12FA6

The 12FA6 is a seven-pin miniature pentagrid converter, or heptode. Tung-Sol specifies combined oscillator and mixer operation in superheterodyne receivers with heater, anode and screens supplied directly from an automotive battery. The heater is nominally 12.6 V, 0.15 A, AC or DC. The application note calls for operation over the 10.0–15.9 V range encountered with a nominal 12 V vehicle source, with adequate design margins; the nominal figure alone does not cover every vehicle condition.

The sheet separates self-excited and externally excited conversion. In self-excited operation, anode and grids 2 and 4 are at 12.6 V; grid 1 receives about 2.5 V RMS oscillation with a 33 kΩ resistor. Grid 3 is listed at 0.50 V, with a note on average contact potential across a 2.2 MΩ resistor. Typical conversion transconductance 320 µA/V, approximate anode resistance 0.8 MΩ, anode current 450 µA, screen current 1000 µA, cathode current 1500 µA, grid-1 current 60 µA. The oscillator note specifies screen feedback: grids-2-and-4-to-cathode voltage is about 13% of grid-1-to-cathode voltage.

With external excitation, grid 1 instead receives 0.85 V RMS, grid 3 is 0 V, and grids 2 and 4 connect to the anode at 12.6 V. Conversion transconductance 300 µA/V, anode current 670 µA, screen current 1200 µA, cathode current 1900 µA, grid-1 current 45 µA. At grid 3 about −3.2 V, conversion transconductance is 10 µA/V. The self-excited case instead gives about −3.0 V for 20 µA/V and −3.5 V for 5 µA/V. These are distinct operating conditions.

The non-oscillating oscillator table uses grids 1 and 3 at 0 V and screens connected to anode at 12.6 V: transconductance 4 mA/V, amplification factor 9, cathode current 4.5 mA, grid 1 about −3.5 V for anode current 10 µA. Ratings headed design-center include anode 30 V, screen voltage and supply 30 V, negative grid 3 up to 30 V and positive maximum 0 V, cathode current 20 mA, maximum grid-3 resistance 10 MΩ. Heater–cathode limit is ±30 V. The 0.50 V grid-3 characteristic has its specific measurement note and does not replace the positive DC rating.

The B7G base shows from below 1 grid 1, 2 cathode, 3 and 4 heater, 5 anode, 6 grids 2 and 4, 7 grid 3. Any mounting position. Maximum diameter 3/4 inch (19.05 mm), seated height 1 7/8 inches (47.625 mm), overall height 2 1/8 inches (53.975 mm). Capacitances use external shield number 316 connected to pin 2: RF and oscillator inputs each 7.2 pF, mixer output 12 pF, oscillator output 20 pF; maxima grid 3–anode 0.25 pF, grid 3–grid 1 of 0.15 pF, grid 1–anode 0.05 pF. Documentation: Tung-Sol, 12FA6, Heptode, Tentative Data, May 1, 1958, two pages.