I had a cool idea today - instead of relying on the termoelectric effect to get electrons into a vacuum, why not use the photoelectric effect? You can get UV LEDs these days, you could focus one onto a peice of metal with a low work function, and hopefully the UV light would knock lots off the electrons off of its surface - this would be your cathode! You could even try amplification by varying the brightness of the LED instead of the voltage on the grid if the LED was fast enough. This seems like a really interesting idea. I'm sure a grid would probably work better though. Think about it - no worn out heaters, just a simple LED you could replace! (Would need clever design of the tube so the LED wasn't in the vacuum). Any thoughts?