From: Steve A.   (steve_ahola@yahoo.com)
Date:	2/20/2001 8:06 AM
Subject: 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