**These curves are great for a couple of reasons. We can look
at
the line itself to see if the tube under test is nonlinear in any
way. Non linearity would show up as squiggles in the line (this
one is excellent). Also, we can use the information printed in
the upper left portion of the chart to create some very well
matched pairs and quads of tubes. It is interesting to see
how individual tubes show their differences here, when otherwise they
test as matched.**

**The charts are easy to interpret. Look at this example:**

**What these lines are showing you is (a) the transconductance (the
slope
of the line), and (b) the plate current (how high up on the chart the
line
is). This one is a good example of two tubes that are matched for
plate
current but not transconductance. The lines show two halves of a
12AX7
that measure 1.7 and 1.8 mA at -0.7 grid volts. But the
transconductance
of one half is 1789 uMhos, the other 2067 uMhos (the slope of one line
is
steeper than the other). If we were matching by plate current
only
then we have a good match. If we were matching by
transconductance only,
we would see a small (less than 300 uMho) difference, and would have no
way
of knowing if the plate current was matched or not. These charts
show
the differences very graphically. Look at the two halves of this
12AU7:**

**The tube halves are matched for transconductance (the lines are
parallel), but not for plate current (the lines are far apart on the
chart). Most
traditional transconductance tube testers would show the two halves of
this
tube as a close match! Check out this one:**

**This is the curve from a NOS RCA 2A3. Do you see the
defect? The tube will not reach complete cutoff. At -40
grid volts, where most
2A3s show less than 1 mA of plate current, this one is still conducting
over
5 mA. This type of defect can not be detected by most tube
testers.**

**Finally, here is a set of curves which shows properly matched
halves of a 6922:**

(Click to
enlarge)

**Now that's matched!**

12-19-2004