AC761

The RFT AC761 is a subminiature triode for microphone amplifiers and low-frequency preamplifiers. The December 1956 document from VEB Funkwerk Erfurt gives provisional technical data and explicitly says the tube is still under development. The date therefore describes the documentation stage, not automatically commercial introduction. Its heater is specified at 4 V and 105 mA, more precise than the 100 mA in an archive summary.

The stated static point has plate 60 V, grid −1.5 V and plate current 2.1 mA. Transconductance is 2.3 mA/V, amplification factor 22 and internal resistance 9.5 kΩ. Limits are separate: maximum operating plate voltage 120 V, cold plate voltage 250 V, plate dissipation 0.5 W and cathode current 5 mA. Thus 250 V is not the normal continuous operating voltage. Capacitances are input 2.5 pF, output 1.2 pF and grid-plate 2 pF.

Connection uses a special subminiature socket. The functional drawing shows four lower connections for cathode, heater ends and plate, with a separate upper grid connection. A coloured spot identifies the plate, without stating its colour. No contact numbers are assigned; a generic wire-ended classification must not obscure this actual arrangement. Maximum dimensions are diameter 10 mm, body length 38 mm and leads 40 mm on both the lower and grid sides. The two lead lengths are recorded separately, not added to invent an unstated overall dimension. Weight is approximately 5 g.

According to the manufacturer, figures printed in light type and not identified as limits are approximate. General operating conditions are referenced but absent from the two pages and are not completed by analogy with other triodes. Documentation: RFT, VEB Funkwerk Erfurt, AC761, Triode für Mikrofonverstärker und NF-Vorverstärker, provisional technical data, December 1956, two pages. The sheet also records a small audio-stage project before its final data were established.