12A8GT

The 12A8GT is a pentagrid converter for AC or DC superheterodyne receivers, designed within the family of valves with 150 mA series heaters. Its history is documented by RMA release no. 156 of 7 January 1939, announcing the commercial release of the 12A8GT and other types by Arcturus. Arcturus calls it Midget and links the new series to lower receiver heater power than 0.3 A heater chains. Its function is radio: the system contains both oscillator electrodes and those converting the received signal.

Tung-Sol describes a T-9 glass bulb and small eight-pin octal base with a metal sleeve; the heater requires 12.6 V and 0.15 A. Arcturus's 1939 sheet instead gives 12.5 V and 0.15 A, a figure to retain in that sheet's context. Tung-Sol connections are pin 1 sleeve, 2 and 7 heater, 3 mixer anode, 4 screen grids, 5 oscillator grid, 6 oscillator anode electrode and 8 cathode; the top cap is the signal grid. Interelectrode capacitances are specified with an external shield connected to the cathode: RF input is 9.5 pF and mixer output 12 pF.

Tung-Sol's 23 September 1943 sheet gives two conversion conditions. At 100 V anode, 50 V screen and −1.5 V signal grid it gives 1.1 mA anode current, approximately 0.6 MΩ anode resistance and 360 µS conversion conductance. At 250 V anode, 100 V screen and −3 V grid the figures become 3.5 mA, approximately 0.36 MΩ and 550 µS. In the latter condition the oscillator anode-electrode supply is 250 V through a properly bypassed 20 kΩ dropping resistor; oscillator-grid resistance is 50 kΩ. Conversion conductance is not transconductance measured on the oscillator section alone without oscillation: the sheet treats these measurements separately.

Changing signal bias demonstrates control of conversion: in the 250 V column Tung-Sol gives 325 µS at −6 V, 100 µS at −10 V and approximately 6 µS at −35 V. Maxima include 300 V anode, 100 V actual screen and 200 V oscillator anode-electrode voltage; screen and oscillator supplies may each reach 300 V, without confusing supply voltage with electrode voltage. Maximum total cathode current is 14 mA and maximum dissipations are 1 W anode, 0.3 W screens and 0.75 W oscillator anode. These distinctions help when reading period radio circuits, where dropping networks and bias are integral to converter operation.