6BC8

The 6BC8 is a miniature medium-mu twin triode with a semiremote-cutoff characteristic. Sylvania, Tung-Sol and RCA specify RF cascode amplifiers in VHF television tuners; RCA’s RC-30 manual also lists push-pull cathode-drive amplifiers. Current and transconductance curves versus bias document its semiremote characteristic: the designation is not a generic promise of audio performance.

The 6BC8 heater is rated 6.3 V, 0.4 A, AC or DC; RCA and Tung-Sol give 6.3 ± 0.6 V. The 4BC8 shares tables but needs 4.2 V, 0.6 A. Sylvania associates controlled 11-second warm-up for series strings specifically with the 4BC8: it must not automatically be transferred to the 6BC8. Any mounting position is allowed.

Per section, typical class A1 operation uses 150 V plate voltage and a 220 Ω cathode resistor, with plate current 10 mA, transconductance 6.2 mA/V and amplification factor 35; RCA and Tung-Sol give approximate plate resistance 5.3 kΩ. About −13 V grid voltage instead refers to transconductance reduced to 50 µA/V, not bias at the 10 mA point. Factor 35 is a triode parameter, not guaranteed gain of a complete stage.

Limits require source identification. Sylvania gives design-center values per section of 250 V plate, 2 W dissipation, 20 mA cathode. Tung-Sol, 1 October 1961, gives design-maximum 250 V, 2.2 W, 22 mA. RCA Data I, October 1966, instead gives 150 V, 2.2 W, 22 mA, whereas the later RC-30 combined 6BC8/6BZ8 entry gives 250 V, 2.2 W, 22 mA. This disagreement does not justify always choosing the larger number. Up to 300 V is conditional on cutoff in a cascode with both sections in series, not a normal operating point; the manual also requires the heater negative to cathode.

The internal shield has a dedicated terminal. RCA specifies capacitances with an external shield connected to the internal shield and distinguishes second-section grounded-grid measurements: 5.5 pF input and 2.4 pF output cannot be directly compared with the first grounded-cathode section’s 2.6 and 1.3 pF. RCA gives grid-to-plate capacitance 1.2 pF per section and plate-to-plate no more than 0.02 pF. Connections and measurement configuration are part of these numbers’ meaning.

The B9A noval base follows bottom-view 9AJ: section 2 plate, grid and cathode at 1, 2, 3; heater 4, 5; section 1 plate, grid and cathode at 6, 7, 8; internal shield 9. RCA and Tung-Sol give maxima of 0.875 inch diameter, 1.937 seated height, 2.187 overall length. Sylvania’s fractional dimensions, respectively 7/8, 1 15/16, 2 3/16 inches, are retained separately in mechanical records. Documentation: Sylvania 6BC8/4BC8 summary and Engineering Data Service, September 1955; Tung-Sol 6BC8, 1 October 1961; RCA 6BC8 Data I, October 1966; RCA Receiving Tube Manual RC-30, pages 191–192.