▼ Menu

    
Extras ▼

The Mazda AC/Pen, introduced in 1930 on the B5 base, was the first technically successful indirectly-heated power pentode capable of enough output to drive a moving-coil loudspeaker at good volume. The B7 based version followed in 1933.

The envelope contains a bright cylindrical anode with clearly visible grids. The electrodes are held in, or clamped to, a glass pinch, from which wires will run to the spring base pins.

Moving-coil loudspeakers (invented in 1926) gave much better sound quality than the horn or moving-iron loudspeakers used previously but initially they had rather low sensitivity due to the difficulty of mass-producing really good, light, magnets at low cost.

Two or three Watts of undistorted audio power was therefore required to drive a moving coil loudspeaker. Larger directly-heated triodes such as the PX4 could readily provide this but at low gain and low efficiency, requiring an extra stage of amplification and wasting costly anode power. Large directly-heated pentodes such as the PT4 gave improved gain and efficiency but direct AC heating of the filament (ie. in a mains-powered set) tended to cause objectionable levels of audible mains hum due to the increased final-stage gain.

The requirement for a powerful indirectly-heated pentode was obvious but early attempts to make such valves resulted in overheating of the inner electrodes due to the combination of heater, screen grid and anode power within a very confined space.

The Mazda AC/Pen was the landmark valve whose design overcame these difficulties at low cost and without causing other problems.

The relatively low heater power (4 W) and the generous anode diameter contributed to its success. The above advertisement uses the AC/PEN as its example of quality. Many original AC/PEN's survive in good working order.

This exhibit is a fairly early example (but not a first edition) with B5 base (+ side terminal), double micas at the top and uncoated anode. The early versions would operate at 200 V and dissipate 8 W.

Later examples had B7 bases, carbonised anodes, and improved ratings. The AC/Pen became a UK industry standard and was widely copied under a variety of names.

The AC/Pen may be found listed under: AC/PEN; ACPen; AC/Pen/5; ACPEN/7, and all possible permutations of these.

The balloon envelope is 57 mm in diameter and, excluding the B5 base pins, is 115 mm tall.

References: Data-sheet, private communication & ¹⁰⁴³. Type AC/Pen was first introduced in 1930. See also 1930 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

PDF scanned from an original document held by the museum