The AR21 is a double diode triode. The type reference indicates a pre CV valve designated for Army Receiving applications. This exhibit was unused and well wrapped, unfortunately all of the grey external metalising had disintegrated and was detached from the envelope.

The design dates from the late 1930s. The most likely use was at the end of the IF chain where the diodes would demodulate the AM signal and rectify an AGC control voltage, and the triode would act as the first audio amplification stage.

Without the protective screening metalisation the inner construction can be seen. The heater cathode assembly runs up the centre of the valve. Above the pinch are the two diodes, Each anode is about 2mm tall. Next is placed a separating screen that the cathode passes through and a spacing mica follows. The top section is the triode. In all cases the anodes are of thin sheet pressed to form a mounting bracket and a cylindrical space for the electrodes within.

The classic envelope is 29 mm in diameter and excluding the IO base pins it is 83 mm tall.

 
Pin Connections

IO	1	2	3	4	5	6	7	8	tc
	m	h	a	ad(2)	ad(1)	-	h	k	g

 
Absolute Maximum Operating Conditions

Triode	Vh	Ah	Va	Vg	mAa	ra	gm
	6.3	0.2	250	-5.5	5	15k	2

This exhibit was last updated on 21 January 2003