DL

The Marconi DL is a low-impedance triode presented in the catalogue of valves for DC mains. The manufacturer proposes it as a small output valve, first low-frequency amplifier or oscillator. Despite “Low Magnification”, referring to low amplification factor, the text stresses that high transconductance can provide considerable stage gain. It is proposed where sensitivity is wanted without a pentode: a catalogue application statement, not a promise of circuit-independent gain.

The sheet gives approximately 16 V heating voltage and 0.25 A current. Archive information describes indirect heating and series operation in DC-mains receivers. The approximate operating table lists three conditions: plate 100 V, grid bias −4 V, plate current 10.5 mA; plate 150 V, −6 V, 18 mA; plate 200 V, −8 V, 25 mA. They are separate points, not values to mix freely.

The characteristics box states maximum plate voltage 200 V and maximum plate dissipation 5 W. Amplification factor is 12; internal resistance 2,660 Ω and transconductance 4.5 mA/V carry a reference to plate 100 V and grid −0.5 V. This differs from the table’s 100 V point at −4 V: the different bias is essential. The curves show current versus plate and grid voltage; their graphical extent does not change the stated 200 V maximum.

For output use Marconi says the DL can provide average domestic volume with a moving-iron loudspeaker. This recalls the transducer envisaged by the catalogue without turning it into an audio-power figure absent from the page. The 5 W is plate dissipation, not output power. Secondary information associates the valve with the European five-contact B5 base, but this page has no pinout or mechanical dimensions; they are not reconstructed by analogy. Documentation: Marconi, D.C. Mains Valves, DL, Low Magnification, page 38; the available page does not state the edition year.