6F31

The 6F31 is a Tesla variable-mu pentode, called a selectode in the sheet of 12 November 1954, intended for receiver RF and intermediate-frequency amplification. Its gradual control characteristic accompanies use in gain-controlled stages. The manufacturer's sheet states that the 6F31 can replace 6BA6, EF93 and 6K4П: this is a documented replacement statement to be read alongside electrical conditions and the circuit.

It is a miniature construction with seven pins set into the glass base and a separately brought-out suppressor grid. The oxide cathode is indirectly heated; the heater requires 6.3 V and 0.3 A, AC or DC. Capacitances measured without external shielding are 5.5 pF input, 5 pF output and 0.0035 pF between anode and control grid. The last figure helps describe an RF amplifier component, where capacitive coupling from output to input matters.

Tesla provides two operating points at 100 and 250 V anode voltage, both with 100 V screen voltage, a 68 Ω cathode resistor and suppressor connected to cathode. Anode currents are respectively 10.8 and 11 mA, screen currents 4.4 and 4.2 mA; transconductance is 4.3 and 4.4 mA/V and internal resistance 0.25 and 1 MΩ. Curves show anode current against bias and anode voltage, as well as screen current. Nominal data therefore apply to specific operating points, not to performance independent of bias.

Operating maxima are 300 V anode voltage, 125 V screen voltage, 3 W anode dissipation and 0.6 W screen dissipation. Cold ratings are 500 V anode and 300 V screen and must not be confused with service limits. The document also lists −50 V as the control-grid voltage limit and 150 V between cathode and heater, for DC or peak AC value. Separating characteristics, curves and limits is essential to understanding the 6F31 as a controllable amplifier valve, rather than merely an equivalent designation.