I've been going through my tube stash and trying to get it organized. First step was to build a spreadsheet so I could get a better handle on how much of what I've been ratholing for all these years.
Found a tube box marked JAN 7864 that felt like it was empty. Nope... Silver Saver anti-corrosion bag folded up inside. Opened that up to find a ceramic tube-ette about the size of two aspirin stacked together. Amazin' piece, had to do a web crawl to get any info on it.
It's a Planar Triode. Typical application is in avionics / microwave around 1.2 Gigs @ 300 mW with ~ 200 volts on the plate. 6.3 volt filament draws 1.5 watts which is more than the maximum plate dissipation. Specs here if anyone is curious: http://www.mpdcomponents.com/pdfs/planarbytype.pdf
Been poking around inside electronic stuff for 40+ years but this is the first time I've come across anything like it. Was also amused to find that web vendors do NOT sell it on-line as there are export restrictions on the part.
Scads of silicon bits that can do the same or better, but the tube version is restricted.