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The 42OT is a tetrode audio output valve that dates from 1937. This exhibit has lost its type identification but retains the etched makers name.

Having measured the heater at 2.0 Amps at 4.0 Volts I looked through all of the B7 based Cossor valves and the 42OT came out as the type. The advert confirmed the identification.

The anode load is given as 8 kΩ, the cathode resistor as 140 Ω and the output power as 3.1 Watts.

The base connections indicate a g3 or beam plate but Cossor describe the valve as a tetrode. The advert shows a pair of vertical rods in the plane of the grids but outside them. The image of the pinch shows a connection from the left rod to the base and so this is probably designed to reduce secondary emission from altering the characteristic curve.

The Cossor name and country of origin are clear. The BVA logo is at the top and the V8 is probably the date code.

The control grid is wound on copper support rods.

The classic envelope is 56 mm in diameter, and excluding the B7 base pins is 114 mm tall.

References: ¹⁰⁴³. Type 42OT was first introduced in 1937. See also 1937 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶