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20.

 
See also:
DV and DL valves
    
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The sticker on the valve questions whether this is a Type 020. Comparing with the above article and Joe Gruber's exhibits shows that it matches the DV-6 and the type 20. especially the latter.
The top of the glass has a ring of letters spelling De Forest but no Type designation can be seen.
The horizontally mounted concentric cylinders with a single strand filament in the centre.
The construction is similar to the European R Type.
This image shows the helical grid fixed to a single support wire. The latter is fixed to the pinch leads at both ends.
The UV base with corroded pins.
The wide glass tube envelope is 34 mm in diameter, and excluding the base pins is 80 mm tall.
Reference: Bill Condon. Type 20. was first introduced in 1920.

 

Pin Connections
UV
1
2
3
4
f
a
g1
f

 

Absolute Maximum Operating Conditions
Triode
Vh
Ah
Va
Vg
mAa
ra
gm
μ
4.0
0.74
60
-1.5
1.0
30,000
0.27
9.7
Updated September 19, 2020.
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