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The D-01A was the first valve to be made by De Forest as an equivalent to another company's product. In this case the RCA UX201A.

The design use was as a detector - with a low anode voltage and a bias of -4.5 Volts - or as a voltage amplifier with a higher anode voltage and a grid bias of -9.0 Volts.

In 1928 this design was superseded by the D401A still with the UX4 base cap.

The original box for this early D-01A

Thanks to Joe Gruber for sending the images.

The balloon envelope is 45 mm in diameter and, including the UX4 base pins, is 112 mm tall.

Reference: Joe Gruber. Type D-01A was first introduced in 1927. See also 1927 adverts.

Absolute Maximum Operating Conditions^¶