The advent of the superheterodyne receiver, in the early 1930s, meant that radio consumers were able to purchase and enjoy a radio far superior to anything that had been offered before. Listeners soon discovered, though, that to realize the benefits of the “super”, accurate tuning was essential. One method that radio designers found to assist set owners was through the use of a device known as the tuning indicator.

Tuning indicators took many forms over the years. The first of these to gain any prominence was little more than a simple voltage-indicating meter. Mounted on the front panel, these instruments indicated correct tuning as the voltage controlling them rose and fell. It was easy enough – simply tune the receiver for maximum deflection of the meter needle (usually to the right). These indicators worked well enough, but they weren’t flashy or interesting, and thus not much of a marketing tool.

Some manufacturers took the tuning meter idea a step farther by using the meter principle and invented a device dubbed the Shadowmeter. This device resembled a small box with a pilot light at one end, and a small rectangular translucent screen at the other. In the middle of the device was the meter mechanism, with a small metal vane mounted in place of the needle. As the controlling voltage actuated the mechanism, the vane would swing back and forth, creating a shadow on the screen. This shadow’s length varied with the controlling voltage and as the receiver was tuned closer to resonance, the shadow widened until it filled the screen.

RCA’s introduction of the tuning-eye tube in 1935 set the standard in tuning indicators for many years. This device consisted of a small dish mounted in the top of an ordinary vacuum tube. The tube was mounted so that its “face” was visible from the front of the radio. During operation, the small dish lit up with a bright green glow all around, except for a shadow in the form of a wedge. The width of this shadow varied, again with the effects of the controlling voltage. The listener only had to tune the radio such that the shadow was at its narrowest to ensure correct tuning. As a tuning indicator and a marketing device, the tuning-eye was an immediate success, and found its way into the radios of many manufacturers.

However, not all manufacturers rushed to adopt the tuning-eye. Some used neon tubes, in which a column of light varied with receiver tuning (Atwater Kent’s “Flash-O-Graph”); others used colored lights where different colors indicated various stages of tuning (General Electric’s “Colorama Tuning”). Others used simply an ordinary panel lamp whose brilliance varied according to how well the receiver was tuned.

Even though vacuum tubes gradually disappeared from electronic designs, tuning indicators did not. The tuning-eye tube survived longer than most; some manufacturers continued to use them, applied to all solid state sets. The tuning meter has returned as well, now more likely to be a bank of light-emitting diodes rather than the traditional meter.


RCA staff, RCA Receiving Tube Manual: Harrison, New Jersey, RCA Electronic Components, 1971.
Philco Radio Corporation notes on servicing the Shadowmeter, 1934
Rider, John F., Servicing Superheterodynes. New York, New York: John F. Rider Publisher, 1934.