EF804S

The EF804S is an audio-frequency pentode intended for stages requiring low hum and microphonics. Telefunken and AEG describe it as a special long-life tube with tighter electrical tolerances than ordinary receiving tubes. Its special cathode is described as resisting formation of an interface layer even when heated for long periods without drawing plate current. The documentation goes beyond static characteristics, covering resistance-coupled circuits, triode connection, noise conditions and mechanical requirements.

Heating is indirect, at 6.3 V and 170 mA, using AC or DC. At the measuring point with a 250 V plate supply, 140 V screen supply, suppressor at zero and a 500 Ω cathode resistor, Telefunken specifies 3.2 ±0.6 mA plate current; AEG gives 3.2 ±0.7 mA. Both specify 0.6 ±0.15 mA screen current, transconductance of 2 ±0.4 mA/V and 2 MΩ internal resistance. The base is noval but has a specific assignment: control grid on pin 9, screen on 8, plate on 7, cathode on 3, suppressor on 1 and heater on 4 and 5. Base shape alone does not establish interchangeability.

The audio examples show different possibilities. As a pentode, with a 250 V supply, 300 kΩ plate load, 1.5 MΩ screen resistor, 2 kΩ cathode resistor and 1 MΩ following-grid resistor, stated gain is 210; tabulated distortion at 4 V RMS output is 0.6%. With the screen connected to the plate, the triode table gives gain of 31 with a 250 V supply, 200 kΩ plate load and 1.5 kΩ cathode resistor, with 0.6% at 4 V RMS. These results refer to the published circuits, not gains or distortion guaranteed for any arrangement.

Telefunken distinguishes hum from noise. Stated hum is below 5 µV with grid impedance below 0.5 MΩ at 50 Hz and a cathode capacitor of at least 100 µF. Equivalent grid noise voltage is approximately 2 µV over 25–10,000 Hz with a 250 V supply and 100 kΩ load, measured using a peak voltmeter and weighting filter according to CCIF 1949. Microphonics guidance also depends on the whole amplifier: 50 mW final output for at least 0.5 mV RMS input, or 5 W for 5 mV, are cited. At 50 mW loudspeaker output, the sheet limits mean tube acceleration to 0.015 g above 500 Hz and 0.06 g below 500 Hz.

These small vibration limits related to microphonics must be distinguished from mechanical robustness: Telefunken specifies prolonged resistance to 2.5 g at 50 Hz and brief resistance to 500 g shocks. The 10,000-hour life is guaranteed as an average over 100 tubes and depends on heater voltage remaining within ±5%. The document defines end of life through plate current falling to 2 mA, transconductance falling to 1.4 mA/V or negative grid current exceeding 1 µA. It also gives an expected failure factor of approximately 1.5 per thousand per 1,000 hours. These are specification criteria and statistical figures that retain their stated conditions.

The principal limits are 300 V plate voltage, 200 V screen voltage, 1 W plate dissipation, 0.2 W screen dissipation and 6 mA cathode current. The separately listed 550 V ratings concern zero-current conditions, not normal operation. Permitted grid resistance depends on dissipation and how bias is obtained. Capacitances are 4.3 ±0.5 pF at the input and 5.5 ±0.5 pF at the output, with less than 0.06 pF grid-to-plate. AEG prohibits using free socket contacts as wiring supports and recommends retaining the tube when necessary: mounting also contributes to the operation of a sensitive stage.