12F8

The 12F8 is a miniature twin-diode pentode designed for car radios powered directly from a 12 V battery. Two detector diodes and a remote-cutoff pentode share the cathode. Tung-Sol and Sylvania describe the pentode as an audio voltage amplifier with heater, anode and screen powered at the battery’s low voltages. Tung-Sol also states that its grid can receive automatic-volume-control voltage, extending receiver control action.

The nominal heater rating is 12.6 V at 150 mA. Sources explain design for the automotive range of 10 to 15.9 V; RCA’s tentative August 1957 sheet recommends 11 to 14 V for longest life and advises against series operation with other heaters. The envelope is miniature T6½ and base nine-contact noval 9FH. Heater 4 and 5, diodes 1 and 6, common cathode 7, pentode anode 3, screen 2, control grid 8 and suppressor 9.

The documented characteristic point has 12.6 V anode and screen, 0 V control grid and suppressor at cathode potential. Anode current is 1 mA and screen current 0.38 mA; internal resistance about 0.33 MΩ and transconductance 1000 µmho, or 1 mA/V. At about −5 V grid bias, transconductance falls to 10 µmho. This shows how bias influences possible gain, without itself being the gain of a complete amplifier.

Design-center limits are 30 V anode and screen, 0 V positive DC grid bias, 10 MΩ grid-circuit resistance and 30 V heater-to-cathode in either polarity. Each diode has a 1 mA maximum; separately, characterization gives 2 mA with 10 V DC applied. The latter is a characteristic measurement point, not a replacement for the service limit. Capacitances without external shield are about 0.06 pF grid-to-anode, 4.5 pF input, 3 pF output and 0.3 pF diode-to-diode.

The documents emphasize battery and charging-system variations. For the described battery-with-charger conditions, RCA requires that design never exceed 90% of design-center anode- and screen-voltage maxima at 13.2 V battery terminals. It thus separates normal supply, expected fluctuations and tube limits. This low-voltage car-radio context, documented by Tung-Sol in April 1956 and Sylvania in July 1956, characterizes the 12F8.