The Vintage Technology Association - Variable Indicator Tubes

Variable Indicator Tubes

Variable indicator tubes are vacuum or gas-filled display tubes whose area of luminescence can be varied by adjusting a control voltage or amperage. The most common type of tuning indicator is the magic eye tube. Like VFDs and CRTs, magic eyes are a descendant of the Crookes tube. Magic eyes have a ray-control electrode which dynamically deflects a beam of electrons traveling from the cathode to a fluorescent target. The ray-control electrode is connected to the plate of an integrated amplifier triode.

Magic eyes were commonly used in consumer tube radios, implemented as a signal strength indicator. The most common tuning eyes are top-viewing with a circular target, though configurations vary widely.

Another type of tuning indicator is the cold-cathode neon bargraph display, or Tunon.

6977 / DM160 (Unknown Mfr.)

At a mere 25mm in length, the 6977 is considered by most tube collectors to be the world's smallest eye tube. This is not strictly accurate: the 6977 has a control grid, but the indicator has no shadow or other measure of an analog value. The control grid simply varies the brightness of the glow, not the glow area. That being said, the 6977 is a unique device, a subminiature triode with a phosphor coating that visually indicates the tube's internal state. The 6977 was originally designed as a logic indicator for early computer systems, but saw nearly as much use as a triode for battery powered radios and similar devices.

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RCA 6HU6 / EM87

The EM87 is a linear "magic eye" style tuning indicator. Unlike more traditional magic eye tubes, which have a circular phosphor coated metal target, the EM87 has a strip of phosphorescent material applied directly to the side of the envelope. The shadow on this tube expands symmetrically outward from the center of the tube's phosphor strip, and will reach all the way to the top and bottom of the strip when fully extended. The tube has a 6 volt filament and a standard 9 pin base.

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Sovtek IN-13

The IN-13 is a neon-filled Soviet bargraph tube used primarily as a VU meter or frequency display in various solid state stereo equipment. The tube is six inches long with a flying lead base. The display is current driven, a source of 4mA is required to extend the bargraph to its maximum length.

Sovtek manufactured massive amounts of neon displays up into the 1990s, and Sovtek display tubes can be bought on eBay in huge quantities.

STC CV359

The CV359 is a bargraph indicator tube, designed for use as a power indicator in microwave applications. The CV359 consists of a glass bulb and long, thin neck which is filled with neon and encased in a protective metal shield. In operation, the tube’s neck is inserted through the side of a waveguide in such a manner as to allow the microwave beam to strike the small bit of exposed glass on the end. This excites the neon gas inside the tube and causes it to glow. A small slot in the tube’s metal shield allows the operator to view the length of the glow discharge.

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Sovtek IV-15

The IV-15 is the Soviet analog of the popular DM160/6977 miniature tuning indicator tube. Like the DM160, the IV-15 does not have a variable length display, only the brightness can be changed. These tubes will illuminate nicely at 9VDC, filament voltage is 1.2 volts. With an envelope height of 25mm the IV-15 is slightly larger than the DM160, but is still a very tiny device.
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Sovtek IV-15
	The IV-15 is the Soviet analog of the popular DM160/6977 miniature tuning indicator tube. Like the DM160, the IV-15 does not have a variable length display, only the brightness can be changed. These tubes will illuminate nicely at 9VDC, filament voltage is 1.2 volts. With an envelope height of 25mm the IV-15 is slightly larger than the DM160, but is still a very tiny device. [View Detail]