That Tony is so cool. I visit often.
Also of interest to the amateur:
CircuitMaker 2000. It's circuit design and simulation software. The indefinate trial "student" version is plenty robust enough to model or modify small circuits, like Tony's.
CircuitMaker Student Version information and download page:
http://www.microcode.com/downloads/student.htm
***
EDIT: How can anybody be so cool? He tells us how to field-build resistors with graphite pencil marks on paper, then explains, "My father, who was with the Dutch resistance during WWII, at that time made repairs like this on many occasion."
Great site.