DH63

The DH63 combines two diodes and a triode in one envelope and is intended for superheterodyne receivers. The Marconi sheet specifies detection, automatic volume control and resistance-coupled low-frequency amplification. The diodes provide the detected signal and control voltage; the triode can serve as the first audio amplifier. All three sections share a cathode, while the triode grid is brought to the top cap. Indirect heating requires 6.3 V at 0.3 A. It must be distinguished from the D63 on the same catalogue page: that type is a double diode without a triode and has two separate cathodes.

The approximate-data box gives a maximum plate voltage of 250 V, amplification factor 70, internal resistance 58 kΩ and transconductance 1.2 mA/V; the last two parameters refer to a plate voltage of 250 V and grid bias −3 V. The application table instead uses supply voltages of 340 and 275 V: these are upstream of the resistive load, not voltages to assign directly to the triode plate. Likewise, factor 70 is a tube parameter and does not equal the gain of the complete stage.

At a 340 V supply, Marconi gives two examples: a 0.1 MΩ plate load and 1,500 Ω bias resistor, with plate current 1.4 mA and gain 29; or a 0.25 MΩ load and 3,000 Ω resistor, with current 0.7 mA and gain 32. At 275 V, a 0.1 MΩ load and 2,000 Ω bias resistor give 1.1 mA and gain 28; with 0.25 MΩ and 3,000 Ω, the figures are 0.6 mA and gain 32. These are four distinct operating conditions. For these applications the following tube’s grid-leak resistance must be at least 0.5 MΩ, and the total effective DH63 grid-to-cathode resistance must not exceed 4 MΩ.

Interelectrode capacitances are specified with a screening can: plate-grid 1.7 pF, grid-earth 2.6 pF, plate-earth 6.2 pF and diodes-earth 7.8 pF. The measurement condition matters, particularly when comparing differently screened versions. Photos show the triode above the diodes; transparent specimens reveal two small diode plates beside the shared cathode and screening around the detector section. In some specimens the heater is wound to reduce mains-hum coupling into the audio signal.

The base is octal with the grid on the top cap. However, Marconi refers to catalogue pages 4–5 for connections; those pages are not included in the available scan, so this sheet does not establish a certified numerical pinout. It prints “Dimensions: 114 × 40 mm” without specifying the height reference plane or tolerance in that line. Descriptions of preserved specimens distinguish classic and tubular envelopes with different diameters and heights; those measurements are not merged into one outline for every DH63.

Constructional variety is an interesting feature: photos include Marconi, Osram, GEC and MWT markings, clear envelopes and others with an internal coating partly hiding the electrodes. Archive sources place the type in 1937 and distinguish the DH63M by external metallisation; this does not make every specimen identical or all related designations automatically interchangeable. Even a 1991 test label concerns the photographed specimen, not its manufacturing date. Documentation: Marconi, DH63, Double Diode Triode, page 27 of the preserved technical catalogue; the edition is not dated on the available page.