out sound engineers and producers if they aren't expecting to hear it. And really, it's just good practice to have all the parts of your rig working correctly.

    Why those resistors aren't on the circuit board however is beyond me. Perhaps the circuit was never intended to work with an active/passive switch. When an audio circuit is biased for a single supply, there are input and output coupling caps. The preamp side of these is usually at 4.5v (or thereabouts), while the pickups/pots/jack side will be at earth potential. And this DC offset will cause pops with typical active/passive switching. Having a 1M2 on the input and a 150k on the output are entirely appropriate values for the job of bleeding the coupling cap's charge to earth. And shouldn't effect the tone.

    So if you remove these resistors, and there aren't any others already on the preamp board, the active/passive switch will very likely pop. There's no harm in removing them to try, just be sure to have your amp turned down quite low when you are testing.

    If there are already resistors on the board, you will register an ohmic resistance between the input/output and earth. If none, then your meter will give an infinite resistance reading. (Obviously, you'll have to remove the external resistors and set the switch to bypass to test this.)

    As for the resistors on the mid pot, well, without seeing the schematic, especially for that MTC-375 module, I'm not sure. The mtc-375 is clearly a passive network, because there's no +ve or earth wires going to it. I'm pretty sure the tct is based on a fender tonestack, so I'd think you could remove the 47k and the black 375 wire all together and it will function as a regular fender amp mid pot. IOW, won't do much. But I'd try it that way anyway. (You can leave the 27k there. With the black wire removed, it won't have any effect.) Then add the the 47k. This will change the sweep of the pot slightly, but more importantly will deepen the inherent mid scoop of the fender tonestack. (IOW, with the pot all the way clockwise, you should hear a slight mid cut when adding the 47K) Then add the module wire and listen for the difference...
     
    Last edited: Mar 17, 2016
  4. bucephylus

    bucephylus Supporting Member Commercial User

    Aug 18, 2002
    General Manager TecPadz LLC
    Thanks for taking an interest in this thread. Here are a few answers and comments.

    Yes, it is strange. I showed the cap markings and resistor values in the second diagram in Post #1. I am pretty sure the cap is 100 uF. Yes, I know that is (absurdly) large. But, since no one so far has been able to explain the idea behind that capacitor, the size is somewhat secondary, no?

    Really? I mean, I own and play a Fender passive Jazz Bass; so, I appreciate what passive basses can do. The Tobias, however, was designed to be played with an active preamp. You will have to perhaps trust me that the non-active Bartoini pickups by themselves, which is what the A/P switch was intended to provide, are not special, do not produce what Fender passives provide, have no other tone controls associated with them, and are almost hopelessly and pathetically weak. In short, these pickups without their preamp could never be anything but backup. I can't see any bass player choosing to do that as part of their "sound;" but, YMMV.

    Yup; no doubt; and, yes, that's the point of this project.

    I'm not sure how to respond to some of those comments, mainly because I really don't know what kind of devices are inside the potted TCT module. However, I find it odd that one of the Bartolini diagrams shows the grounding resistors on the switch, while the other doesn't show them. I'm also not sure why switching the preamplifier in and out of the circuit should inherently cause a pop. My Alembic Series I electronics do not cause a pop when the preamplifier is toggled off and on. But, as I said, we don't know what is inside the black box; so, maybe you are correct. Bartolini's comments, please refer to Post #8, agree with you.

    Ahhh; now, you have arrived at the MOST interesting part.

    "Shouldn't" is the entirely correct word to use. I get that, and that is clearly Bartolini's thought as well. But, please look carefully at what I did for re-wiring in Post #9. I apologize for the lousy drawing; but, the red rectangles are meant to indicate where I removed wires and the yellow and green wires are my adds. The changes effectively wire the switch out of the circuit by using the center posts as junction points, and removing the shorting jumper on the lower switch contacts leaving them open.

    As I stated earlier, I initially left that odd RC network on the upper switch position so that I could test it's effects on the sound of the bass vs having no grounding networks just by actuating the switch. Well, of course, the capacitor was messing up the tone. So, I cut the capacitor out of the circuit. Now I can switch those grounding resistors in and out of the circuit. Maybe they are not intended to affect the sound of the bass; but the empirical result is pretty much night and day. Those resistors definitely make the bass MUCH less dynamic.

    In fact, I am pretty much inclined just to leave the switch wired the way I currently have it. Putting the A/P capability back in place would be nice, since that was the original thought of the preamp layout, and I am to some extent trying to get the circuit back to what it should be. OTOH, my main concern with just removing the resistors is more that it might somehow internally damage the TCT module; and I am not interested in that outcome. OTOH, I am not going to compromise the sound of the instrument so that I can have an A/P capability that will never get used. So, I am kind of caught in the middle.


    Thanks for the suggestions. Based on EG's comment above, it seems likely that the MCT is actually a 250 Hz module. Otherwise, you are correct. I may think about testing you ideas down the road. However, two factors are probably going to cause me not to try further changes. First, those are the bias resistor values recommended by Bartolini; and while your hypothesis about the components in the module may be correct, Bartolini are not likely to recommend incorrect biasing values. Second, the addition of the biasing resistors has improved the tone to the point that I don't need to change anything. I now get a very credible and powerful p bass tone from the neck pickup, a very cutting staccato sound from the bridge pickup, and all the slap and plectrum sounds I could want from the balanced pair of pickups. In short, the Bartolini recommended bias values provide the tones I am after.

    Again, thanks for putting some effort into what has perhaps become a somewhat arcane thread at this point. I really appreciate your thoughts and enjoy discussing the project.
     
  5. Oh, I see. I doubt it's a 100uF cap. If it's rated at 1kv, you're obviously mistaken.

    It's probably 100nF and you've misread it. (I need a magnifying glass to see most part codes these days :bored:) Perhaps you could post a pic?

    If it is, then yeah it will roll off lots of tone. (Imagine a passive tone pot with double the capacitence and rolled all the way down.) Perhaps the original owner played lots of reggae? :thumbsup:

    Yeah, if there's already say a 1M resistor across the input on the board, adding another externally will halve the input z and, (after removing that cap!) you could hear a difference I guess. Just test it the way I mentioned.

    As for the 150k output resistor, well that would imply the preamp has a fairly low output z. As does the first bart diagram with a 25k vol pot.

    But that final diagram you've posted, has a 250k vol pot, which in active mode would act quite strangely, even more so with the 150K across it I'd think.

    Yeah, that's fair enough. I've come across those really low output barts a few times. And yeah a passive switch with those is pretty useless.

    Anyway, I'm glad you've found some good sounds with all your tweaks.

    If it was me I'd get rid of the mid pot, the a/p switch and probably the mid boost. Place a fixed resistor in place of the pot (the tonestack won't function otherwise), then put in another more flexible and modern mid control. The switch could be used to select frequencies, or perhaps run the pickups in series.
     
  6. bucephylus

    bucephylus Supporting Member Commercial User

    Aug 18, 2002
    General Manager TecPadz LLC
    Sure! (My eyes have seen better days as well; LOL.
    Capacitor for Web.JPG

    LOL; ya think?

    Certainly, not knowing what is "behind the curtain" makes it a pretty big guessing game.

    And, that's what I think a lot of people ultimately decide, which explains why the number of early Tobias basses in their original configurations is becoming small. In this case, I think I'm pretty close to what Mike intended; and the bass sounds great. So, I'm good with it.

    Thanks again!
     
  7. evilgus

    evilgus Supporting Member

    Feb 27, 2006
    Sydney, Australia
    Out of interest, what pickups are in the bass? The drop-shoulder Barts? They are usually pretty high output, but some of the later variants are much lower.

    EG
     
  8. bucephylus

    bucephylus Supporting Member Commercial User

    Aug 18, 2002
    General Manager TecPadz LLC
    Yup. Drop shoulder.
     
  9. ex-tension

    ex-tension

    Jun 11, 2009
    Last edited: Mar 19, 2016
  10. bucephylus

    bucephylus Supporting Member Commercial User

    Aug 18, 2002
    General Manager TecPadz LLC
  11. actually I was thinking it looked like 100p. a lot of small value caps are just marked in picofarads.

    but I would suggest measuring it. a 10 or even 100p cap wouldn't make anything like the tonal difference you've described.
     
  12. ex-tension

    ex-tension

    Jun 11, 2009
    Yeah, me too, I thought it's a 100pF. But it looks like if there is a tolerance letter right next to the value, the third number is the multiplier.
    If it said only '100' instead of '100M' it's would be 100pF.
     

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