DC70

The DC70 is a directly heated subminiature UHF triode for battery-powered radio equipment. Philips describes an oscillator for frequencies around 500 MHz and also amplification, super-regenerative detection and mixing in portable transceivers, balloon sondes, Citizens Radio and professional equipment. The small envelope and leads soldered straight into the circuit suit compact construction without a conventional valve holder. Any mounting position is allowed.

The filament requires 1.25 V, 0.20 A DC. Mullard explicitly describes intermittent push-to-talk service: life depends mainly on filament operating hours and temperature. About 200 hours may be expected under these conditions, whereas continuous operation at typical supplies has relatively short life. This is not a general life guarantee. Philips’ November 1960 revision likewise ties approximately 200 hours to intermittent service and stresses strong dependence on filament voltage.

Static data must be kept separate by document. Philips gives 150 V plate and −4.5 V grid: plate current 12 mA, transconductance 3.4 mA/V, amplification factor 14 and plate resistance 4 kΩ. Mullard Issue 3 gives 14.5 mA, 3.75 mA/V, factor 15 and 4 kΩ at the same plate and grid voltages. With external shielding, Philips capacitances plate-grid, grid-filament and plate-filament are 1.4, 1.3 and 1.9 pF; Mullard gives 1.5, 1.3 and 1.9 pF respectively. These small capacitances describe the high-frequency construction; the two sheets are not merged into one operating point.

For oscillation at 500 MHz, Philips gives 150 V plate, 20 mA cathode current and 450 mW output. Mullard’s separate class C RF telegraphy oscillator table gives 500 MHz, 150 V, plate current 18.7 mA, grid current 1.3 mA, grid resistor 6.8 kΩ and load power 0.55 W at 20% efficiency. For class C RF amplification at 200 MHz, Mullard lists 150 V plate, −18 V grid, plate current 16.8 mA, grid current 3.2 mA, load power 1.2 W and efficiency 48%; drive measured at the grid is about 200 mW, excluding tuned-circuit losses. Doublers and triplers are also covered: these are RF applications with their own conditions, not audio output powers.

Limits include 150 V plate, 2.4 W plate dissipation, 5 mA grid current and absolute maximum filament voltage 1.35 V. The 20 mA maximum cathode current does not apply indiscriminately to every multiplier: Mullard limits the tripler to 15 mA and the push-push doubler above 400 MHz to 2 × 12.5 mA. Maximum grid-filament resistance is 500 kΩ. An earthed metal mounting clip may reduce output by about 10% up to 200 MHz. Solder joints must be at least 5 mm from the glass seal and lead bends at least 1.5 mm away.

Mullard’s eight-wire subminiature layout is B8D/F: grid 1, negative filament 4, positive filament 5, plate 8; 2, 3, 6 and 7 unconnected. These are direct-solder leads, not a conventional rigid-pin base. Philips specifies diameter 9.3–10.16 mm and maximum glass length including tip but excluding wires 44.3 mm; Mullard gives 44.4 mm, cylindrical portion 35.4–38.2 mm and wires at least 38 mm long. Dimensions stay with their respective drawings and are not added to invent a maximum overall height. Documentation: Philips DC70, sheets dated 4 April 1959 and 11 November 1960; Philips Subminiature U.H.F. Triode DC70, pages 3–4; Mullard DC70 Subminiature U.H.F. Triode, Issue 3, six pages of data, applications and curves.