...on grandma's old tube radio with the "magic shining green thing" in the front panel which showed the quality of the signal reception?
If you're looking on the well known selling platforms for "Magic Eye" than you'll find not only a lot of tubes but also offers of VU meter kits. But here as against the most available offers you need not to supply all the voltages (normally 6 V and 250 V), you do not need to build a case, solder adapter wires for the tube's connections, have only a mono display or only one tube type which can be fitted, etc...
Mr. Nixie has taken all these kits, put them in a pot, added a hefty dose of the legendary Mr. Nixie Magic, and finally the Magilyzer EM800 was created.
The Magilyzer EM800 is an analog audio spectrum analyzer, based around an analog analyzer chip. It offers the display of six selective audio bands: 63 Hz, 150 Hz, 330 Hz, 1 kHz, 3.3 kHz and 10 kHz.