Prowess Amplifiers offers resources for the guitar tube amplifier builder. We have a huge schematics inventory, projects section and forum.

http://www.prowessamplifiers.com/      (popularity:  ; clicks: 9; comments: 0)      

Semiconductors: physics, manufactoring, devices

http://www.semiconductorsinfo.net/      (popularity:  ; clicks: 9; comments: 0)      

Introduction to thermodynamics, description of its laws and processes

http://www.termodynamics.com/      (popularity:  ; clicks: 10; comments: 0)