U19

The U19 is a half-wave vacuum rectifier for high-voltage power supplies, with a directly heated oxide-coated filament. Our sources place its introduction in 1947; the preserved documents include a Marconi sheet dated July 1954. The filament requires 4 V and approximately 3.3 A. The British B4 base connects the filament to pins 3 and 4, while 1 and 2 are not connected; the anode connects to the top cap. Photographed specimens show ceramic supports and insulators, filament tension springs and a carbonised anode, described by our sources as helping to dissipate heat. One GEC specimen also carries the CV187 marking.

The sheets specify 2.5 kV RMS as the maximum applied AC voltage and 7.1 kV as the maximum peak inverse voltage. Limits in individual editions must nevertheless be distinguished. For the single-phase half-wave circuit, the Osram sheet specifies maximum DC current 250 mA, peak current 2 A and anode limiting resistance at least 100 ohms. Marconi in 1954 instead specifies maximum RMS current 250 mA, peak current 1.5 A and source resistance at least 500 ohms. Both give a 4 µF input capacitor and a minimum 10 H inductance for a choke-input filter. These figures cannot be freely combined between different circuits and editions.

Two U19 valves can rectify both half-cycles using a centre-tapped secondary. Marconi documents an example delivering 500 mA in total, with input of 2.5 + 2.5 kV RMS and source resistance at least 700 ohms. With a 4 µF capacitor input it gives 2.5 kV DC output; with choke input it gives 2.1 kV and, for this particular operating point, minimum inductance 4.25 H. The same sheet considers a four-valve circuit and includes output-voltage curves against current. Comparing capacitor and choke input makes clear how much the result depends on the power-supply circuit as well as the rectifier.

Both documents require the filament to be powered at least 20 seconds before applying anode voltage when peak inverse voltage exceeds 5 kV. Marconi also warns that a load short circuit or flashover, even very brief, may destroy the rectifiers in circuits with low DC resistance. Vertical mounting is preferred; horizontally, the filament’s major axis must remain vertical, avoiding excessive shocks and vibration. The Marconi drawing gives maximum diameter 52 mm and maximum overall length 186 mm: photographs of individual specimens document varying actual dimensions and construction details.