12DF7

The 12DF7 is a miniature high-mu twin triode designed for low-level input amplification in tape recorders and high-quality audio preamplifiers. Tung-Sol describes it as a low-microphonics version of the 12AX7 that may replace that type. Microphonics concerns sensitivity to mechanical vibration; the sheet provides no numerical noise figure to turn into a guaranteed specification. The available document is dated June 1, 1957; that dates the document rather than independently proving launch year.

Both sections have the same stated class-A1 operating points. At anode 100 V and grid −1 V, each triode has anode current 0.5 mA, amplification factor 100, anode resistance 70 kΩ and transconductance 1.25 mA/V. At anode 250 V and grid −2 V, current is 1.2 mA, amplification factor remains 100, anode resistance falls to 55 kΩ and transconductance rises to 1.6 mA/V. These are triode parameters at the stated points: factor 100 does not automatically equal complete-stage gain.

The sheet includes a resistance-coupled amplifier table for supplies of 90, 180 and 300 V. At 180 V, for example, with anode resistor 100 kΩ, load 100 kΩ, grid resistor 100 kΩ and adequately bypassed cathode resistor 1 kΩ, it gives gain 40 and maximum output 15 V RMS at 5% total harmonic distortion. Table gain is measured at 2 V RMS output. This is a circuit example with defined loading and bias, not a universal valve output level.

Per-section design-center maximums are anode 300 V and anode dissipation 1 W; maximum positive DC grid voltage is 0 V and negative bias extends to −50 V. Heater–cathode limit is 180 V. Stated capacitances are grid–anode 1.4 pF and input 1.6 pF for either triode; output is 0.4 pF for section 1 and 0.3 pF for section 2. These small capacitance differences remain distinct despite common operating points.

The heater operates at 12.6 V, 0.15 A between pins 4 and 5, or 6.3 V, 0.3 A between pin 9 and joined pins 4 and 5. Noval B9A base, bottom-view connection 9A: 1 anode, 2 grid, 3 cathode of section 2; 6 anode, 7 grid, 8 cathode of section 1; 4 and 5 heater ends, 9 centre tap. Any mounting position. Tung-Sol gives maximum overall length 2 3/16 inches, 55.5625 mm, maximum seated height 1 15/16 inches, 49.2125 mm, and maximum diameter 7/8 inch, 22.225 mm. Documentation: Tung-Sol, 12DF7 Twin Triode, June 1, 1957, three pages, including resistance-coupled amplifier table and characteristic curves.