DF33

The DF33 is a variable-mu pentode for controlled RF and IF amplifiers. Mullard therefore describes it for stages whose gain is adjusted through grid bias. Direct heating requires 1.4 V and 50 mA; the manufacturer explicitly specifies DC operation. The filament is not a heater isolated from a separate cathode: its ends appear directly in the electrode diagram.

The typical class A point has anode and screen at 90 V and control grid at zero. Anode current is 1.2 mA, screen current 300 µA or 0.3 mA, plate resistance 1.5 MΩ and transconductance 750 µA/V or 0.75 mA/V. At −4 V grid, Issue 2 gives 5 µA/V. The December 1958 page expresses the same result as a 150:1 reduction in transconductance. This quantitative control description belongs with the stated anode and screen conditions.

Both sheets publish maximum anode and screen voltages of 110 V each. Capacitances are 3.8 pF input, 9.5 pF output and less than 0.007 pF anode-to-control-grid. Curves show anode current versus anode voltage and bias with 1.4 V filament and 90 V screen. Extending the voltage axis beyond 110 V does not change the tabulated limit: a characteristic curve does not authorize exceeding ratings.

The base is octal, with control grid at the top cap. Following Mullard's bottom view, 1 is metallization, 2 positive filament, 3 anode, 4 screen and 7 negative filament joined to suppressor; other positions have no function indicated. The top cap is separate from base pins. Drawings give maximum diameter 30 mm and maximum overall length 102 mm, from top cap to the lowest outline extremity. These manufacturer dimensions remain separate from measurements of individual specimens.

Documentation consulted: Mullard DF33, Variable-mu R.F. pentode, Issue 2, sheets DF33 1546/1–1546/2, two pages of data, connections, dimensions and curves; Mullard DF33 technical page of December 1958, with control ratio and outline drawing.