6B8

The 6B8 is a metal-envelope double-diode pentode designed to combine detection, automatic volume control and amplification in radio receivers. Its pentode can amplify RF, IF or audio; the diodes share its cathode but have separate plates. Ken-Rad highlights receivers where tube count matters. Its sheet associated with RMA release 77 is dated 15 June 1936, documenting the 6B8’s presence that year.

The indirectly heated cathode requires a 6.3 V AC or DC, 0.3 A heater. The base is octal, 8E, with control grid on the top cap. Pin 1 is the metal shell, 2 and 7 heater, 3 pentode plate, 4 and 5 diode plates, 6 screen, 8 cathode; suppressor connects internally to cathode. RCA allows any mounting position and grounds the shell in the cited application circuits. The glass 6B8G must be distinguished: Sylvania gives metal-version input 6 pF, output 9 pF and grid-plate 0.005 pF, versus G values 3.6, 9.5 and 0.01 pF under the stated conditions.

The representative class A1 point uses plate 250 V, screen 125 V and grid −3 V. RCA, Ken-Rad and Tung-Sol share 10 mA plate current, 2.3 mA screen, plate resistance about 0.6 MΩ, amplification factor about 800 and transconductance 1325 µmho, or 1.325 mA/V; cathode-current cutoff is around −21 V. RCA limits grid-circuit resistance to 1 MΩ. Limits are edition-specific: the earlier RCA manual and Ken-Rad give 250 V maximum plate, RCA of 2 September 1941 gives 300 V, while Tung-Sol of 1 October 1938 also presents a 335 V column with screen 110 V and plate current 8.5 mA. These must not be merged into one operating regime.

RCA 1941 and Sylvania specify actual screen maximum 125 V, screen supply 300 V, plate dissipation 2.25 W and screen dissipation 0.3 W. RCA application notes explain screen voltage’s role in control: extended cutoff permits moderate gain adjustment through bias. With resistor-fed screens the voltage can rise as grid bias becomes more negative; the earlier manual permits special conditions up to 200 V with bias more negative than −3 V. This belongs to that circuit and edition, not a replacement for other tables’ 125 V limit.

The diodes allow half-wave detection with one plate or two parallel plates, or full-wave detection. RCA and Ken-Rad note about twice the rectified voltage from half-wave operation in the described circuits. One diode can serve control alone: an RC network sets timing, while negative diode bias delays onset. RCA also considers the pentode as a DC control amplifier; Sylvania mentions reflex use. For audio, Tung-Sol gives a resistance-coupled example with 250 V supply, 0.5 MΩ plate load, 2800 Ω cathode resistor, 2.5 MΩ screen resistor and voltage gain 90. This is that circuit’s result, not an invariant tube gain.