I had these pics of my JTs from 2009, both conventional and supercharged JTs. The conventional JT is lucky if it can be 50 percent efficient. My Supercharged JT will do upwards of 80 percent easily, using much less battery current while giving the same amount of light output.
I believe I have seen only one or two experimenters use my Supercharged Joule Thief circuit; for the most part it has been ignored. The circuit is almost as simple as the conventional JT. All it takes is a diode and a capacitor to make one. The diode can be a 1N914 or 1N4148, and the capacitor can be from 560 pF to 1000 pF or 1nF (ceramic disks are marked 561 or 102). The Supercharged JT picture shows how they are connected. This uses a BC337-25 transistor, but a 2N4401 or PN2222A will work fine.
Back to experimenting…