EF22

The EF22 is a variable-mu pentode for amplifying radio-frequency or intermediate-frequency signals. Making the control-grid voltage more negative progressively reduces gain, a useful function for automatic control in receivers when signal strength changes. Philips sheets also allow its use as an audio amplifier with resistance-capacitance coupling and, if required, variable gain. The indirectly heated element requires 6.3 V at 0.2 A, and the base is an eight-contact Loctal type. Philips notes that its electrical characteristics are close to those of the EF9, but this does not make the two types interchangeable without checking the socket and circuit.

One Philips operating point for RF or IF amplification specifies 250 V on the plate, a 90 kΩ resistor in the screen-grid supply and a 325 Ω cathode resistor. Without gain control, the screen is at about 100 V, plate current is 6 mA and mutual conductance is about 2.2 mA/V. As control-grid bias becomes more negative, mutual conductance can fall to one hundredth of its initial value. The documentation distinguishes a fixed-voltage screen supply from a resistor feed, in which screen voltage follows tube current: the latter improves behaviour with respect to cross modulation while gain is being controlled. The graphs show that screen-circuit choice matters, as well as the control voltage.

Philips gives the EF22 an equivalent noise resistance of about 6.2 kΩ and proposes it for sensitive receivers with an RF preamplifier. The same documentation illustrates its use as an audio amplifier with two resistor networks and tables showing how gain and distortion change with control voltage at different output amplitudes. The pentode can therefore perform different jobs, but its data must be read in relation to the selected circuit. During restoration, check the screen-grid connection, cathode resistor, control voltage and socket before interpreting a change in gain as a fault in the tube.