The Early Mullard Naming Convention

The Type No. AC044 merits comment because it is so frequently quoted as ACO44, even in some later official Mullard publications.

AC---	Directly-heated filament designed for AC operation
--04-	The nominal mu value ie. 4
----4	Indicates the filament voltage ie. 4

Further examples

AC042	Same as AC044 but with 2-volt filament
AC064	Similar to original AC044 but with mu = 6
AC084	Similar but 400-volt anode rating and mu = 8
AC104	Similar to original AC044 but mu = 10