DBS1

The DBS1 is a small Mullard battery tube identified by its manufacturer as a screened tetrode. This matters because a secondary index calls it a pentode, while the original title and drawing show a tetrode with control and screen grids. The sheet is marked Tentative Data. Its directly heated filament is nominally 1.5 V, 70 mA, specific to DBS1 rather than interchangeable with similar-named small Mullard types.

Mullard gives two points, both at 120 V plate. Screen 45 V, control grid −2 V: plate current 0.8 mA, transconductance 0.43 mA/V, internal resistance 0.65 MΩ. Screen 60 V, control grid −2.7 V: 1.5 mA, 0.58 mA/V, 0.5 MΩ. The columns describe different screen and bias conditions; values are not interchangeable and do not predetermine circuit gain.

Stated limits: plate 120 V, screen 60 V, maximum control-grid circuit resistance 2 MΩ. Capacitances: plate-control grid 0.07 pF, input 5.7 pF, output 3.4 pF. No screen current, dissipations, audio power or distortion are given; they are not arbitrarily derived. Screened tetrode describes construction, not equivalence with other tubes.

A crucial connection detail is the top cap: the drawing assigns it to the plate, not the control grid. The four-contact base carries two filament ends, screen and control grid, the latter on the larger drawn contact. No pin numbers or explicit viewing direction are printed, so source function letters are retained. Maximum diameter 16 mm, overall height including pins and cap 67 mm. Documentation: Mullard, DBS1, Screened Tetrode, Tentative Data, one page, printing reference 11238; no explicit date.