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The 21A6 is the Cossor implementation of the design that is also type PL81. It was designed for universal mains sets without a mains transformer and with the heaters on a series chain. The main design function was as a line output valve. It can also be used successfully as a series pass valve in stabilised voltage circuits. Such circuits place heavy demands on the cathode and so does the line output circuit where the valve effectively acts as a switch and draws heavy current when turned on.

As a line output valve in television service, the peak anode surge voltage is 7,000 Volts. The anode dissipation is 8 Watts.

The control grid is wound on copper rods for good heat conduction and these supports art topped with radiating fins.

The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 72 mm tall.

References: Data-sheet & ¹⁰⁴⁰.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.