A look at the operation of the Sure Electronics magic eye module which can be purchased from the usual online outlets.
A cool little device for use as an audio VU meter that is basically plug and play.
(High voltages are present, so be careful when you are playing with any types of valves!!).
If you visit ebay at the following link: https://www.ebay.com/sch/i.html?_nkw=magic+eye+audio+level
You can scroll down through the results and find the module in the video.
Its more expensive than the cheapest ones as it only needs low voltage, the cheap ones have to be supplied with 250V from an external source.