A recent telephone call from William PA0WFO raised an interesting question of the correct filament voltage for the valves in the Wireless Set No.38. He pointed out that when fed via the suggested stabilised mains power unit described in Volume 2, of which the LT should be set to 3V, the actual filament voltage via the dropping resistors is about 2.4V. This is on the high side as the maximum tolerance for the valves is 10% (2.2V). When the set was designed it was taken in account that the battery voltage would drop steadily. However, when fed from a stable source such as the mains power unit it is advisable to reduce the voltage of the LT to 2.5V, resulting in approximately 2V for the filaments. See R3 in the circuit diagram below. Although 2.4V would not easily burn out the filament, it may, however, affect the emission of the valves sooner or later.

Suggested Mains Power Supply Unit as printed in Wireless for the Warrior Volume 2. This circuit is also suitable for Wireless Sets Nos.18, 46, 48 and 68. Adapted to give other voltages, it may also be used to power other battery sets such as Wireless Sets Nos. 31 and 88.
The required 3V LT (set to 2.5V as suggested above!) is obtained from the rectified 6.3V, regulated by an LM317-T. In case of using a No.18 or No.68 Set, remember the setting of the filament potentiometers in the transmitter unit. Note that in this mains power supply unit with its simple type of regulation using a 150V stabiliser valve, the limits of regulation (about 3-30mA) are only suitable for the No.38 Set or the other sets on receive.

END