DLL21

The DLL21 is a directly heated double output pentode for battery radio receivers. Two sections in one bulb form a class-B push-pull output stage with low standing plate current. Philips compares it with double triodes: at low battery voltages, triodes may need grid-current drive and a stage capable of supplying it, whereas the described pentode arrangement operates without grid current. Its advantage therefore also concerns power consumption and the drive chain, not just the number of bulbs.

The DLL21 permits a choice of how much filament to use. At 1.4 V between pin 1 and either 7 or 8, current is 100 mA; at 1.4 V between 1 and joined pins 7 and 8, it is 200 mA; between 7 and 8 with pin 1 free, it requires 2.8 V at 100 mA. These are wiring arrangements of one tube, not three versions interchangeable without rewiring. Filament supply is DC. Philips sheets of 1953 additionally distinguish series-chain voltages: 1.3 V for one section or two parallel sections, and 2.6 V for two series sections.

In the economical 1.4 V, 100 mA arrangement, push-pull operation with plate and screen at 90 V, bias −5.75 V and 30 kΩ plate-to-plate load yields 0.3 W at 2.8% total distortion, requiring 4.8 V RMS per grid. At 120 V, −8.7 V and the same load, output is 0.6 W at 3% with 6.8 V RMS per grid. Standing plate current is 1 mA per section in both cases. These are complete-circuit results under the stated conditions, not power independent of supply and output transformer.

With both filament branches in parallel at 1.4 V, 200 mA, plate-to-plate load is 15 kΩ. At 120 V plate and screen, −8.2 V grid bias and 7 V RMS per grid, output is 1.2 W at 5% distortion; at 135 V, −9.4 V and 7.6 V RMS, 1.5 W at 3.8%. The 2.8 V, 100 mA filament arrangement has its own data: at 120 V, −8.1 V, 15 kΩ and 6.4 V RMS per grid, 1.1 W at 2.8%; at 135 V, −9.5 V and 7.4 V RMS, 1.5 W at 3.6%. Similar supplies do not justify interchanging the table rows.

For class B Philips specifies a bias battery: a resistor in the negative HT return would increase bias too much on strong signals and cause excessive distortion. The manual recommends 1.4 V, 100 mA to conserve dry cells; 200 mA is discussed for accumulator supply through a resistor, and 2.8 V for battery/mains sets. Switching to economical operation also requires appropriate loudspeaker matching. Parallel connection of both sections for class A is described, but considered uncommon because of higher plate-current consumption.

Limits apply to each section: maximum plate and screen voltages 135 V, plate dissipation 0.5 W, screen dissipation 0.1 W without signal and 0.4 W at maximum output. Maximum cathode current is 12 mA at 1.4 V/100 mA, and 25 mA at 1.4 V/200 mA or 2.8 V/100 mA. Maximum external grid-to-filament resistance is 1 MΩ. The −0.2 V grid limit is specified at +0.3 µA grid current and does not replace operating bias. The 1.1–1.5 V filament range applies to the 1.4 V section; the 1940 sheet separately gives 2.2–3 V for the 2.8 V arrangement.

The base is octal: 1 common filament connection and suppressors, 2 first control grid, 3 first plate, 4 common screens, 5 second control grid, 6 second plate, 7 and 8 filament ends. Dimensions above the base seat exclude pins: maximum height 85 mm; maximum diameter 36 mm in the manuals and 1940 and 1953 sheets, but 35 mm in the 1948 drawing. Editions remain separate; 85 mm is not overall height including pins. Documentation: Philips Miniwatt DLL21, November 5, 1940 sheets and curves, January 6, 1948 and October 10, 1953 sheets; Philips, “DLL21 Double output pentode” and “DLL21 Dubbele eindpenthode”, pages 107–116.