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Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

Anatomy of the Fender TBX Tone Control

Discussion in 'Tele-Technical' started by Phostenix, Jul 31, 2010.

  1. Phostenix

    Phostenix Tele-Afflicted

    Jul 28, 2010
    Phoenix, AZ
    After reading lots of differing & conflicting explanations of how the Fender TBX tone control works, I decided to take apart my Strat & find out. This is a '94 Strat Plus with Lace sensors. I don't know if there are variations of the TBX. I desoldered the resistor & cap so that I could get accuarate measurements.

    So, here's the deal. It looks like a double stack pot with 1 shaft (not dual concentric), but the pots are NOT standard pots by any definition. There is a detent position at the center of the pots' rotation. Fender only uses pins 2 & 3 of the pots (the wiper & the terminal on the clockwise end of the pot). I think it's best to think of these pots as 3 separate functions - 0-5 on the knob, the detent postion in the middle, and 5-10 on the knob.

    Measuring between pins 2 & 3, the bottom pot (with the shaft up) is a standard 250K audio taper pot from 0-5 on the knob (CCW end to just before the detent position). At the detent position, it goes open (like a no load tone pot). From 5-10 on the knob (the detent position to the CW end), it remains open & out of the circuit.

    Measuring between pins 2 & 3, the top pot (with the shaft up) is a 0 ohm connection from 0-5 on the knob (basically a solid wire connecting the pins). At the detent, it remains 0 ohms. After the detent from 5-10 on the knob, it is a 1M ohm linear pot.

    I've made 3 schematics of the 3 phases of the tone control.

    0-5 on the knob before the detent, it looks like a standard Fender tone control circuit with an 82k resistor added to ground:


    [​IMG]



    At the detent position, the standard tone control is removed from the circuit due to the bottom pot going open at the detent, but the 82K resistor to ground (in parallel with the pickup(s)) stays:


    [​IMG]



    5-10 on the knob after the detent, the 1M linear pot comes into the circuit in series with that 82k resistor. As you turn the knob, the added resistance reduces the effect of that resistor's load on the pickup(s) until at 1M, it has almost no effect. I found that removinging the TBX control (with a switch) did provide a little more brightness than even the full CW setting of the TBX, though.


    [​IMG]


    I find that I usually like the TBX set at just beyond the detent (6 on the knob), so I might try increasing that 82K resistor a little.
     

  2. Phostenix

    Phostenix Tele-Afflicted

    Jul 28, 2010
    Phoenix, AZ
    Here's a pic of the TBX layout.

    [​IMG]
     

  3. 1962guitargeek

    1962guitargeek Friend of Leo's

    Oct 9, 2008
    eastern n.c.
    I recommend the Doc's mod for the TBX. I can't explain it well, but it makes the control much more usable, IMO...

    linky
     

  4. Phostenix

    Phostenix Tele-Afflicted

    Jul 28, 2010
    Phoenix, AZ
    Interesting mod. Thanks!
     

  5. 1962guitargeek

    1962guitargeek Friend of Leo's

    Oct 9, 2008
    eastern n.c.
    Hey Phostenix


    Here's some text I read when I stumbled across this mod some time ago. I should have posted it with the pic....

    linky
     

  6. Phostenix

    Phostenix Tele-Afflicted

    Jul 28, 2010
    Phoenix, AZ
    I remember reading that a while back when I was trying to get info on the TBX. I don't think I'll have time today to draw up the modified circuit he made, but it makes sense to me. I'd be curious to see what it sounds like. Up to the detent position it ends up roughly equivalent to a tone circuit with 120 ohm pot (instead of the standard 250k) & a .022uF cap. At the midpoint, it's a fixed 220k resistor & the .022uF cap. After the detent, the 1M pot adds in series to the fixed 220k & .022uF cap, reducing its affect.
     

  7. DenisS

    DenisS Tele-Meister

    Age:
    61
    436
    Nov 27, 2009
    USA
    Good anatomy. Keep that knowledge flowin'. Thanks.
     

  8. Phostenix

    Phostenix Tele-Afflicted

    Jul 28, 2010
    Phoenix, AZ
    TBX Mod Schematics

    OK, I went ahead & made the schematics for the blueguitar mod linked to above.


    From 0-5, it acts like a standard tone control with a 220K resistor in parallel with the pot, reducing its value to about 120K. That should make the guitar sound darker than a standard tone control.


    [​IMG]



    At the detent position, the 250K pot is removed and it's a 220K resistor in series with a .022uF cap to ground - pretty much equivalent to a standard tone control turned all the way up to "10".


    [​IMG]



    After the detent, a 1M pot comes into the circuit in series with the resistor & cap, reducing its effect on the guitar tone until it has almost no effect at 10.


    [​IMG]


    So, the mod should (by my analysis) give you a tone control that is darker than normal from 0-5, about the same as a normal tone control turned all the way up at the detent, and brighter than normal (to almost a no load pot) from 5-10. Sounds like a mod worth trying.
     

  9. bluesax

    bluesax NEW MEMBER!

    1
    Aug 1, 2010
    Tempe, AZ
    Hello Phostenix,
    New to the forum and new to guitar electronics but I have a question for you. I'm trying to bring back a Strat with a dead middle pickup and I noticed today that it has one of the TBX tone pots. I'm assuming that it's wired correctly (it couldn't affect the pickup) but I'm looking at your picture and have a resistor question. My tone pot is exactly like yours except the resistor is reversed. Does that make any difference? Also, mine has a spot on one end that looks like a tiny brown 'bubble' if that's the right word. How can you tell if a resistor is bad? And, last one, this is an 82K resistor, right? I see in some of your other posts you talk about a 220K. Sorry for the simple questions but I'm just learning. Thanks.
     

  10. Phostenix

    Phostenix Tele-Afflicted

    Jul 28, 2010
    Phoenix, AZ
    Resistors can go in either way, so that's not a problem. I'm not sure what you mean about the brown bubble. Resistors can go bad, you would need to check it with an ohm meter (or a digital multimeter). To be sure, you'd need to de-solder one end of the resistor to measure it. The "stock" resistor in the TBX is 82K - the rings would be gray - red - yellow like in the picture above (the gold one means a tolerance of + or - 5%). The 220K resistor is a modified version of the TBX from blueguitar.org.
     

  11. Phostenix

    Phostenix Tele-Afflicted

    Jul 28, 2010
    Phoenix, AZ
    Tonight I decided to experiment a little. I wanted the detent position of the TBX to be a little brighter & I wanted the 10 postion to be closer to the no load sound. I tried 3 different values in place of the 82K resistor. I didn't change the circuit other than to change the resistor value. I brought the wire that connects to the TBX control out of the guitar so that I could switch the whole thing out to compare it to a no load setup.

    I started with a 220K ohm resistor. It was definitely brighter at the detent, but the difference between that & full CW wasn't much. At full CW, though, it was very close to no load.

    Then I tried 150K. It probably turned out to be the best compromise, but it still had a noticeable difference between 10 & no load.

    I went to 100K & it wasn't much different than the origianl 82K and was a little further away from the sound of no load at 10.

    I put the original 82K back in for comparison & then decided to switch the whole thing out with a Fender no load pot. I was surprised at how close the standard 250K pot with .022uF cap of the no load gets to the brightness of the no load position. It isn't a big, ubrupt change. It's actually just what I wanted - that little bit of extra clarity for certain things. I think I'm going to stay with the no load.
     
    Last edited: Aug 2, 2010

  12. Fenderman71

    Fenderman71 TDPRI Member

    4
    Dec 17, 2014
    United Kingdom
    A good tone mod for a fender is to have a second tone pot (I did this to my twin-humbuckered strat copy), along with the other tone as a master. Wire this second pot in the usual way using a 22nF cap in series with a spare single coil mounted under the scratchplate (preferably a cheap unit minus it's bar magnet to give enough room).

    Luckily my guitar has a "swimming pool" rout so this only needed a little chiselling to make it fit. - It works a treat with humbuckers especially if you're unable to coil-tap them.

    Well worth a try!


    Tim
     

  13. sssteeve_a

    sssteeve_a TDPRI Member

    Age:
    67
    5
    Jun 12, 2016
    California
    Very interesting! With the bar magnet removed from a cheap strat pickup you have a dummy coil so vertical orientation would be important in determining whether it was in or out of phase with the real pickups. In other words you might try installing the pickup upside down to see which works better for you.
    I never thought of adding a tone cap in series with a dummy coil... thanks for the suggestion!

    Steve Ahola
    The Blue Guitar
     

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