12SJ7GT

The 12SJ7GT is a sharp-cutoff detector and amplifier pentode, the tubular-glass version of the 12SJ7 family. Tung-Sol’s sheet covers 6SJ7, 6SJ7GT, 12SJ7 and 12SJ7GT together and states identical electrical characteristics except for heater and capacitance ratings. It also specifies biased-detector, RF-oscillator and mixer use in properly designed circuits. Sharp cutoff distinguishes it from variable-mu pentodes: the document describes a defined cutoff behavior without assigning a universal gain-control function.

The 12SJ7GT heater requires 12.6 V at 150 mA AC or DC; the same sheet’s 6SJ7 and 6SJ7GT need 6.3 V at 300 mA. For GT, Tung-Sol depicts a T-9 glass envelope and eight-contact octal base with metal sleeve. All connections are at the base, without a top terminal. The maker recommends keeping heater-to-cathode potential as low as possible and never exceeding 100 V.

Both typical class A1 points keep screen at 100 V, control grid at −3 V and suppressor connected to cathode. At 100 V anode, the sheet gives 2.9 mA anode current, 0.9 mA screen, about 0.7 MΩ internal resistance and 1575 µmho transconductance, or 1.575 mA/V. At 250 V anode these become 3 mA, 0.8 mA, about 1.5 MΩ and 1650 µmho, or 1.65 mA/V. Grid voltage for cathode-current cutoff is −9 V in both cases. These are operating-point characteristics, not gain values directly applicable to every circuit.

Listed limits are 300 V anode, 125 V screen, 2.5 W anode dissipation and 0.3 W screen dissipation. Screen voltage is distinguished from maximum screen-supply voltage, 300 V: the latter does not permit 300 V directly on the electrode. With shell connected to cathode, maximum grid-to-anode capacitance is 0.005 pF; GT input and output capacitances are about 6.3 and 10 pF, versus 6 and 7 pF for metal versions. This is a real construction difference that related designations should not conceal.