Get outside of your comfort zone. Build a vacuum tube lab power supply. They're actually very stable and just fun. Now here's something for those with an interest in tubes and don't want to mess with high voltages. You don't have to. In the 50s they came up with a 12v tube to use in car radios. So you can play with vacuum tubes and learn some old analog design with safe voltages.
Here are a few.
6GM8 -- Twin Triode with 6.3V on filaments and plates
12AC6 -- Remote Cutoff Pentode
12AD6 -- Pentagrid Converter
12AE6 -- Dual Diode & Medium-Mu Triode
12AE7 -- Medium-Mu Triode & Lo-Mu Triode
12AF6 -- Remote-Cutoff Pentode
12AG6 -- Pentagrid Converter
12AJ6 -- Dual Diode & Hi-Mu Triode
12AL8 -- Medium-Mu Triode & Power Tetrode
12BL6 -- Remote-Cutoff Pentode
12CN5 -- Remote-Cutoff Pentode
12CX6 -- Remote-Cutoff Pentode
12CY6 -- Remote-Cutoff Pentode
12DE8 -- Diode & Remote-Cutoff Pentode
12DK5 -- RF Pentode
12DK7 -- Dual Diode & Power Tetrode
12DL8 -- Dual Diode & Power Tetrode (sep. cathodes)
12DS7 -- Dual Diode & Power Tetrode
12DU7 -- Dual Diode & Power Tetrode
12DV7 -- Dual Diode & Power Tetrode
12DV8 -- Dual Diode & Power Tetrode (sep. cathodes)
12DW8 -- Diode & Dissimilar Dual Triode
12DY8 -- Medium-Mu Triode/Remote-Cutoff Tetrode
12DZ6 -- Remote-Cutoff Pentode (With Curves)
12EA6 -- Remote-Cutoff Pentode
12EC8 -- Med.-Mu Triode/Semiremote Cutoff Pentode (9 pin)
12EG6 -- Dual-Control Heptode (Pentagrid Amplifier)
12EK6 -- Remote-Cutoff Pentode
12EL6 -- Dual Diode & Hi-Mu Triode
12EM6 -- Diode & Power Tetrode
12EZ6 -- RF Pentode
12F8 -- Dual Diode & Remote-Cutoff Pentode
12FA6 -- Pentagrid Converter
12FK6 -- Dual Diode & Lo-Mu Triode (With Char. Curves)
12FM6 -- Dual Diode & Medium-Mu Triode
12FR8 -- Pentode & Triode & Diode
12FT6 -- Dual Diode & Triode
12FX8 -- Medium-Mu Triode & Pentagrid Converter
12GA6 -- Pentagrid Converter
12F8 -- Dual Diode & Remote-Cutoff Pentode
12G8 -- Dual Triode
12J8 -- Dual Diode & Power Tetrode
12K5 -- Power Tetrode
12U7 -- Dual Medium-Mu Triode (9 pin)
8056 -- High-Frequency Triode (Nuvistor)
list from:
http://www.junkbox.com/electronics/lowvoltagetubes.shtmlMy current project I'm gathering the materials for is building a vacuum tube geiger counter to monitor background levels. I'm use to working on and with them so the 400-950v needed doesn't bother me. The space is clear around it and it's switched off before anything goes near it until it's in an enclosure. What's nice is there's lots of USSR vintage 400v regulator tubes matched to the Russian geiger tubes I'm using.
You'll learn a lot getting stuff working with tubes that will carry over to your future projects.