KF3

The KF3 is a variable-slope pentode for RF and IF amplification in battery receivers. Philips describes a selective-control tube providing strong gain reduction over a relatively short negative-bias range. It is particularly useful in short-wave superhets: when converter-octode control is avoided to prevent frequency pulling, the KF3 can provide automatic control in amplifier stages. Philips Miniwatt’s 8 November 1935 sheet already documents its data and curves.

Heating is direct, parallel battery-fed: manuals and the 1948 sheet specify 2 V at about 45 mA; the first 1935 sheet instead gives 50 mA. The European P side-contact base has control grid on the top cap. Suppressor and external metallizing have separate connections; especially on short waves Philips recommends very short, low-inductance ground leads. Maximum 1948 dimensions are height 102 mm, diameter 40 mm. External connections are part of the intended RF performance.

Normal points use plate and screen both 90 or 135 V, control grid −0.5 V, suppressor 0 V. Respectively: plate current 1 and 2 mA, transconductance 0.5 and 0.65 mA/V, amplification factor 1000 and 850, internal resistance 2 and 1.3 MΩ. Screen current at 135 V is 0.6 mA; at 90 V older German/French texts give 0.3 mA, the English manual 0.2 mA. Control data also differ: 1948 gives 5 µA/V at −9 V in the 90 V regime and 6.5 µA/V at −13.5 V in the 135 V regime; 1937 gives below 2 µA/V at −10 and −15 V. These belong to their respective editions.

The 1948 limits are plate and screen 135 V, plate dissipation 0.5 W, screen 0.2 W, cathode 5 mA and control-grid circuit maximum 3 MΩ. November 1935 allowed plate 0.7 W and grid 2.5 MΩ; French Bulletin 30 gave maximum plate/screen 150 V with 0.7 W and 2.5 MΩ. Capacitances in 1948 are input 6.2 pF, output 5.2 pF, grid-plate below 0.006 pF; the older bulletin gave 5.7 and 5.1 pF. Differences are not resolved by always picking the largest value.

Philips discusses a practical compromise: moderate slope saves current in a properly apportioned receiver. If the audio stage is omitted and a KL4 output tube follows the KB2 double diode directly, the maker advises against a controlled KF3 IF stage immediately ahead: large detector-drive voltages can cause overmodulation. It prefers the fixed-slope KF4 there. With a sensitive audio stage, KF3 can again serve as controlled IF because less signal is needed. This is receiver architecture, not an assertion of inadequate output power.

For RF/IF, Philips stresses low capacitances, high damping resistance and weak plate feedback, preserved by effective shielding. The French bulletin reports about 37 kΩ parallel input resistance and 0.16 MΩ at the plate at 10 m wavelength, hot: RF measurements under those conditions, distinct from megohm static internal resistance. Lowering screen voltage can narrow the control range but the note warns of increased characteristic-curvature distortion. Current, slope and cross-modulation curves complete the description of a control tube whose use depends on its circuit.