W42

The W42 is an indirectly heated variable-µ pentode designed for radio-frequency and intermediate-frequency amplifiers in AC mains receivers. Changing grid bias controls gain; the Osram manual states that its characteristic provides enough range for automatic volume control without modulation distortion at normal input signal levels. The heater is rated at 4 V and approximately 0.6 A, equivalent to 2.4 W. Some collected historical information gives a different current, but the manufacturer’s primary table is the reference used here. The control grid connects to the top cap: according to the manual, this reduces input capacitance and can simplify wiring layout in the receiver.

The recommended conditions are 250 V anode voltage, 100 V screen voltage and −3 V grid bias, with an average anode current of 7.6 mA, screen current of 1.9 mA and transconductance of 1.5 mA/V. The table also gives 1.75 mA/V at zero grid bias and only 0.0045 mA/V at −40 V. The voltage limits are 250 V at the anode and 125 V at the screen; the 150 V value found in a summary sheet is not the value in the manual used here. The curve page shows anode characteristics at 100 V screen voltage and currents versus bias at 250 V anode voltage. Grid-to-anode capacitance is about 0.005 pF with other electrodes grounded; capacitances to the other electrodes are 10.4 pF from the anode and 5.1 pF from the grid. The manufacturer recommends a potentiometer network to keep screen voltage substantially constant and limits the total effective resistance between grid and cathode to 2 MΩ.

The bulb is carbonised and supplied without external metallisation. If additional screening is required, the manual suggests a can about 42.5 mm in diameter and 78 mm long, extending from the bottom of the bakelite base to the centre of the grounded internal screen in the bulb dome. It recommends following these dimensions to retain the benefit of low grid-to-anode capacitance, a practical detail for understanding installation in contemporary receivers. The base has seven pins: viewed from below, pin 1 has no connection indicated, pin 2 is the anode, pin 3 the suppressor grid, pins 4 and 5 the heater, pin 6 the cathode and pin 7 the screen grid. The manual gives a maximum diameter of 41.5 mm and maximum length of 120 mm including pins. The photographs also show a specimen missing its top contact; its recorded dimensions are 41 mm diameter and 103 mm height without pins, distinct from the catalogue maxima.