12SF7GT

The 12SF7GT is a diode-pentode for radio reception, associated by archive information with RF and IF stages using gain control. Tung-Sol's April 1946 booklet, entitled Tentative Data, documents pentode and diode behaviour separately. Both systems in one bulb combine controllable amplification and rectification; the cathode is common. Values must be read as those of the preserved tentative sheet.

Indirect heating requires 12.6 V and 0.15 A AC or DC. The valve has a T-9 glass bulb, octal base and any mounting position. Pin 1 connects to the metal base shell and inter-pin shield; 2 to control grid, 3 to cathode, suppressor grid and the shield shown in the diagram, 4 to screen grid, 5 to diode anode and 6 to pentode anode. Pins 7 and 8 serve the heater. With external shield connected to cathode, control-grid-to-pentode-anode capacitance is at most 0.004 pF and diode-anode-to-control-grid at most 0.002 pF.

For the pentode in class A1 with screen at 100 V and control grid at −1 V, Tung-Sol gives two operating points: at 100 V anode voltage, anode current 12 mA, screen current 3.4 mA, approximate anode resistance 0.2 MΩ and transconductance 1.975 mA/V; at 250 V anode voltage, respectively 12.4 mA, 3.3 mA, 0.7 MΩ and 2.05 mA/V. For both it gives approximately −35 V grid voltage when transconductance drops to 10 µmho, or 0.01 mA/V. The curve shows progressive changes of transconductance and currents with bias, helping explain pentode control without assigning fixed gain.

For the diode the sheet requires at least 0.8 mA with 10 V DC applied. Another curve relates developed DC voltage and rectified current to load and RMS input signal, confirming circuit dependence. Listed limits are 300 V pentode anode voltage, 100 V screen-grid voltage, 3.5 W anode dissipation, 0.5 W screen dissipation and 90 V heater-to-cathode. Maximum screen-supply voltage is instead 300 V: a separate entry from the 100 V allowed at the electrode. This distinction and the common cathode are essential details for reading this combined valve correctly.