EF5

The EF5 is a variable-mu pentode for radio-frequency and intermediate-frequency amplification, particularly associated with receivers using automatic gain control. Philips also calls it a selectode: the grid can be made progressively more negative to reduce amplification as the received signal grows. The design balances transconductance, plate current, control range, cross-modulation and modulation hum. The last aspect matters in AC/DC receivers, where a mains-frequency voltage can appear between heater and grid. Archived Miniwatt sheets dated 12 April 1937 already document the type, without establishing its first introduction by themselves.

At 250 V plate and 100 V screen voltage, with approximately −3 V grid bias, Philips gives 8 mA plate current, about 2.6 mA screen current, 1.7 mA/V transconductance and 1.2 MΩ internal resistance. With 85 V screen voltage and approximately −2 V bias, plate current is 7.5 mA and transconductance rises to 1.85 mA/V; at 60 V screen voltage it falls to 1.4 mA/V, with 4 mA plate current. Lower screen voltage therefore shortens the control range but also changes cross-modulation. The English and German editions describe ranges of approximately −3/−46.5 V, −2/−39 V and −2/−29 V; the 1937 sheets and French and Dutch versions instead give endpoints of −50, −45 and −35 V. These figures remain associated with their editions.

Philips recommends supplying the screen through a voltage divider. With a simple series resistor, when control reduces screen current almost to zero, the resistor voltage drop decreases and screen voltage can rise excessively, opposing control. The French bulletin includes automatic-control and manual-sensitivity circuits and divider tables for different supplies. It also calls for adequate shielding to limit capacitive and inductive coupling between input and plate: low internal capacitance does not remove the need for careful construction.

Heating is indirect, at 6.3 V and 200 mA, using AC or DC and parallel or series supply. Philips also considers car radios: for approximately 12.6 V supply it describes two 6.3 V, 200 mA heaters in series. The usual base is European P with side contacts; the control grid uses a top connection, while suppressor and metallization have separate connections useful for grounding on short waves. Photographs document red-coated specimens and one with gold metallization and an octal base: designation alone therefore does not identify every specimen's base. Specified capacitances are 5.4 pF input, 6.9 pF output and less than 0.003 pF grid-to-plate.

Limits also require attention to the edition. The 1937 Miniwatt sheet gives 250 V operating plate voltage, 2 W plate dissipation, 125 V screen voltage, 0.4 W screen dissipation and 15 mA cathode current; maximum grid-circuit resistance is 2.5 MΩ and heater-to-cathode voltage 75 V. The German discussion instead gives 300 V plate voltage, 3 MΩ grid-circuit resistance and 100 V heater-to-cathode voltage. Cold ratings of 550 V plate and 400 V screen are listed separately and are not normal amplification voltages. The sources present characteristic curvature as the reason for assigning the EF5 to controlled RF and IF stages.