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The designation VI103 or Valve Indicating 103, is a UK military type that predates the CV or common Valve naming system introduced in 1941. The VI103 specification is virtually the same as the commercial Y63 in specification.

This valve is a magic eye tuning indicator.

The illustrations above show the pattern of the green shadow in use. In (A) the valve is shown with zero grid bias. i.e. no signal to generate an AGC voltage. In (B) the negative grid bias is showing the 'in tune' condition. The tuning would be adjusted for minimum size. In (C) the valve is shown overloaded. The green areas have overlapped and a patch of extreme brightness is displayed.

In operation the normal practice was to ground the cathode, take the target to +250 volts and run the anode from the 250 volts through a 1M ohm resistor. The grid is then connected to the AGC line.

The classic envelope is 37 mm in diameter and, excluding the IO base pins, is 87 mm tall.

References: Data-sheet, ³⁰⁰², ¹⁰⁴³ & ¹⁰⁴⁰. Type VI103 was first introduced in 1937. See also 1937 adverts.

Thanks to Frank Philipse for supplying the above PDF datasheet.