The AC/HL indirectly heated amplifier triode. Early versions used the shortpath construction method. This later version has been made as a normal triode with box anode etc.
The AC/HL was a very long-lived valve. Many originals survive in working order and during the late 1960s it was claimed that a specimen removed from obsolete BBC studio equipment still met the original specification for a new valve after over 250,000 hours in service! Note that the Type No. AC/HL was also used by other brands of valve, including Lissen.
The oblique stroke is sometimes omitted in listings (ACHL instead of AC/HL).
The zinc paint has broken away and all that remains is a small piece on the centre right. The connecting wire is visible on the lower left.
The lower mica with box anode above.
Another view. This was very difficult to light.
This exhibit tests to 62% with a gm of 3.4.
The classic envelope is 44 mm in diameter and, excluding the B5 base pins, is 94 mm tall.
References: Private communication, datasheet & 1043. Type AC/HL was first introduced in 1929.