ECH41

The ECH41 is a triode-hexode intended for frequency conversion in superheterodyne radio receivers. The triode forms the local oscillator; the hexode mixes its signal with the received signal and feeds the intermediate-frequency circuit. The triode grid is internally connected to the hexode's third grid: this link makes the two sections part of one conversion system rather than two independently usable tubes. The ECH41 has a Rimlock base and an indirectly heated 6.3 V, 0.225 A heater. Philips sheets show medium- and long-wave applications and particular arrangements for short waves.

At the Philips operating point, with an oscillator voltage of about 8 V RMS, the hexode's conversion conductance is about 500 µA/V and its internal resistance about 2 MΩ. The effective slope of the oscillating section is about 0.55 mA/V, distinct from the initial slope of the non-oscillating triode, about 1.9 mA/V. The document gives a conversion gain near 90 only for an intermediate-frequency transformer with a quality factor of 140, 100 pF tuning capacitors and critical coupling. It is a result of that specified circuit, not a guaranteed property of the tube alone.

The Philips article pays close attention to stable conversion. It proposes feeding the hexode screen grids from a potential divider and shows how the same divider can also feed the screen grid of an EAF42 or EF41 intermediate-frequency amplifier. For short waves it considers intermittent loss of oscillation and frequency drift when automatic gain control acts. An additional coil in the oscillator network allows the illustrated circuit to maintain a more even oscillator voltage across the band without excessively tight coupling. These are design suggestions for particular circuits; they help explain why replacement or repair also requires checking surrounding coils and components.

A distinctive feature of the documentation is a Philips circuit using the ECH41 as a phase inverter, with an overall voltage gain near 10 under the conditions shown. That example does not remove the internal grid connection or turn the ECH41 into two freely separable sections. The sheet dated 7 July 1953 also separates the sections' limits: the hexode has a maximum plate voltage of 300 V and plate dissipation of 0.8 W, the triode 175 V and 0.9 W. Each figure applies to its respective electrode and operating circuit. Documentary sources: Philips, ECH 41 Triode-hexode frequency changer, pp. 64–75; Philips ECH41, sheet 939 4307–4308, 7 July 1953.