A type 1629 'magic eye' tube, driven by a 6SL7 (on top). Both are mounted on a diecast aluminum box that I primed & painted with enamel spray paint. A calibration pot is mounted on back (photo on right) to adjust for full illumination at full power output of amp (onset of clipping).
These are the 'output level' indicators for my 2A3 power amps. Besides looking cool as they move to the music, they are also quite useful; showing when the amps are reaching full power output & starting to distort. As these amps are single-ended-triode, with no feedback loops, they generate very mild (and relatively un-obvious) distortion when over-driven.
The eye tubes in these photos have several thousand hours of use on them, so the upper portion of them is rather dim compared to new eye tubes.
Technically referred to as 'electron ray' tubes, the term 'magic eye' was coined by RCA, who first marketed them in 1935. Originally they were used as tuning indicators in their more expensive radios.