The GI-21 is the larger counterpart to the GI-10, both manufactured by National Union as part of their “Inditron” line of display tubes. This places it among the earliest Nixie-style tubes ever produced, even predating the term “Nixie” itself that was coined by Haydu Brothers (later Burroughs) in the late 1950s.
Like the GI-10, the GI-21 does not feature a dedicated anode cage, which became standard in later Nixie tubes. Instead, to illuminate a specific digit, all other digits must be held at anode potential while the desired digit is connected to ground. Interestingly, the GI-21 includes four metal rods connected to one of the pins on its phenolic base. In some examples, such as the one shown below, these rods can function as a dedicated anode, significantly simplifying the required driving circuitry. However, this functionality appears inconsistent across different units, suggesting that it may have been an experimental or partial implementation of a dedicated anode that did not fully meet expectations.
Aside from that, the GI-21 shares an almost identical construction with the GI-10, including the use of ceramic-coated rods to support the various cathodes. Like the GI-10, its digits are not stamped from sheet metal but are instead hand-formed from wire which is a design feature typical of early Nixie tubes and can also be found in tubes such as the British GN-1.
Key Specifications
| Property | Description |
|---|---|
| Manufacturer | National Union |
| Time period | Mid 1950s |
| Digit height | ~17mm |
| Envelope diameter | ~29mm |
| Socket | ? |
References
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