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The Mullard 90C1 is a voltage regulator discharge tube. The tube will strike at 115 Volts and will regulate to 90 Volts for currents of between 1 and 40 mA.

The specifications show a regulation of 14 Volts for this range of currents. This would be a wide tolerance by any standard. It is probable that this was intended to supply something like an oscillator that required sensibly constant current.

Special quality versions were made and given the type designation M8206 on a B7G base and M8207 with flying leads.

The internal coating obscures most of the envelope but close-up the construction is of a half cylinder cathode with a central rod anode.

The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 47 mm tall.

References: Data-sheet, Data & ¹⁰⁴⁰. Type 90C1 was first introduced in 1956. See also 1956 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.