12DE8

The 12DE8 is a miniature diode-pentode with independent cathodes. Tung-Sol describes a remote-cutoff pentode for RF or IF amplification in car radios, with heater, anode and screen supplied directly from the battery. RCA confirms receivers powered by six-cell storage batteries. The diode adds a rectifying section with its own cathode; the pentode is not thereby an audio power-output stage.

Nominal heater 12.6 V, 0.200 A; Tung-Sol permits AC or DC and requires design margins for automotive variation 10.0–15.9 V. RCA gives the same range and recommends 11–14 V for longest life. The pentode A1 point uses anode and screen 12.6 V, suppressor connected to cathode at the socket. Average control-grid voltage is −0.8 V, developed across 2.2 MΩ. Anode current 1.3 mA, screen current 0.5 mA, transconductance 1.5 mA/V, approximate anode resistance 300 kΩ.

Remote-cutoff operation permits reduced stage sensitivity: at control grid about −6 V, transconductance is 10 µA/V. Tung-Sol also gives anode curves at different grid biases with screen and heater 12.6 V and suppressor 0 V relative to cathode. Both documents list diode current 20 mA with 5 V applied; RCA explicitly identifies an instantaneous test condition. Maximum average diode current remains 5 mA; test current is not an available continuous operating value.

Pentode design-center limits: anode 30 V, screen 30 V, cathode current 20 mA, grid-circuit resistance 10 MΩ. Heater–cathode voltage is limited to 30 V in either polarity. Without external shield, pentode input 5.5 pF, output 5.7 pF, maximum grid–anode 0.006 pF; diode input 3.7 pF, output 5.7 pF. Maximum couplings: diode anode–pentode grid 0.018 pF, diode anode–pentode anode 0.012 pF, diode cathode–pentode grid 0.13 pF, diode cathode–pentode anode 0.0016 pF.

B9A E9-1 noval base, 9HG bottom view: 1 pentode control grid, 2 diode cathode, 3 diode anode, 4 and 5 heater, 6 pentode anode, 7 suppressor and internal shield, 8 screen grid, 9 pentode cathode. Pin 7 is separate from cathode pin 9, to which it connects in the stated operating point. Any mounting position. Maximum diameter 7/8 inch (22.225 mm), overall height 2 3/16 inches (55.5625 mm), seated height 1 15/16 inches (49.2125 mm). RCA separately gives body excluding tip 1 9/16 ±3/32 inches (39.6875 ±2.38125 mm), diameter 0.750–0.875 inch. Preserved photographs show RCA and Westinghouse specimens. Documentation: Tung-Sol, 12DE8, Diode-Pentode, Tentative Data, September 1, 1957, two pages with curves; RCA, 12DE8, Diode–Remote-Cutoff Pentode, March 1961, two pages.