Sensibly equivalent¶ to:
The 12A5 has a conventional indirectly-heated cathode but its heater is centre-tapped so that it could be used in (American) car radios suitable for either 6V or 12V systems. Car radios of this era used 'vibrator' HT power packs which were not very suitable for generating HT voltages in the 250-300V region. The 12A5 was therefore designed to operate well from lower HT voltages and therefore needed a 'juicier' cathode to provide extra current.
The classic envelope is 37 mm in diameter and, excluding the UX7 base pins, is 90mm tall.
References: Data-sheet & private communication. Type 12A5 was first introduced in 1940. See also 1940 adverts.
Absolute Maximum Operating Conditions¶
Updated February 16, 2013.