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VT-255

Sensibly equivalent to:
705A
    
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This 705A carries the US Government procurement code of VT-255 as well as the maker's name.
The 705A is an EHT power rectifier with a PIV rating of 35,000 Volts. The average rectified current is 100 mA with a peak of 500 mA. The construction is all glass with the lead-out wires being fixed to the main pins that are held in a ceramic disc outside the envelope. The special base clamps to this ceramic disc.
The anode is made from three sheets stitched together. Two of the side wings provide cooling and the third is also the connection point for the anode support. The anode is fixed only to the top of the envelope.
A detailed view of the construction. The thoriated tungsten filament is made as two helical sections with the central support forming the return connection.
Another view of the lower end of the filament. Note that the coating on the outer surface of the anode does not go the the base - this suggests a spray coating.
The wide glass tube envelope is 58 mm in diameter, and excluding the B4A base pins is 116 mm tall.
References: Datasheet & 1040.

 

Pin Connections
B4A
1
2
3
4
tc
f
fct
f
-
a

 

Absolute Maximum Operating Conditions
Rectifier
Vh
Ah
Va
mAa
5.0
5.0
12.000
500
Thanks to Frank Philipse for supplying the above PDF datasheet.
Updated November 13, 2017.
shape:wide.glass.tube construction:all.glass type:rectifier.hw age:1940.1950 base:b4a heater:5.0v pins:4 pin:1.f pin:2.fct pin:3.f pin:4.- pin:tc.a
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