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This SP4 is of similar age and appearance to the SD20 but it is a screened pentode rather than a screen-grid tetrode and therefore has a more modern type of electrode assembly.

Equally important is its relatively thick heater (1 Amp rating) and improved heater-cathode insulation technology. Moreover, unlike the SG20, the heater-cathode insulation would never need to withstand high voltage.

These differences made the SP4 a much more successful valve which was marketed in this 5-pin (from 1933) or (SP4) 7-pin base (from 1934) versions and is a relatively common valve in collections. Apart from its ungainly size, the main weakness of the SP4 was that it was unsuitable for AGC applications and therefore had no place in the 'modern' type of superhet which became the norm from the mid-1930s onwards. Mullard describe the valve as an HF pentode for use as a speech detector or RF amplifier.

Image from an ebay sale.

The anode and outer screen are made of mesh and the three grids are wire. The anode appears to be cylindrical.

The balloon envelope is 46 mm in diameter and, excluding the B5 base pins, is 130 mm tall.

References: Data-sheet & ¹⁰⁴³. Type SP4 was first introduced in 1933. See also 1933 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Andy Cowley for supplying the above PDF datasheet.