The Mazda AC/HL is an indirectly heated audio amplifier triode and the design dates to 1929. These valves were rugged and had long service lives. In the late 1920s and early 1930s Mazda produced the leading indirectly heated valves. The AC/Pen of 1930 set the standard that others followed.
From the top of the dome the rear wing of the anode can be seen.
The short path construction method is visible in this picture. The anode wings and the closely wound grid are defining features.
