The AC/HP5 is a mystery with no information found within the museum or on-line.
On the paper label someone has written SP4 and this is a good clue. B5 based valves from the early 1930's with side contacts and a top cap are rare. Without the top cap the use would be as an audio output valve. With the top cap - probably the anode - the case becomes clearer for an HF pentode. The AC suggests a 4 V 1 A heater. The HP looks to be HF pentode and this matches the SP$ but the latter does not have the side terminal. The AC/HP5 seems to have all the grids brought out to pins including the suppressor grid. Most other HF pentodes of the time have g3 strapped internally to the cathode.
The screw top cap and the side terminal. Thrower in his 6,500 valve references does not list a valve with B5 st and tc.
The B5 base pins.
Thanks to Mark Mallison for sending the images.
The classic envelope is mm in diameter and, excluding the B5 base pins, is mm tall.
References: Observation. Type AC/HP5 was first introduced in 1933. See also1933 adverts.
