SP42

The Mazda SP42 is a screened high-frequency pentode designed for the video output stage of mains-powered television receivers. Ediswan's sheet of 1 March 1939 describes a small metallized bulb and British Octal base. The heater needs 4 V and 0.95 A. Video service explains the attention given to capacitances and synchronization-circuit conditions: this entry presents the function actually described by the manufacturer, with its loads and voltages.

Listed maxima are 200 V plate and 200 V screen. Transconductance of 9 mA/V is specified at 200 V plate, 100 V screen and zero control-grid voltage, not for every operating point without conditions. One table setup with about 115 V screen-to-cathode, −1.25 V bias and 2600 Ω plate load gives idle currents of 20 mA plate and 5 mA screen, with 50 Ω total cathode resistance. Other setups change screen voltage, bias, feedback resistance and load: each column is a consistent combination.

Application notes distinguish common DC-restoring circuits, recommending 115 V screen and plate load around 3000 Ω. Mazda then describes a sync separator fed from a resistor in the video-output cathode, with the CRT shield DC-connected to the SP42 plate. Connecting the screen grid directly to the 115 V stabilizer and supplying the timebase from that stabilizer makes synchronization independent of mains-voltage variations under the stated conditions. Supply selection therefore also serves image stability.

For that arrangement the sheet gives initial bias −1 to −1.25 V, plate load around 4000 Ω and total cathode resistance around 1000 Ω. Under certain conditions greater sync output can be obtained by increasing the latter to 2000 Ω, with extra screen voltage obtained through a resistor in series with the stabilizer. These are circuit-specific instructions, not isolated values for every television. Capacitances are 7 pF plate-to-earth, 10 pF grid-to-earth and 0.004 pF plate-to-grid; earth includes the remaining low-potential electrodes and metallizing joined to cathode.

Viewed from the free end of the base: heater on 1 and 8, cathode 2, plate 3, screen 4, suppressor grid 5, metallizing 6; 7 is omitted and the control grid goes to the top cap. Separating the grid connection from the base is a visible construction feature. Mazda maxima are overall length 95 mm and diameter 32 mm. The drawing distinguishes individual terminals and external coating without confusing the physical base family with a generic pinout.

Documentation consulted: Mazda SP42 A.C. Mains H.F. Pentode, Ediswan Radio, Sheet 16/1, 1 March 1939, data, typical operation, applications, dimensions, connections and average curves.