DPT

The Marconi DPT is an indirectly heated output pentode presented among DC mains valves. The catalogue page recommends it for three- or four-valve receivers with only one low-frequency stage, emphasizing substantial output at 200 V with comparatively small grid input. The manufacturer mentions moving-coil and medium-impedance moving-iron loudspeakers. These are source application statements, without an associated numerical audio-power figure.

Operating figures are explicitly approximate. Heater current is 0.25 A and voltage approximately 16 V. The tabulated point has plate and screen 200 V, grid −10 V, plate current 40 mA and screen current 6 mA. Separately, amplification factor 90, internal resistance 30 kΩ and transconductance 3 mA/V are given at plate and screen 100 V, grid 0 V. These conditions must remain distinct from the 200 V point.

Maximum stated voltage is 200 V for both plate and screen. Maximum plate dissipation is 8 W: heat allowed at the plate, not 8 W output power. The graph includes a dashed constant-dissipation curve at 8 W and plate characteristics for several grid biases, measured with screen at 200 V. No output load or distortion figure is supplied to establish guaranteed audio performance.

Archive classification identifies an old British seven-contact M7A base. The primary page gives no pinout or mechanical drawing; dimensions stored elsewhere are not attributed to a nonexistent manufacturer drawing on this page. A commercial detail survives in the heading: the printed price is 25/−, or 25 shillings. Documentation: Marconi, D.C. Mains Valves, DPT, catalogue page 39, table and curves. The available page bears no publication date and is not used to fix a definite introduction year.