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The CV1258 is a half wave rectifier with flying leads. The bulb is spherical with tubes and pinches at each end. The maximum PIV is 14 kV and the anode dissipation is 150 Watts. The total emission is 240 mA.
The envelope also has etched into it the original Navy valve code of NU1, indicating that it was the first UK naval power rectifier. The construction matches the M-OV T1 transmitting valve but minus the control grid.
The mounting of the rectifier is by means of a bronze strip assembly that connects to the end tubes with metal straps and buckles. The white insulation is asbestos tape. The flying leads have ceramic beads as initial high temperature insulation. Away from the bulb the insulation sleeve is woven cotton that has been varnished.
The anode connection end. The anode cylinder has four strip supports that return to a collar that is clamped around the pinch stem. Sealed into the pinch is the support for the filament tension spring. The bronze spring support can be seen at either end and fixes to an insulating block that in turn holds the neck strap.
Another view of the anode connection end. The anode connection would be the lower end once mounted.
The etched lettering is augmented by a paper label reading NU1. The lettering is CV1258, NU1, Serial No. 8397C and Year 1944. The T1 design dates from 1917.
The filament end. The pinch holds the filament supports and the pinch stem tube is flared out to seal with the envelope in a rounded disc.
The top end of the filament with the tension spring.
The balloon envelope is 120 mm in diameter, and excluding the base pins is 240 mm tall.
References: Data-sheet, 1005 & 1047.


Absolute Maximum Operating Conditions
Thanks to Frank Philipse for supplying the above PDF datasheet.
Updated March 22, 2017.
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