6BK5

The 6BK5 is a miniature beam power valve designed chiefly for audio output stages of radio and television receivers with small available driving voltages. General Electric emphasizes power sensitivity, transconductance and plate efficiency; Tung-Sol also documents television video-amplifier service. Its 6.3 V, 1.2 A heater, for AC or DC, heats an indirectly heated coated cathode. Any mounting position is allowed.

The class A1 audio example agrees in all three sheets: 250 V anode and screen, −5 V control grid and 5 V peak audio drive. Quiescent currents are 35 mA anode and 3.5 mA screen; at the stated maximum signal they become approximately 37 and 10 mA. Transconductance is 8.5 mA/V and plate resistance about 100 kΩ. A 6500 Ω load gives 3.5 W at about 7% total harmonic distortion. The small drive voltage belongs to these conditions, not an output available with any power supply.

Design-center ratings specify 250 V for anode and screen, 9 W anode dissipation, 2.5 W screen dissipation and no positive DC control-grid bias. Grid-circuit resistance must not exceed 0.1 MΩ with fixed bias or 0.5 MΩ with cathode bias. RCA and General Electric give 100 V peak heater-to-cathode voltage for the 6BK5. Tung-Sol instead gives 200 V total DC plus peak, limiting the DC component to 100 V when the heater is positive relative to cathode. These specifications remain associated with their manufacturers.

Capacitances also require attention to the source: without external shielding, General Electric and RCA give 0.6 pF control-grid-to-anode capacitance, Tung-Sol 0.06 pF; input and output are 13 and 5 pF in all three documents. Curves describe anode and screen currents against bias and anode voltage; General Electric and Tung-Sol also plot output and distortion against load. GE's shared sheet includes 12BK5 and 25BK5: their heaters differ, and the stated 11-second controlled warm-up belongs to the 12BK5, not the 6BK5.

The T-6½ glass envelope uses the nine-pin noval E9-1 base, diagram 9BQ viewed from below: 1 anode, 2 unconnected, 3 and 7 control grid, 4 and 5 heater, 6 cathode and beam-forming electrodes, 8 screen, 9 unconnected. Drawings give maximum diameter 7/8 inch, maximum seated height 2 3/8 and maximum overall length 2 5/8: 22.225, 60.325 and 66.675 mm. General Electric and RCA separately dimension base-seat to bulb top excluding the tip: 2 inches ±3/32, or 50.8 ±2.38125 mm.

Documentation consulted: General Electric 6BK5–12BK5–25BK5, ET-T937, April 1955, four pages; Tung-Sol 6BK5, four pages with data of 1 July 1957 and curves of 1 April 1953; RCA 6BK5, tentative data of 1 May 1955, two pages.