I know this isn't exactly an audio device, but it can be used to understand all things pertaining to audio. This is a vacuum tube operated device that functions as three measurement tools. It is a VTVM (vacuum tube volt meter) which is still one of the best methods of making accurate voltage measurements without loading down the "device under test" (DUT). It is also Watt-meter, with built-in 4, 8, 16 and 600 Ohm loads and it is an intermodulation distortion meter (IM distortion). You can mix two frequencies, 60Hz and 6,000Hz in any ratio with it you can measure power output, noise, gain, overload and intermodulation characteristics. It uses 12AX7, 12AU7, 12AT7, 6C4 and 6X4 tubes (all British made Mullards!) and many precision value resistors.
I repainted the back enclosure and replaced the front panel binding posts and did a general sprucing up, outside of the scratch on the meter bezel it looks pretty new.