EFF51

The Philips Miniwatt EFF51 is a twin RF pentode for push-pull amplifiers at metre and decimetre wavelengths and broadband stages. The 1 May 1948 sheet specifies use up to 500 MHz. Both systems share cathode and suppressor connections; anodes, control grids and screens have separate terminals. Philips Special Valves also describes ultra-short-wave frequency conversion with a separate external oscillator, for example a triode-connected EF51. It is therefore more than simply a pair of audio output pentodes.

The descriptive chapter gives heating 6.3 V, 0.6 A; the May 1948 sheet instead gives 6.3 V, 0.75 A. Other parameters also differ. For each system, the 1948 point at anode 250 V, screen 200 V, control −2 V gives anode current 6 mA, screen 1.2 mA, transconductance 7.5 mA/V, internal resistance 0.35 MΩ and equivalent noise resistance 800 Ω. At anode 300 V, screen 225 V and control −2 V: 10 mA, 1.8 mA, 9 mA/V, 0.25 MΩ and 750 Ω. The chapter gives slopes 8 and 10 mA/V and noise resistance 600 Ω for corresponding points; versions remain separate.

The 1948 sheet documents both systems in push-pull at anode 300 V, screen 225 V, current 2 × 10 mA: gain 2.7 times at wavelength 1 m and 4.7 times at 1.5 m. At 1.5 m, input damping resistance is 750 Ω, output 4700 Ω. The application chapter explains that 0.1 µH inductors in each screen lead can raise input damping resistance to about 4000 Ω at 1.5 m. This belongs to the described arrangement, not a fixed tube value.

Parallel connection is also documented: the Philips chapter gives combined transconductance 20 mA/V at total plate current 20 mA, equivalent noise resistance 300 Ω. Cascading both sections in one envelope is discouraged because mutual coupling could cause self-oscillation. Philips also says EFF51 supersedes EFF50, differing in its new nine-contact base. This documented relationship does not justify substitution without considering the different socket.

The 1948 sheet gives per-section capacitances input 9.6 pF, output 4.5 pF, anode-grid below 0.04 pF and grid-heater below 0.1 pF; the chapter instead gives cold input 9.4 pF and output 5.5 pF. Tabulated 1948 limits include anode 300 V, plate dissipation 3 W, screen 225 V and 0.5 W, control-grid dissipation 0.05 W, cathode current 15 mA and peak 50 mA. Zero-current anode and screen limits 550 V remain separate from operating values. Grid circuit up to 1 MΩ, heater-cathode circuit 20 kΩ, heater-cathode voltage limited to 50 V.

Nine-pin B9G base with central guide: 1 and 9 heater, 2 first anode, 3 first screen, 4 first control grid, 5 common cathodes and suppressors, 6 second control, 7 second screen, 8 second anode. Philips drawings give maximum body diameter 34 mm and maximum height above pin plane 63 mm, separately showing lower projection 15 mm and base width 38 mm. These are not interchangeable with overall height. Documentation: Philips Miniwatt Special Valves, EFF51 USW Push-Pull Amplifier Valve for Metre- and Decimetre Waves, two descriptive/application pages; Philips, EFF51 Twin H.F. Pentode, two multilingual sheets, 1 May 1948.