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Interservice Valve Bases

    
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Left to right: I/S1, I/S2, I/S3 & I/S4

These four base types were introduced with the aim of reducing inter-electrode capacitance and, by employing a tubular bulb, allowing a receiver to occupy less space. We believe that they were used on bright emitter valve designs between 1918 and 1923. The general arrangement was a vertical cylindrical anode allowing for the relatively small diameter tubular envelope usually 25.4 mm (one inch). By contrast the R type of the same period used a large spherical envelope of about 50 mm (two inch) diameter.

The I/S1 was acorn shaped with a moulded base. Electrical contact with the filament was made by top and bottom contacts and two side plates on the base connected to the grid and anode respectively. The flat side contacts were 7 mm square. The I/S1 was also found on some commercial valves such as the Mullard ORA-B.

This arrangement was designed to offer a lower capacitance connection to the electrodes and thus improve high frequency performance. Types Q and V24 from the same era used metal caps stuck to the glass envelope to form a low capacitance mounting.

The valves using this base were mainly constructed for the UK military. Navy type NR5 being an example, but some commercial valves also used this base such as the Mullard ORA-B

Although the I/S1 was the most common of the Inter-service bases, others were also used. The most common being:-

The I/S2 that also had a moulded base. In this arrangement two stud contacts were at the bottom with two side plates, one on each side. An example is the AR3

The I/S3 was used for the Army AR valve series. These featured three studs on the moulded base and two opposing side contacts.

The I/S4 was also used for the Army AR valve. This had two round short pins on the moulded base and two opposing round side connectors. An example can be found here

Information courtesy of Keith Thrower from his book. British Radio Valves. The Vintage Years. 1904-1925

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