AC101

The Telefunken AC101 is a triode for early audio amplifier stages, emphasizing low noise, low microphony and long-term stability. The maker especially specifies microphone amplifiers and similar uses. The booklet contains a shared AC100/101 table and two AC101-specific pages: electrical relationship does not remove the base difference, European five-pin for AC101, postal type for AC100. AC101’s grid goes to the top cap; the functional base diagram explicitly looks from below but does not number contacts.

The oxide cathode is indirectly heated at 4 V, about 650 mA. The normal point, plate 250 V, current 7 mA, has mean bias −5.5 V; the specific page permits −3.5 to −7 V. Mean transconductance about 2.7 mA/V, minimum 2.2; amplification factor about 30, internal resistance 10.5 kΩ. Self-bias cathode resistor 770 Ω. The shared table also gives amplification factor 27–33.5 and internal resistance 8.5–12 kΩ, spreads at that point rather than new operating voltages.

Limits are plate voltage 250 V, plate dissipation 2 W, cathode current 10 mA; a separate note gives cold switch-on maximum 300 V. Heater-cathode coating voltage is limited to 50 V. Grid resistor maximum 1 MΩ fixed bias, 1.5 MΩ self-bias. AC101 capacitances: grid-cathode 3.5–5.5 pF, grid-plate 2–3 pF, plate-cathode 4–6 pF, grid-heater about 0.005 pF. These describe electrical conditions without assigning an audio output power absent from the sheet.

Additional tests are not continuous-service ratings: plate 250 V, grid zero gives about 27 mA, minimum 18 mA, while operating cathode-current limit stays 10 mA. At grid −13 V, the specific page limits residual plate current to 0.1 mA. Grid-current onset is characterized at −1.3 to +0.5 V for 3 × 10⁻⁷ A = 0.3 µA, plate 250 V, heater 4 V; it is not a generic cutoff voltage. Current-voltage curves complement bias/static-point comparison without deriving new values from graph scale.

The shared drawing gives AC101 diameter 35.8–37 mm and base-plane-to-cap height excluding pins 103 ±4 mm; its specific mechanical page confirms maxima 37 and 107 mm. Lower base diameter 30 mm, weight about 40 g. Specified holder Telefunken Lg.-Nr. N355 has mounting dimensions belonging to the holder, not the bulb. Documentation: Telefunken AC100/101, NF-Triode für Anfangsstufen, shared sheet; Telefunken AC101, Triode für NF-Vorstufen, mechanical and technical pages, the latter June 1944, FNV 38. Our sources’ historical 1939 report remains distinct from the consulted document date.