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The ‘Magic Eye’

em81_tube — EM81 ‘Magic Eye’ tuning indicator tube – Photograph taken by Luigi Amaglio 2014.

A ‘Magic Eye’ is a specialised vacuum tube that was typically used in old radios as a visual indicator of reception signal strength to aid the user when tuning into a radio station. In the case of the EM81 tube type utilised on the Binson Echorec, visual indication is in the form of green ‘fan’ which close up as the signal level increases, eventually overlapping to indicate overload (see figure 1 below). The green light is caused by a stream of electrons, emitted from the cathode and controlled by the grid, hitting the “target” electrode, which is coated in a layer of phosphor. The phrase “magic eye” became a trade-mark of Radio Corporation of America (RCA) in the mid 1930s, but the device was also known as the “tuning eye” or “cat’s eye” as well as its technical name, the cathode-ray indicator. Occasionally, sceptics would describe the magic eye as the “idiot lamp”, the implication being that radio owners who were not savvy enough to tell if their radio was tuned-in properly using their ears were idiots.

The EM81 on the front panel of the Echorec gives an indication of the signal level being ‘printed’ onto the magnetic drum. The idea is to allow the musician to monitor their instrument levels so they can record the highest levels possible without overload distortion or saturation, therefore achieving maximum fidelity. Again, this is something that can be achieved by ear but it is useful to have a visual indication as a back up confirmation. The majority of tube Echorec machines were factory-fitted with Philips “Miniwatt” EM81 indicator tubes. The sensitivity or range of the magic eye is factory set using an internal trim-pot, which can be accessed by removing the rear panel from the machine. CAUTION is advised as dangerously high voltages are present in the tube circuitry. More information on adjusting the trim-pots can be found on our Echorec B2 Trimpot Adjustment and Echorec Export Trim-pot Adjustment pages. On a final note, the brightness of the magic eye will gradually become dimmer over the years as the tube ages but it will still function correctly.

em81_magic_eye_tube_animation — Fig. 1 – The EM81 magic eye in action!

For more information on magic eye tubes we highly recommend taking a look at the excellent website All About Magic Eye Tubes.

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The ‘Magic Eye’