12C8GT

The 12C8GT combines two diodes and an amplifier pentode with a common cathode. Brimar lists it as a replacement type and explicitly refers to 6B8GT for information and curves. The referenced page describes simultaneous detector and IF or low-frequency amplifier service. Its pentode is semi-variable-mu: the maker allows some automatic-volume-control bias without appreciable distortion within the described operating context.

The 12C8GT heater requires 12.6 V at 150 mA, whereas the referenced 6B8GT uses 6.3 V at 300 mA. The base is octal and control grid goes to the top cap. Base connections are heater 2 and 7, pentode anode 3, diode anodes 4 and 5, screen 6, cathode with suppressor 8. Brimar specifies pin 1 connected to the metal shell. Our archived photograph shows a Brimar tubular-glass version: external form and connections belong together.

In Brimar’s referenced sheet, limits are 300 V anode, 125 V screen, 2.25 W anode dissipation and 0.3 W screen dissipation; control-grid resistance must not exceed 1 MΩ. At 250 V anode, 125 V screen and −3 V grid, it gives 9 mA anode current, 2.3 mA screen, 0.6 MΩ internal resistance and 1.12 mA/V transconductance. Cutoff is −21 V. These are data from the Brimar documentation referenced for 12C8GT, not figures to confuse with other editions or makers in the family.

Resistance-coupled operation has three examples. At anode and screen supply voltages of 90, 180 and 300 V, Brimar keeps a 250 kΩ anode resistor and 1.2 MΩ screen series resistor. Cathode-bias resistors are 3500, 2000 and 1600 Ω respectively; stated gains are 55, 70 and 80 with peak output voltages of 33, 55 and 100 V. Supply voltages are not actual electrode voltages after resistor drops; this distinction is essential, especially for the screen.

For GT, Brimar gives about 4.5 pF input, 10 pF output and at most 0.005 pF grid-to-anode capacitance with a close-fitting shield connected to cathode. Comparison with G figures on the same sheet shows that mechanical versions may have different capacitances. Two diodes, amplification and gain control explain its interest for compact receivers; not every circuit necessarily uses every function identically.