Please enjoy this short video of me going ham on this poor 6v3a diode tube

In this video I'm using a large double-convex lens to heat up the cathode of a 6v3a diode to stimulate emission. Because these were designed to be heated from inside with a filament, the cathode doesn't really have line-of-sight to the outside of the envelope, but there is enough protruding through the top mica that I was able to get a bead on it. Considering this tube is not optimized for conversion, half a volt of open-circuit voltage is more than I was expecting. Of course there's almost no current available, and if you were to leave it this way it would eventually fail as the anode heated up and began emitting (or the envelope would eventually shatter) because there's no cooling. 

That said, there it is, glass tube solar thermionic conversion!