6BC4

The 6BC4 is a miniature medium-mu triode designed for cathode-input radio-frequency amplifiers in UHF television tuners covering 470–890 MHz. Its documented application is therefore ultra-high-frequency amplification. A particularly short bulb and multiple grid connections distinguish its appearance and terminal arrangement. The indirectly heated cathode requires a 6.3 V, 0.225 A heater supply, AC or DC; any mounting position is permitted.

The nine-pin noval base uses connection diagram 9DR. Pins 1 and 9 connect to the plate, 2, 3, 7 and 8 to the grid, 4 and 5 to the heater, and 6 to the cathode. These repeated connections belong to the same electrodes of a single triode: they do not signify two independent sections or four grids. Cathode input, expressly specified by the manufacturers, is essential when interpreting tuner circuits using this tube.

The typical class-A1 operating point specifies a 150 V plate supply and a 100 Ω cathode resistor. The stated characteristics are 14.5 mA plate current, amplification factor μ of 48, 4.8 kΩ plate resistance and 10 mA/V transconductance. μ is not the actual gain of every circuit, which depends on its loads and connection. Likewise, 150 V is the supply voltage for that operating point, not automatically the actual plate-to-cathode voltage. Approximately −10 V grid voltage refers to reducing plate current to 10 µA, not to bias at the 14.5 mA operating point.

Design-center limits are 250 V plate voltage, 2.5 W plate dissipation and 25 mA cathode current. The 2.5 W rating describes heat dissipated at the plate, not delivered audio power. Heater-to-cathode voltage may reach 75 V peak in either polarity. With cathode bias, grid-circuit resistance is limited to 0.5 MΩ; fixed bias is not recommended. Characteristic curves do not supersede these limits: a voltage axis extending beyond the maximum does not permit operation beyond it.

Approximate interelectrode capacitances without an external shield are 1.6 pF grid-to-plate, 2.9 pF grid-to-heater-and-cathode, 0.26 pF plate-to-heater-and-cathode, and 2.7 pF heater-to-cathode. They help explain the UHF context and must retain their terminal definitions. Maximum dimensions are 7/8 inch diameter, 1¾ inches overall height and 1½ inches above the base seat: 22.225, 44.45 and 38.1 mm respectively. RCA additionally specifies base seat to bulb top excluding the tip as 1⅛ inches ±3/32, or 28.575 ±2.38125 mm. This dimension does not replace the height including the tip.

Documentation: RCA, 6BC4, tentative data dated June 14, 1954 and curves dated February 12, 1954; Tung-Sol, 6BC4, September 1, 1955, plate 4448; RCA Receiving Tube Manual RC-30, page 190. These dates identify the consulted documents and are not assumed to be commercial introduction dates.