DL65

The Philips Miniwatt DL65 is a subminiature output pentode for hearing aids, intended for milliwatt-level power. Direct DC heating uses a parallel supply of 1.25 V, 13 mA. The file preserves tentative data dated 11 November 1949 and a Philips version of 11 November 1953 covering both DL65 and DL67. Shared electrical figures do not erase the two types’ different connection and outline drawings.

Typical characteristics refer to plate and screen 22.5 V and control grid −0.2 V: plate current 0.475 mA, screen current 0.10 mA, transconductance 0.42 mA/V and internal resistance 0.4 MΩ. Screen-to-control-grid amplification factor is 9. Capacitances are control grid 2.5 pF, plate 2.2 pF and plate-grid less than 0.2 pF. “Less than” is retained, not converted into an exact capacitance.

For output, the manufacturer gives two 22.5 V circuits with a 0.1 MΩ plate load. In the first, grid-leak resistance is 10 MΩ and Rk zero: idle plate/screen currents are 0.5/0.095 mA; with 0.45 V RMS input they become 0.34/0.09 mA, output 1.8 mW at 10% total distortion. The note says the tube’s effective input resistance is approximately 3.5 MΩ in this case, so it is not simply the external 10 MΩ resistor.

The second circuit has a 3 MΩ grid leak and 4 kΩ bias resistor Rk. Idle plate/screen currents are 0.20/0.04 mA; at 0.67 V RMS input, 0.19/0.07 mA, output 1.6 mW and total distortion 10%. Dedicated curves show current, distortion and drive as output rises in each configuration. The examples are not merged into one operating point or extended to an arbitrary load.

Maximum ratings are plate voltage 45 V, plate dissipation 25 mW, screen voltage 45 V, screen dissipation 6 mW, cathode current 0.6 mA and grid-leak resistance 10 MΩ. The specified filament-voltage range is 0.9–1.55 V. The −0.2 V grid entry refers to grid current +0.3 µA and is not a cutoff voltage. Maximum plate dissipation 25 mW is not available output power; the sheet distinguishes them.

The DL65 has five wire leads. In the numbered drawing: 1 positive filament joined to g3, 2 plate, 3 screen g2, 4 control grid g1, 5 negative filament. The 1953 version identifies the reference with a red spot. DL65 dimensions are maximum width 7.9 mm, maximum body height 36 mm and minimum lead length 32 mm: not one overall height or an invented tolerance. Secondary designation B5A does not describe an ordinary five-pin rigid-pin socket. Documentation: Philips Miniwatt, DL65, tentative data 11 November 1949; Philips, DL65/DL67, 11 November 1953; thirteen-page file including both versions, curves and collection index.