The tuning indicator tube, or "Magic Eye"
as it is was introduced by RCA, was used to display the optimum signal tuning point
of a tuner/receiver, or the peak signal level of a preamplifier or tape
recorder. The main benefit in tuning reception was for FM tuners where the
exact tuning point was harder to determine than for AM. They were cool looking
then and still are.
Scott typically used indicator tubes as
replacements for FM tuning signal strength (mA) meters on many (300/320, 314, 345,
350-A, & 399) tuner and receiver types. The 122 preamp also featured
indicator tubes to show the effective signal levels of the Dynaural Noise Suppressor.
The indicator tube is a standard thin glass
tube envelope with a phosphor strip on the inside of the side glass. The
electrode structure is based on a double triode with common cathode. The anode
of the second triode is called the target and within the electron stream is a
deflector electrode designed to alter the path of the electron beam, it would
normally be connected to the anode. In the EM84 the quiescent condition was
two green bars, one at each end of the window. As the signal strength
increased the bars would elongate and eventually overlap to form a bright
green region. The optimum point for tuning was a minimum gap, and for
monitoring peak levels the operator was told to have the bars just
barely touching (but not overlapping) on peaks.
Note: original RCA-designed "Magic Eye" tuning
indicators featured round "end window displays" with variable
percentages of the display circumference illuminated as levels increased.
Also see: Introduction
to the RCA's "Magic Eye"