Sensibly equivalent¶ to:
The Mazda AC/Pen, introduced in 1930 on the B5 base with a side cap connection, 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.
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 (i.e. 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.
This exhibit differs from those usually seen as the anode is a rectangular box not a cylinder. The envelope has a shoulder to hold the top mica but is as wide as the earlier balloon shaped valves.
The control grid is wound on copper supports and above the top mica is a heat radiator to lower the grid temperature. All three grids are shaped so that the wires pass flat across the working faces of the anode. This looks to be a redesign of the whole valve as the original AC/Pen's had helical grid windings. The positioning of the Type designation in the outer printed frame leaves no room for a suffix, clearly making this an AC/Pen - probably a post-war valve for replacement purposes.
Side view. The screen grid connection on the side of the base has lost the screw cap.
Top view showing the mica fingers holding the electrodes firmly against the shoulder of the glass envelope.
The inner grids have sharp bends to form the flat part across the anode face. The suppressor grid is much more rounded at the ends.
The classic envelope is 56 mm in diameter and, excluding the B5 base pins, is 125 mm tall.
References: Data-sheet, private communication & 1043. Type AC/Pen was first introduced in 1930. See also 1930 adverts.
Absolute Maximum Operating Conditions¶
PDF scanned from an original document held by the museum
Updated June 11, 2022.