Electron-Ray Indicator Eye Tube
The EM80 / 6BR5 Vacuum Eye Tube is a vacuum tube which gives a visual indication of the amplitude of an electronic signal, such
as an audio output, radio-frequency signal strength, or other functions. The magic eye
(also called a cat's eye, or tuning eye in North America) is a specific type of such a tube with
a circular display similar to the EM34 illustrated. Its first broad application was as a tuning
indicator in radio receivers, to give an indication of the relative strength of the received
radio signal, to show when a radio station was properly tuned in.
The magic eye tube was the first in a line of development of cathode ray type tuning indicators
developed as a cheaper alternative to the needle movement meters. It was not until the 1960s
that needle meters were made economically enough in Japan to displace indicator tubes. Tuning
indicator tubes were used in vacuum tube receivers from around 1936 to 1980 before vacuum tubes
were replaced by transistors in radios. An earlier tuning aid which the magic eye replaced was
the "tuneon" neon lamp.
The EM80 / 6BR5 magic eye tube is a miniature cathode ray tube, usually with a built-in triode
signal amplifier. It usually glows bright green, (occasionally yellow in some very old types,
e.g., EM4) and the glowing ends grow to meet in the middle as the voltage on a control grid
increases. It is used in a circuit that drives the grid with a voltage that changes with signal
strength; as the tuning knob is turned, the gap in the eye becomes narrowest when a station is
tuned in correctly.
The purpose of magic eye tubes in radio sets is to help with accurate tuning to a station; the
tube makes peaks in signal strength more obvious by producing a visual indication, which is
better than using the ear alone. The eye is especially useful because the automatic gain control
(AGC) action tends to increase the audio volume of a mistuned station, so the volume varies
relatively little as the tuning knob is turned. The tuning eye was driven by the AGC voltage
rather than the audio signal.
When, in the early 1950s, FM radio sets were made available on the UK market, many different
types of magic eye tubes were made available, with differing displays, but they all worked the
same way. Some had a separate small display to light up indicating a stereo signal on FM.
Magic eye tubes were used as the recording level indicator for tape recorders, and it is also possible to use them
(in a specially adapted circuit) as a means of rough frequency comparison as a simpler alternative to Lissajous figures.
A magic eye tube acts as an inexpensive uncalibrated (and not necessarily linear) voltage indicator, and can be used
wherever an indication of voltage is needed, saving the cost of a more accurate calibrated meter.
At least one design of capacitance bridge uses this type of tube to indicate that the bridge is balanced.
It is quite easy to swap a worn out EM80 with a new 6E1N or EM81. The pinning is identical so just
replace without any changes to the existing wires. Bear in mind that the dimensions of the
russian type are slightly different to the EM80 but in most cases this won´t be a problem
with mounting installation.
Electrical Data:
Heater Voltage ................................ 6.3 V
Heater Current ............................... 0.27 A
Preferred Substitutes ......................... 6DA5, 6E1P, CV1352, EM80.