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The 4120AA is a Barretter or current stabiliser. The main use for this tube was to stabilise the filament current in telephone repeater equipment.

The current range is 370 mA to 490 mA and the voltage drop over this range is 5.5 - 12 Volts.

This tube was made by STC in England as the prefix 4 indicates. ITT being the parent company.

The coiled iron wire resistance wire.

The wide glass tube envelope is 28 mm in diameter, and including the ES base is 97 mm tall.

Reference: Data-sheet. Type 4120AA was first introduced in 1956. See also 1956 adverts.

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.