12J5GT

The 12J5GT is a general-purpose medium-mu triode documented as both a resistance-coupled audio amplifier and a superheterodyne oscillator. Ken-Rad stresses transconductance and low output capacitance for high-frequency oscillator service. Its sheet references RMA release No.169 of March 28, 1939; preserved documents then include Tung-Sol data from January 1948, an RCA note from September 1949 and a JETEC sheet from August 1952. These references trace its technical documentation over time.

The indirectly heated cathode uses 12.6 V at 150 mA AC or DC. The envelope is tubular T-9 glass with an octal base; main connections are heater 2 and 7, anode 3, grid 5 and cathode 8. Pin 1 is unconnected and some base versions physically omit it, according to JETEC. RCA relates it to 6J5GT but specifies heater and base differences, referring to 6P5GT for 12J5GT base and connections. Similar designations therefore do not replace mechanical and electrical checks.

Tung-Sol gives two typical class A1 points. At 90 V anode and 0 V grid: 10 mA, about 6700 Ω internal resistance and 3000 µmho transconductance, or 3 mA/V. At 250 V anode and −8 V grid: 9 mA, about 7700 Ω and 2600 µmho, or 2.6 mA/V. Amplification factor is 20 in both cases. This is a triode parameter; actual stage gain also depends on load, bias and circuit.

Tung-Sol’s resistance-coupled amplifier table illustrates this. With both anode resistor and following load at 100 kΩ, supply voltages of 90, 180 and 300 V require cathode resistors of 3300, 2200 and 1800 Ω respectively; stated gain is 14. RMS outputs at 5% total harmonic distortion are 13, 26 and 40 V. Gain, however, is measured at 2 V RMS output: these indications use different measurement conditions. The maker also requires coupling capacitors chosen for the desired frequency response and adequate cathode-resistor bypassing.

Tung-Sol and JETEC limits include 300 V anode, 2.5 W anode dissipation, 20 mA cathode current, 1 MΩ grid-circuit resistance and 90 V heater-to-cathode. With a close-fitting shield connected to cathode, Tung-Sol gives about 3.8 pF grid-to-anode, 4.2 pF input and 5 pF output. The earlier Ken-Rad sheet has different capacitances and a 250 V anode limit: these remain tied to their sheet rather than becoming an artificial unified specification. Measurement and publication differences belong to the type’s technical history.