DL98

Mullard describes the DL98 as an RF output pentode for portable equipment: class C amplifier, oscillator and frequency multiplier. The sheet documents operation at 100 MHz. Its purpose is RF amplification in the stated arrangements, not linear audio power inferred from an output designation. Available documents include November 1961 data and four pages of July 1959 curves.

The directly heated filament has two sections: series 2.5 V, 165 mA; parallel 1.25 V, 330 mA. In series, Mullard requires shunting the lower section with a resistor to bypass cathode current from the upper section. The filament midpoint is thus part of the circuit, not merely an accessory terminal. Any mounting position is allowed.

The static point at anode/screen 90 V and grid −9.9 V gives plate current 15 mA, screen 1 mA, transconductance 1.7 mA/V and plate resistance 22 kΩ. Without external shield: input capacitance 4.6 pF, output 7.6 pF, anode-grid below 160 millipicofarads, or 0.16 pF. The table also characterizes anode 135 V, screen 120 V at plate current 30 mA; this must remain distinct from the following page’s absolute plate current maximum 25 mA and does not authorize continuous operation beyond it.

At 100 MHz as class C amplifier or oscillator, anode/screen 90 V, grid −18 V, plate current 15 mA and screen 4.8 mA give load output 0.45 W with drive 30 mW. The second point, anode 150 V, screen 135 V, grid −39 V, plate 25 mA, screen 6.2 mA, gives 1.25 W with drive 70 mW. Mullard explicitly states this is at the absolute anode voltage, screen voltage and plate current limits. Other maxima include plate dissipation 3 W, screen 1.1 W, cathode current 32 mA, control current 1.5 mA, negative control voltage 75 V and control-cathode resistance 100 kΩ. Dissipation is not useful RF output.

Seven-pin B7G base: 1 screen, 2 and 6 filament midpoint with suppressor, 3 control grid, 4 and 5 filament ends, 7 anode. Mullard drawing gives maximum diameter 19 mm, body height 47 mm excluding pins, overall length 54 mm. Midpoint connections are distinct from filament ends; do not assume a generic B7G output pinout. Documentation: Mullard, DL98 R.F. Output Pentode, D1–D2, November 1961; Mullard, DL98, C1–C4 curves, July 1959.