12FM6

The 12FM6 combines two detector diodes and a medium-mu triode sharing a cathode in a miniature envelope. It is intended for car radios supplied directly from the automotive battery: General Electric describes detector, automatic-volume-control rectifier and transistor-driver functions. Tung-Sol specifies the triode as an audio voltage amplifier into a low-impedance load. It is an interesting example of circuits where a tube performs detection and preliminary amplification while interacting with transistor stages.

Heating is indirect, nominally 12.6 V and 0.15 A. RCA’s October 1959 sheet allows 10–15.9 V AC or DC and recommends 11–14 V for life. General Electric says extremes are tolerable briefly and better supply regulation improves reliability. Any mounting position is allowed. The seven-pin miniature base uses 7BT: triode grid 1, common cathode 2, heater 3 and 4, second-diode anode 5, first-diode anode 6 and triode anode 7.

Tables give two triode conditions with 12.6 V anode. With grid connected to zero and no grid resistor, anode current is 1.8 mA, transconductance 2400 µmho, plate resistance 5600 Ω and amplification factor 13.5. With a 2.2 MΩ grid resistor, current is 1 mA, transconductance 1300 µmho, resistance approximately 7700 Ω and factor 10. The transconductances equal 2.4 and 1.3 mA/V. This shows the importance of the bias circuit even when anode voltage is unchanged.

Sheet limits use the design-center system: triode anode 30 V, cathode current 20 mA, grid circuit resistance no more than 10 MΩ and 30 V heater to cathode; RCA specifies the latter as peak in either polarity. Each diode allows 1 mA continuously. The also-tabulated characteristic of 2 mA with 10 V applied to a diode does not replace that continuous-service limit. Unshielded capacitances are triode grid–anode 1.7 pF, input 2.7 pF, output 1.7 pF and diode-anode to diode-anode 1.1 pF. General Electric curves examine the triode further; the Precision 660 table instead preserves three separate test rows, one per diode and one for the triode.