Here's the poop on the TBX... 
 

Doc:
 
    I was able to pick up a TBX pot with a broken shaft for free today so I analyzed it with
 my DMM to figure out exactly what the heck is going on. Let's call the section next to the
 shaft A and the other section B. (Since the CW terminals are not used in the TBX wiring
 I will ignore them.)
 
Section A (1M): from 0 to the detent position, it is like a dead short— no resistance to 
speak of. Once you crack the detent in the CW direction the resistance increases
 from 97k to 220k to 435k to 635k to 923k. The wiper of section A is connected to the
 guitar signal from the volume pot. The CCW terminal is jumpered to the wiper of 
section B and also run to ground through an 82k resistor.
 
Section B (250k): at 0 the resistance is 0 ohms and as you move to the detent position
 it increases from 19k to 44k to 109k to 175k. From the detent position on up to 10,
 this section is open. The wiper is connected to the CCW terminal of section A
 and the CCW terminal is wired up to ground through a .022uF cap.
 
Conclusions:
 
#1. At the detent position the tone cap is not in the circuit. What you have is the
 82k load to ground. Added together with a 250k volume pot it brings the total load
 down to 61k; with a 500k volume pot the total load is 70k and with a 1M pot it is 75k.
 Assuming that your pickups are around 7k dc resistance, I believe that you would
 want the total load to be at least 70k- right? That might explain why 
I think that the TBX works better with a 500k volume pot...
 
#2. As you turn the TBX from the detent position towards 10, the tone cap is out
 of the circuit and you are varying the added load from 82k all of the way up to ~1M.
 At 10, adding the 1M load to ground to a 250k pot results in a total load
 of ~200k; when added to a 500k pot it results in a 333k resistance to ground.
 So basically a TBX set to 10 with a 500k volume pot should be a little bit hotter
 than a 250k volume pot and a "no load" tone control set to 10.
 
#3. As you turn the TBX from the detent position towards 0, it acts much 
like a regular tone control, except there is that 82k resistor to ground which
 will load down the signal.
 
#4. Leaving the TBX at the detent position might not be that good of an
 idea... after analyzing the results here I think it would work better if you
 cracked it CCW or CW, either to get a more traditional sound
 (with some of the signal going through the tone cap) or to reduce
 that gawdawful 82k load to ground a bit.
 
 
Questions and observations:
 
#1. So what is the function of the 82k resistor to ground? Does it
 "smooth out" the response between the two different ranges? Why
 not use something like a 220k resistor, just to lessen the loading down of the signal?
 
#2. I believe that the TBX was introduced at roughly the same time as the
 mid boosts used in the EC and Buddy Guy strats, so maybe it is designed
 to work with a ss preamp as well as a passive pickup
 (in other words, a compromise). Perhaps increasing the load resistor from 82k
 to 220k would be too extreme but something like 120k or 150k might work better
 when used with vintage output single coil pickups.
 
#3. I have experimented with using .015uF and .047uF caps instead of the 
stock .022uF tone cap... the jury is still out on which value I prefer. For a tele
 it is cool to have that .05uF tone cap just to get the bassy sound from the neck 
pickup. For a strat the .015uF might be a little too midrangy (the EC "Woman" tone
 control was used mainly with humbuckers, I believe.)
 
    Respectively submitted by:
 
Steve Ahola


I didn't remember that the 250k trace actually open circuits at the detent, and stays
 open from there to full CW rotation. Seems like you should be able to wire the
 dual pot so that the grounded tone cap is connected to the far end of the 250k
 trace, connect the 1meg pot so the two resistance traces will be strung in series.
 The detent would provide a wide open 250k standard tone pot loading. Opening
 up further adds more resistance in series with the 250k & the cap to reduce
 loading up to a value of 1,250k in series with a cap to ground.
 
I got curious enough to experiment on an AmStd Telecaster with 250k
 volume control and a TBX tone control. The fact that there was a shunt 82k
 resistor in the circuit, and the cap was not connected beyond the 5.5 detent
 bothered me. I lifted the grounded end of the 82k and repositioned it on the 
250k CCW terminal where the cap is connected. This made the control continuous
 with no abrupt open at the detent, but provided more loading than I preferred.
 I wanted the detent to be similar to the wide open standard 250k Fender tone
 control, always connected in the circuit across the volume control. Removing 
the 82k and replacing it with a 220k resistor pretty much did the trick. 
 
I used a Fender black numbered amp knob and measured the resistance of the
 modified TBX tone control, between top and the ungrounded end of the .022uf
 cap. Here's what I got:
 
1.......0 ohms
2.......6k
3.......20k
4.......84k
5.......115k
5.5.......218k (detent position: normal 250k wide open)
6.......270k
7.......540k
8.......850k
9.......1120k
10......1205k (essentially no-load)
 
From 1 to 5.5 with a .022uf cap it seems to be a nice smooth normal tone 
control, but of course within a compressed rotation range. At the detent, 
it's pretty much like a normal 250k tone pot wide open. Further rotation
 reduces tone pot loading considerably, and although still electrically connected
 across the circuit at full CW rotation, the R-C of 1.2meg-.022uf is a practical
 open circuit with single coils and a 250k volume control.
 
Doc