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AR21

Sensibly equivalent to:
CV1055 EBC33 NR48 OM4 VR55
    
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The AR21 is a double diode triode equal to the Mullard EBC33. 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 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 2 mm 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.
References: Datasheet, 1040, 1043 & 3002. Type AR21 was first introduced in 1938.

 

Pin Connections
IO
1
2
3
4
5
6
7
8
tc
m
h
a
a(d2)
a(d1)
-
h
k
g1

 

Absolute Maximum Operating Conditions
Triode
Vh
Ah
Va
Vg
mAa
ra
gm
6.3
0.2
250
-5.5
5
15k
2
Thanks to Frank Philipse for supplying the above PDF datasheet.
Updated February 16, 2013.
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