12DT7

The 12DT7 is a miniature high-mu double triode designed for resistance-coupled voltage amplifiers in audio circuits where low noise and hum are important. Tung-Sol describes it as otherwise similar to the 12AX7, without making that phrase a universal guarantee for every substitution. The available sheet is marked tentative and dated March 1, 1964; this dates the document, not independently the start of production.

The two class-A1 characteristic points apply to each triode. At anode 100 V and grid −1 V, anode current is 0.5 mA, amplification factor 100, anode resistance 80 kΩ and transconductance 1.25 mA/V. At anode 250 V and grid −2 V, current rises to 1.2 mA, anode resistance falls to 62.5 kΩ and transconductance becomes 1.6 mA/V, with amplification factor still 100. That is a triode parameter, not automatic gain in any circuit.

The document includes specific noise tests. A mechanical test for each section uses anode supply 250 V, anode load 220 kΩ, cathode resistor 3 kΩ bypassed by 100 µF and grid resistor 20 kΩ; the note describes a hand mallet and an amplifier covering 30–12,000 Hz. A separate equivalent-noise-and-hum test parallels both sections with supply 100 V, anode load 110 kΩ, unbypassed cathode resistor 2.4 kΩ and grid resistor 0 Ω. These are measurements in defined circuits, not directly transferable to a preamplifier with different connections or bandwidth.

Per-section design-center maximums are anode 300 V, anode dissipation 1 W, positive DC grid bias 0 V and negative bias 50 V. Total DC-and-peak heater–cathode limit is 180 V in either polarity; with positive heater the DC component is limited to 100 V. Without an external shield, grid–anode capacitance is 1.7 pF and input 1.6 pF for either triode; output is 0.46 pF for section 1 and 0.34 pF for section 2.

The heater operates in series at 12.6 V, 0.15 A or in parallel at 6.3 V, 0.3 A. 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. For 6.3 V use pin 9 and joined pins 4 and 5; for 12.6 V use 4 and 5. Any mounting position. Drawing maximums are overall length 2 3/16 inches, 55.5625 mm, seated height 1 15/16 inches, 49.2125 mm, diameter 7/8 inch, 22.225 mm. Documentation: Tung-Sol, 12DT7 Double Triode, tentative data March 1, 1964, two relevant pages in the preserved file.