EBF2

The EBF2 combines a variable-mu pentode and two diodes sharing a cathode in one envelope. Philips describes it primarily for intermediate-frequency amplification: in a superheterodyne receiver the pentode can amplify the signal before detection, while the two diodes provide separate functions within the same component. In the published circuit, one diode detects the signal and the other rectifies the voltage for automatic gain control. The documentation specifies shielding between pentode and diode sections to reduce unwanted interaction; the assignment of the diodes describes that circuit, not a requirement for every receiver.

Variable transconductance lets the pentode progressively reduce gain as control-grid bias becomes more negative. In Philips's example with 250 V on the anode, the screen is fed from the same supply through 95 kΩ and sits at about 100 V; a 300 Ω cathode resistor contributes to bias. With −2 V on the grid, stated anode current is 5 mA and transconductance is 1.8 mA/V. At −38 V grid bias, transconductance falls to 18 µA/V: the documentation uses these points to illustrate a control ratio of about 100 to 1. These are conditions of a particular circuit; Philips warns that fixing the screen at 100 V rather than allowing it to vary through the resistor changes the control curves.

Heating is indirect, at 6.3 V and 0.2 A, and the sheets allow AC or DC supply, in series or parallel. The connection drawing shows a European side-contact base and the control-grid connection on top of the valve. The 1938 sheets and application chapter give maximum dimensions of 32 mm diameter and 93 mm height; the 1953 sheet instead gives 33 and 92 mm. In the described application the two diodes also share the pentode cathode potential: consequently the automatic-control delay remains close to cathode bias, about 2 V in the example, unless special circuits are provided. An application detail concerns muting: Philips requires bypassing the bias resistor with about 25 µF. Without it, characteristic curvature may rectify the signal and generate residual audio preventing complete muting. The chapter also proposes pairing with the EFM1, combining audio amplification and electronic tuning indication, to concentrate several functions in a simple receiver.

The limits must be read with their stated conditions. For the pentode Philips distinguishes 550 V on a cold anode from 300 V in operation; maximum anode dissipation is 1.5 W and maximum cathode current is 10 mA. For the screen grid the permitted voltage depends on anode current: 125 V at 5 mA, versus 300 V when it is below 2 mA; maximum screen dissipation is 0.3 W. The 1938 Philips Miniwatt sheets and later curves document the valve in that period, while another consulted sheet dates from 1953: these are document dates and do not by themselves establish the first year of sale. Documentary sources: Philips technical sheets and chapters on the EBF2.