2AF4A

The 2AF4A is a miniature indirectly heated triode designed as a UHF oscillator in television receivers. In General Electric and Tung-Sol sheets, plate and grid each lead to two pins: the parallel connections reduce internal lead inductance at high frequencies, without making the tube a twin triode. The 2AF4A heater requires 2.35 V and 0.6 A. General Electric also relates the 2.35 V version to series-heater operation, but expressly distinguishes its heater rating from that of the 6.3 V 6AF4A.

Both sheets give 150 V as the design maximum plate voltage and 2.5 W as maximum plate dissipation. At an average amplifier point with 80 V on the plate and 150 Ω in the cathode circuit, they give 17.5 mA plate current, amplification factor 13.5, transconductance 6.5 mS and plate resistance about 2.1 kΩ. These values belong to one test condition and do not automatically describe the television oscillator. The manufacturers also differ in a limit: Tung-Sol gives 24 mA maximum cathode current, General Electric 22 mA.

In its JETEC No. 400 UHF oscillator test at 1000 MHz, General Electric specifies a 100 V plate supply, a 220 Ω plate-circuit resistor and a 10 kΩ grid-circuit resistor; it gives 17.5 mA plate current and 700 µA grid current. Tung-Sol gives its own point at the same frequency, with 100 V labeled plate voltage, about 17 mA plate current and 750 µA grid current: its table does not label the 100 V as a supply voltage. General Electric’s 100 V is the supply voltage, not a measurement of the actual plate voltage after the resistor. The stated 11-second heater time is defined by a controlled starting test, not a general warm-up time in a television.

The 2AF4A uses a T-5½ envelope and an E7-1 seven-contact miniature base, diagram 7DK: plate on pins 1 and 7, grid on 2 and 6, heater on 3 and 4, cathode on 5. Tung-Sol shows the socket from below. For this shorter version both manufacturers give maximum diameter 19.05 mm, maximum overall length 44.45 mm and maximum height excluding pins 38.1 mm. The same Tung-Sol sheet also shows the unsuffixed 2AF4, which is physically longer; its dimensions must not be assigned to the 2AF4A. Sources: Tung-Sol 2AF4/2AF4A sheet, 1964; General Electric 6AF4-A/2AF4-A/3AF4-A ET-T1257A sheet, 1956.