12K8GT

The 12K8GT is a frequency converter combining a triode oscillator and a hexode mixer in one envelope. Tung-Sol specifies universal AC/DC superheterodynes with 150 mA heaters and describes construction intended to reduce oscillator-mixer interaction, aiding stability on the broadcast band and at higher frequencies. This is a maker-stated feature within the receiver circuit context.

The heater requires 12.6 V at 150 mA AC or DC. A T-9 glass envelope, octal base and top cap for the signal grid are used. Cathode is common; base connections are heater 2 and 7, hexode anode 3, screens 4, oscillator grid 5, triode anode 6 and cathode 8. Brimar refers to 6K8GT for information and curves while retaining 12K8GT heater ratings on its own page: that reference does not make heater supplies interchangeable.

Tung-Sol tabulates converter service at 100 or 250 V hexode anode. Both use 100 V screens and oscillator anode, −3 V signal grid and a 50 kΩ oscillator-grid resistor. Hexode anode currents are 2.3 and 2.5 mA, screen currents 6.2 and 6 mA, while oscillator anode current is 3.8 mA and its grid current 0.15 mA. Conversion transconductance is 325 and 350 µmho, or 0.325 and 0.350 mA/V; hexode internal resistance is about 0.4 and 0.6 MΩ.

Conversion transconductance differs from ordinary triode transconductance. The sheet gives about 3000 µmho for the non-oscillating oscillator section at 100 V anode and 0 V grid. It also shows control: at about −30 V signal-grid bias, conversion transconductance falls to roughly 2 µmho under the tabulated conditions. This illustrates variation through bias, without confusing the reduced value with normal operating-point transconductance.

Tung-Sol limits are 300 V hexode anode, 150 V hexode screens, 125 V oscillator anode and 16 mA total cathode current. Maximum dissipation is 0.75 W for each anode and 0.7 W for hexode screens. Screen supply may reach 300 V, distinct from the 150 V allowed at the electrode. With external shield at cathode, signal-grid capacitance to mixer anode is at most 0.08 pF, to oscillator anode 0.05 pF and to oscillator grid 0.2 pF. These small couplings illustrate the design’s attention to interaction between the functions.