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Mazda were rightly proud of their AC operated valve pentodes and made their status plain in the name AC/S2Pen.

This exhibit is a later 1930s pentode version of the earlier AC/S2 screen-grid (tetrode) type. Redesigned as a pentode it gave a more linear characteristic over a larger range of signal strengths and made performance more tolerant to the range of HT voltage occurring in typical mains-powered sets.

The AC/S2Pen would be used in the final RF or IF stage of a radio or early TV, immediately before the detector stage where the signal strength fluctuates and can sometimes be quite high (tens of volts). For the 1930s the gm was respectable at 4.6.

The valve is also suitable as a high-gain audio amplifier capable of providing the relatively large signal voltages required at the grid of a PX4 type triode output stage.

The late 1930s witnessed the birth of TV, and behind the scenes RADAR, both technologies required gain at high bandwidths.

The Mazda ACS2/Pen was a fairly early product of this race. It was a great improvement on its screen-grid predecessor (AC/S2) but was in turn soon overtaken by classic video pentodes such as the EF50 which not only had higher gm and better gain-bandwidth product but was more compact, easier to make, and less power-hungry.

The AC/S2Pen, then, was a fairly short-lived valve linking 'wireless' technology to the new imperatives imposed by high-gain video amplification. One aspect of this old vs new status is the design of the top cap which will accept either a small spade terminal, as used in screen-grid wireless sets) or the newer, American-style, push-on connector.

The valve is sometimes listed as AC/S2/Pen; ACS2PEN; etc.

The top cap connection is the screw top Bakelite moulding variety that gave way to the metal top cap during the mid 1930s.

The balloon envelope is 45 mm in diameter and, excluding the B7 base pins, is 109 mm tall.

References: Data-sheet & ¹⁰⁴³. Type AC/S2Pen was first introduced in 1933. See also 1933 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

PDF scanned from an original document held by the museum