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AC/PenSensibly equivalent¶ to:See also:
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![](../pics/aab0111.jpg)
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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.![](../pics/aab0111a.jpg)
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
| 1 | 2 | 3 | 4 | 5 | tc | ![](../static/key.gif) a | g1 | h | h | k | g2 |
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Absolute Maximum Operating Conditions¶
| Vh | Ah | Va | Vs | Vg | mAa | mAs | ra | gm | Pout | D | ![](../static/pdflogo.gif)
| 4.0 | 1.0 | 250 | 250 | -15.5 | 32 | 6 | 75k | 2.7 | 3.3W | 7% |
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PDF scanned from an original document held by the museum |
Updated June 11, 2022.
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