6AD4

The 6AD4 is a high-mu subminiature triode with a coated unipotential cathode and flexible leads. Sylvania’s registration sheet describes a T-3 bulb and R8-1 button base, distinct from ordinary miniature tubes with rigid pins. It refers to RTMA release 773A of January 29, 1951: a documentary reference, not proof of commercial introduction. The two available pages describe characteristics and ratings without assigning specific equipment or circuit applications.

The heater nominally requires 6.3 V and 150 mA. The design-center ratings table allows AC or DC heating and ±10% heater-voltage tolerance. Maximum plate voltage is 150 V, maximum plate dissipation 0.3 W and maximum cathode current 2 mA. The stated heater–cathode limit is 90 V; maximum grid-circuit resistance is 1 MΩ with cathode-resistor self-bias. These ratings must be distinguished from the typical operating point on the following page.

The characteristic point uses a 6.3 V heater, 100 V plate and 820 Ω cathode resistor. Plate current is 1.4 mA, transconductance 2000 µmho, equivalent to 2 mA/V, plate resistance 35 kΩ and amplification factor 70. This last figure is a triode parameter; it does not mean that a complete amplifier necessarily provides a gain of 70. The sheet also gives approximately −3 V grid voltage for 10 µA plate current, a low-current reference distinct from the normal operating point.

Interelectrode capacitances show the importance of the measurement arrangement. Without an external shield the figures are 0.8 pF grid–plate, 1.7 pF input and 0.7 pF output; shielded values are respectively 0.7, 1.9 and 2.2 pF. The note specifies an external shield 0.405 inch in diameter, approximately 10.287 mm, connected to the cathode. This diameter describes the test shield, not the tube bulb. Mounting in any position is permitted.

Functional basing 8DK assigns lead 1 to the grid, 5 to the cathode and 8 to the plate; heater leads are 3 and 6, while 2, 4 and 7 have no connection. The source describes flexible leads, not an octal base. Sylvania distinguishes maximum bulb diameter 0.400 inch (10.16 mm), maximum bulb-only length 1.375 inches (34.925 mm), and minimum lead length 1.500 inches (38.1 mm). These values are not combined into a total height including leads. Documentation: Sylvania Electric, Type 6AD4, High Mu Triode, RTMA Registration Data, two pages, referring to RTMA 773A of January 29, 1951.