6BL7

The designation 6BL7 identifies a twin-triode family documented mainly as 6BL7GT in datasheets. Two independent systems share one glass envelope with an eight-pin octal base and indirect heating. The intended period application is particularly interesting: generating vertical scanning and amplifying its signal in television receivers with wide-angle picture tubes. Tung-Sol explicitly describes using the two sections together as deflection generator and amplifier; the name 6BL7 alone does not establish a universal audio output power.

The 6BL7GT heater requires 6.3 V at 1.5 A, AC or DC. Each triode has separate grid, anode and cathode connections: one set occupies pins 1, 2 and 3, the other 4, 5 and 6; the heater is on 7 and 8. RCA data of 1 October 1953 give, for each section as a class A1 amplifier, 250 V anode, −9 V grid and 40 mA anode current. At this point they list amplification factor 15, anode resistance 2150 Ω and transconductance 6.2 mA/V. Tung-Sol tentative data of October 1950 use the same voltage and current point but give 14, 2000 Ω and 7 mA/V instead: values must be read with their manufacturer and edition.

For vertical deflection, RCA distinguishes a 600 V maximum anode supply from 500 V maximum DC at the anode. For a 525-line, 30-frame-per-second television system, the design-center specification permits positive anode pulses up to 1800 V, lasting no more than 15% of a scanning cycle. Maximum anode dissipation is 10 W for one section, but only 12 W total for both: doubling the individual limit would be wrong. Dissipation and power delivered to a load are also different quantities. The pulse limit is not an allowable continuous voltage.

RCA’s typical vertical circuit makes the distinction concrete. At 450 V DC anode, 1200 Ω cathode-bias resistance and 11 mA anode current, the output contains an approximately 270 V peak-to-peak sawtooth component and a positive pulse component of about 600 V. The grid receives a 36 V peak-to-peak sawtooth component and a 44 V negative pulse. These are television waveform components, not audio sine-wave figures. Family photographs show the two adjacent anodes, supporting micas and, in one Sylvania specimen, cooling fins attached to the grids: details consistent with managing substantial currents and dissipation in a compact envelope.