Talk-Polywell.org

"Now, if for some reason you loose the ability to make silicon..."

http://en.wikipedia.org/wiki/Rocket_Ship_Galileo

Yes, it is a Heinlein 'Juvenile /YA' tale. Yes, they use a fission reactor with zinc powder as reaction mass. Yes, they run into Luft46 N*zis.

Pertinently, they salvage broken valves from their space-ship's radio room, improvise a 'naked' transmission vacuum tube and --literally-- tell the world...

They could have used bits of wire and steel plate. Broken valves are not a requirement. Unless efficiency and fidelity and longevity matter.