E810F

The E810F, also marked 7788 in Philips and Siemens sheets, is a special-quality pentode with very high transconductance for wideband amplifiers. Philips presents it for professional equipment; Siemens emphasizes output stages of wideband amplifiers. Archived descriptive sources also place it in measuring instruments, but do not establish a universal bandwidth for every circuit. Primary documentation includes a Siemens sheet from 1961 and Philips sheets from 1962 and 1968, allowing specifications and test conditions to be followed without attributing undocumented sonic qualities to the tube.

Heating is indirect, nominally 6.3 V and 340 mA, with parallel AC or DC supply. Philips gives an initial current range of 320–360 mA; Siemens specifies 340 ±17 mA. The noval base has cathode at pins 1 and 3, first grid 2, heater 4 and 5, second grid 6 and 9, anode 7, third grid and screen 8. Philips specifies gold-plated pins in 1968; Siemens approximately 11 g weight and any mounting position. The double cathode and screen-grid connections should be preserved when wiring the stage.

At its main measuring point Philips uses a 135 V anode supply, 165 V second-grid supply, third grid 0 V, first-grid supply +12.5 V and a 360 Ω cathode resistor. The positive first-grid supply does not equal grid–cathode voltage. Stated figures are 35 mA anode current, 5 mA screen current with an initial range of 4.4–5.6 mA, transconductance 50 mA/V with a 42–58 mA/V range and internal resistance 42 kΩ. Equivalent noise resistance is 110 Ω at 45 MHz; input resistance is 415 Ω at 100 MHz. Siemens gives approximately 100 Ω for noise resistance and approximately 600 Ω at 100 MHz for input resistance: distinct documentary figures, not values to merge.

An interesting feature is the difference between cold and operating capacitance. Philips gives first-grid capacitance to all other electrodes except the anode as 14.5 pF at zero cathode current, but 24 pF at 40 mA cathode current and 100 MHz. Without an external shield, output capacitance is 3.5 pF and grid–anode capacitance no greater than 0.036 pF; with the test shield they become 4.1 pF and 0.032 pF respectively. The frequency-valued figure of merit is 250 MHz without a shield and 245 MHz with it in the Philips sheet; this is not a promise of stage bandwidth. The curves also include triode-connected operation.

Philips gives an output-stage example in class A: anode supply 155 V, second-grid supply 165 V, third grid 0 V, first-grid supply +12.5 V, a 360 Ω cathode resistor bypassed by 1000 µF and a 560 Ω anode resistor. At 35 mA anode current and a 40 mA peak-to-peak swing, stated total distortion is 7.5%. This is the result for that particular regime, not a general judgement of the tube. Philips life tests run for 10000 hours on production samples; the sheet distinguishes initial and end-of-life values, including minimum transconductance 35 mA/V. It also documents 500 g test shocks and 2.5 g vibration at 50 Hz for 32 hours in each of three directions, explicitly distinct from normal service conditions.

Ordinary limits include anode 250 V and 5 W, second grid 200 V and 1 W, cathode current 50 mA, cathode–heater voltage 100 V and bulb temperature 200 °C. First-grid dissipation is limited to 10 mW averaged over one second. Its circuit resistance depends on bias: 0.2 MΩ with fixed bias, 0.6 MΩ with a 47 Ω cathode resistor or 3.5 MΩ with 360 Ω. Philips permits 65 mA cathode current and a 220 °C bulb only when a 1000-hour life expectancy suffices. Long life requires accurate heating: Siemens specifies ±5%, while Philips in 1968 warns that variations outside 6–6.6 V shorten life. The archived Precision 660 table also provides specific tester settings, which remain instrument data rather than amplifier operating conditions.