SP22

The SP22 is a screened Mazda pentode for battery radio receivers, with a 2 V, 0.1 A filament. The Ediswan Radio sheet dated 1 March 1939 presents it for fixed-gain amplification, frequency changing and detection. It specifies HF or IF stages with fixed bias and self-oscillating frequency changing with cathode injection; HF amplification requires an aerial control. Mazda describes its characteristics as identical to those of the SP210 except for interelectrode capacitances. However, the SP22 has a small metallised bulb and British octal base: this electrical comparison does not imply identical connections or dimensions.

Maximum anode and screen voltages are 150 V. The general transconductance of 1.7 mA/V refers to 120 V on both electrodes and zero bias. All three tabulated points use a 120 V anode: with a 60 V screen and zero bias, anode current is 0.55 mA, screen current 0.16 mA, transconductance 0.9 mA/V and dynamic anode resistance 2.7 MΩ; at 80 V screen and zero bias, 1.1 mA, 0.35 mA, 1.2 mA/V and 1.35 MΩ; at 120 V screen and 1 V bias, 1.1 mA, 0.38 mA, 1.2 mA/V and 1.35 MΩ. Listed capacitances are 12.5 pF anode to earth, 7.75 pF grid to earth and 0.0055 pF anode to grid, with metallising and other electrodes at earth potential joined to the cathode in the measurement definition. Curves show anode current and transconductance against bias.

For grid detection, the sheet recommends a screen voltage of 50–60 V, obtained from an anode-battery tap or preferably through a dropping resistor from the full supply to limit overload. The grid leak is 0.5–2 MΩ, returned to the positive filament terminal. For smooth reaction control, it suggests returning the leak to a centre-tapped potentiometer across the filament supply. Mazda also requires the metal coating always to be earthed to avoid instability. Viewed from below, pins 1 and 8 are filament, 3 anode, 4 screen, 5 suppressor and 6 metallising; 2 and 7 are omitted and the top cap is the control grid. Maximum overall length is 103 mm and diameter 32 mm. The photographed specimen bears Mazda and SP.22 markings on the front, while its back is covered with essentially unmarked zinc paint; its 87 mm height excludes the pins and does not replace the datasheet overall dimension.