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 (mid-1930s) 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.

 
Pin Connections

UX7	1	2	3	4	5	6	7
	h	a	g2	g1	k	hct	h

 
Absolute Maximum Operating Conditions

Pentode	Vh	Ah	Va	Vs	Vg	mAa	mAs	ra	gm	Pout	D
	6.3	0.6	180	180	-25	48	14	35,000	2.4	3.4W	11%

Thanks to Frank Philipse for supplying the above datasheet

This exhibit was last updated on 12 June 2007 {find valves by name}