This device was designed to be a benchtop amplifier, a replacement audio output transformer, an AM demodulator, and a simple power meter all in a box. It was primarily designed for radio troubleshooting purposes, but found it's way into almost every kind of test bench where audio needed to be followed through a circuit. The device is a 4 tube unit, consisting of a 6X4 rectifier, a 12AX7 preamp, a 6AQ5 audio output, and a 1629 eye tube. The 6X4/6X5 tubes had a history of being somewhat flaky, and early examples would sag internally, killing power transformers. Later tubes don't seem to have this problem. Of particular interest is the eye tube. These are synonymous with technology from the 30s-50s, and are a quick and easy visual indicator. They're a simple phosphor target that has a shadow that grows and contracts depending on the bias voltage on the grid. This is where the "eye" name comes from, as it resembles an eye blinking (opening and closing) from a side profile. See this page for more information about eyetubes. https://www.rfcafe.com/references/popular-electronics/the-tuning-eye-how-it-works-jan-1955-popular-electronics.htm This wasn't the only kind of eye, some had their phosphor (called the taget) on the side and made a bargraph-type display, later tubes designed for specific uses like AM/FM tuners had multiple targets. The 1629 eye tube was designed for WWII service, and we're just now using up that stock. EICO devices could be purchased as a kit or pre-assembled, this one appears to have been factory built as it's well made and has no bad solders. All tubes appear to be original, and there are very few components that may give trouble in the circuit, the primary one of concern being the large black capacitor at the top of the chassis. This unit is fully operational, and was opened to replaced the crumbling line cord.