6BK8

The 6BK8 is a low-noise miniature audio pentode for early stages of high-gain amplifiers. Amalgamated Wireless Valve Company documentation also permits the first stage following a low-level microphone or tape reproducer, based on its specified microphony standard. It amplifies weak signals before power stages. The preserved document refers to JETEC release 1462, May 9, 1955, a registration-document date.

Heater 6.3 V, 0.2 A. Limits: plate maximum 300 V, screen maximum 200 V, cathode current maximum 6 mA, peak heater–cathode maximum 150 V. At plate 250 V and screen 140 V the sheet gives plate resistance 2 MΩ and transconductance 1.85 mA/V at 3 mA plate current. Cold, externally unscreened capacitances: grid–plate 0.025 pF, grid–heater 0.0025 pF, input 4 pF, output 5.5 pF.

Two practical conditions at plate 175 V: first voltage gain 180, cathode current 0.6 mA, cathode resistor 2.2 kΩ, grid resistor 1 MΩ, plate resistor 220 kΩ. Second gain 110, cathode current 1.2 mA, cathode resistor 1 kΩ, grid resistor 470 kΩ, plate resistor 100 kΩ. These are gains of the specified circuits, depending on resistors and currents, distinct from the tube’s transconductance alone.

A specific note quantifies hum: under the stated conditions, with control grid returned to ground through 470 kΩ, total hum referred to the grid does not exceed 1.5 µV. AC heating requires a centre-tapped winding; no variable hum-balancing potentiometer is required. This is hum under specified conditions, not a general equivalent-noise figure for every circuit. The tube is internally screened; an additional screening can may be used as needed. Any mounting position is allowed and a retaining device is recommended.

Noval E9-1 base, 9BJ bottom view: 1 screen grid, 2 internal shield, 3 cathode, 4 and 5 heater, 6 plate, 7 internal shield, 8 suppressor, 9 control grid. Internal electrostatic shield and screen grid have distinct functions. Bulb T6½. Maximum overall height 2 3/16 inches (55.5625 mm), seated height 1 15/16 (49.2125 mm), diameter 7/8 (22.225 mm). Check the specific drawing for connections and dimensions before replacement. Documentation: Amalgamated Wireless Valve Company, 6BK8, Miniature A.F. Pentode, two pages of characteristics and application notes, JETEC release 1462, May 9, 1955.