EF800

The EF800 is a special-quality pentode for radio-frequency and intermediate-frequency stages. Our sources place it among professional long-life tubes, with characteristics close to the EF80 and the same connection arrangement. Tesla also documents use as a wideband and video amplifier, self-oscillating mixer and sync-pulse separator. Relationship to a common type does not remove differences between sheets and manufacturers: electrical characteristics, tolerances and limits must be compared for the version actually used.

Heating is indirect, at 6.3 V, using AC or DC. Telefunken specifies approximately 275 mA; AEG and Tesla give 295 mA. The common typical point uses 170 V plate and screen voltage, suppressor at zero and a 160 Ω cathode resistor: plate current 10 mA, screen current 2.5 mA, transconductance 7.5 mA/V and internal resistance approximately 400 kΩ. AEG and Tesla give approximately −2 V bias. Measuring tolerances include +1.5/−1 mA plate current, +0.5/−0.3 mA screen current and ±1 mA/V transconductance.

The three sheets give approximately 1 kΩ equivalent noise resistance and approximately 3 kΩ input resistance at 100 MHz, the latter with pins 1 and 3 connected. These are different quantities. The 50 MHz curves also show how additional input capacitance, input resistance and transconductance vary with current: RF amplification also depends on the load the tube presents to the preceding circuit. Plate and grid characteristics and cross-modulation and hum-modulation curves are available. The noval base connects the cathode to pins 1 and 3, control grid to 2, heater to 4 and 5, internal shield to 6, plate to 7, screen grid to 8 and suppressor to 9.

Operating voltage limits are 250 V plate and 250 V screen, with maximum cathode current 12.5 mA. Telefunken and Tesla specify 1.7 W plate dissipation and 0.45 W screen dissipation; AEG gives 2.5 and 0.7 W respectively. These values remain separate. The separately listed 550 V ratings concern absence of current, not normal amplification. Maximum grid resistance is 1 MΩ with automatic bias and 0.5 MΩ with fixed bias. Input and output capacitances are approximately 8.1 and 3.4–3.6 pF depending on the sheet, while control-grid-to-plate capacitance does not exceed 0.007 pF.

Special quality includes tight tolerances and attention to reliability. Telefunken guarantees 10,000 hours as an average over 100 tubes and specifies approximately 1.5 failures per thousand per 1,000 hours; heater voltage must stay within ±5%. It also describes a special cathode free from the interface layer that could form during prolonged heating without plate current. Tesla documents mechanical tests including individual 250 g shocks lasting approximately 1 ms, five in each of three positions, and tests at constant 10 g acceleration. These are test conditions, not operating recommendations. Electrical end-of-life criteria refer to 8 mA plate current and 5.4 mA/V transconductance; the negative grid-current threshold is above 0.5 µA for Telefunken and above 1 µA for AEG and Tesla. These criteria do not assign a certain lifetime to every specimen.