6AB7

The 6AB7 is a high-transconductance semi-remote-cutoff pentode paired with designation 1853 in RCA, Tung-Sol and Sylvania documentation. RMA release 166 of March 18, 1939 explicitly relates the names: RCA registered 6AB7 and 6AC7 as identical to its 1853 and 1852 respectively, double-marked 6AB7/1853 and 6AC7/1852. Attached documentation copyrighted 1938 presents the 1853 for amateurs and experimenters building television receivers, chiefly RF and IF stages including automatic gain control, also mixer and oscillator service at low voltage. This documented origin explains the later sheets’ television-amplifier heading.

The envelope is metal with eight-contact octal base and all connections at the base. The oxide cathode is indirectly heated at 6.3 V and 0.45 A, AC or DC. Bottom view: pin 1 shell, 2 and 7 heater, 3 suppressor, 4 control grid, 5 cathode, 6 screen, 8 anode. RCA explains that a shielded grid lead avoids a top cap while retaining low grid-anode capacitance; adjacent grid and cathode pins simplify wiring and reduce the input inductive loop. With shell at cathode, capacitances are 8 pF input, 5 pF output, maximum 0.015 pF grid-anode.

Two A1 conditions use 300 V anode, −3 V control grid and 0 V suppressor. First: screen directly supplied at 200 V; second: screen supply 300 V through 30 kΩ to limit screen to 200 V at normal current. Both give 12.5 mA anode, 3.2 mA screen, approximate internal resistance 0.7 MΩ and transconductance 5 mA/V. Approximate amplification factor 3,500 is a pentode parameter, not any stage’s gain. To reduce transconductance to 50 µmho takes approximately −15 V grid in the first condition and −22.5 V in the second; the series resistor extends the control range.

RCA’s December 1, 1941 sheet and Tung-Sol list maxima 300 V anode, 200 V screen, 300 V screen supply, 3.75 W anode dissipation, 0.65 W screen. Initial RCA documents and Sylvania instead specify combined anode-plus-screen maximum 4.4 W with the same 0.65 W screen limit; combined power is not anode dissipation alone. The specified cathode-bias resistor has minimum 190 Ω. Tung-Sol limits grid resistance to 0.25 MΩ fixed bias and permits 0.5 MΩ with full cathode bias and series screen resistor. Notes also require low heater-cathode potential and dissipation checks under line fluctuations, especially fixed bias.

The original document discusses input capacitance and conductance varying with anode current. RCA proposes leaving part of the cathode resistor unbypassed: these changes shrink but effective transconductance is reduced and effective grid-anode capacitance increases. With that arrangement screen and suppressor must be at signal ground; in RF and IF the suppressor connects directly to ground to reduce feedback. The example uses 190 Ω cathode resistance tapped approximately 70 Ω from the cathode end. Later curves separately show anode current, screen current and transconductance, explaining the screen supply’s contribution to circuit behavior.