Magic Eyes

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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.

Jukka Tolonen's
Glass Audio
on Indicator Tubes
Click to "SuperSize" Photo

    Low-Level Signal

High-Level Signal

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" 


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