DA41

The DA41 is a Marconi power triode for audio amplification, specifically designed for class B. The July 1954 sheet describes a thoriated-tungsten filament and carbon plate, a documented construction distinct from metal-plate specimens in the photo archive. Its signature rating is 175 W from a push-pull pair, not one tube. Secondary sources also link the family with transmitter modulation, but the examined document explicitly presents an audio amplifier.

Under approximate data, the manufacturer gives filament 7.5 V, 3.1 A, plate voltage 1 kV, plate dissipation 40 W. Amplification factor 62 and transconductance 3.6 mA/V are taken at 1 kV plate and 40 W dissipation. Capacitances: plate-grid 5.9 pF, plate-cathode 1.8 pF, grid-cathode 10 pF. This is an approximate table, not a complete set of absolute maxima. The secondary 2.5 A figure remains distinct from Marconi’s 3.1 A.

Typical operation is per pair: plate supply 1 kV, grid bias 0 V, plate current 44 mA at zero signal and 280 mA at maximum signal. Plate-to-plate load 7 kΩ; specified input 220 V peak between grids, not 220 V peak-to-peak. Maximum-signal grid current 30 mA, output 175 W, distortion 5%. Currents belong to the per-pair table and are not individually attributed to one tube. Plate dissipation 40 W is also distinct from audio power delivered to the load.

The curves include plate characteristics with positive-grid traces marked up to +120 V, and grid current at 1,000 V plate with different indicated resistances. The push-pull operating graph relates output, plate current, grid current, drive requirement and distortion at 1,000 V supply, 0 V grid and 7,000 Ω plate-to-plate load. It supports the tabulated conditions, not treating graph-axis endpoints as ratings.

The base belongs to the UX4 family, but the plate is on the top cap. The primary diagram identifies two filament contacts, grid and a dummy contact, without pin numbers or an explicitly named viewing direction. Its functional arrangement is retained without invented numbering. Maximum overall height 155 mm; height to base bottom excluding pins but including top cap 141 mm. Bulb diameter 52 mm, cap diameter 14 mm, base diameter 35 mm are not marked maximum. Stated weight is a quarter pound, printed as 113 g.

Stored photographs and descriptions show actual construction differences: one specimen has a ceramic base; another has a different base, a plate formed from sheet metal, filament-tension springs and close-spaced grid. Specimen measurements must therefore remain separate from the 1954 sheet. Document date does not establish introduction year, and secondary catalogue associations are not guaranteed equivalences. Documentation: Marconi’s Wireless Telegraph Company Limited, Triode Type DA41, AF Amplifier, July 1954, four pages including curves and rear cover.