E55L

The E55L is a special-quality output pentode with very high transconductance, intended for wideband amplifiers and industrial applications requiring stable characteristics. Philips also lists it as 8233. One specimen described in our sources came from the final stage of a Tektronix oscilloscope vertical amplifier, with one tube per channel driving the cathode-ray tube plates. This is a concrete example of its wideband output role, not a specification for all oscilloscopes. Available sheets include Philips from 1961 and 1968 and Mullard sheets from 1962 and 1965.

Heating is indirect, 6.3 V and 600 mA, with parallel AC or DC supply. The base is nine-pin magnoval B9D; Philips specifies gold-plated contacts in 1968. Anode is at pin 1, third grid 2 and 6, second grid 3, heater 4 and 5, cathode 7 and 9 and first grid 8. The 1968 Philips drawing gives maximum diameter 30.2 mm and total height 71.2 mm. Photographs and descriptions identify beam-forming plates inside the anode, although the datasheets retain the pentode designation.

The nominal pentode point uses anode 125 V, second grid 125 V, third grid 0 V and first grid −3 V. Stated figures are anode current 50 mA, screen current 5.5 mA, transconductance 45 mA/V and internal resistance 20 kΩ. Input resistance is 1 kΩ at 50 MHz. A separate service circuit uses 140 V anode and screen supplies, a +12 V first-grid supply and a 270 Ω cathode resistor, maintaining approximately −3 V grid to cathode. Here Philips gives initial anode current 48–52 mA and transconductance 38–52 mA/V. The −55 V limit figure is not the nominal bias of this regime.

The documentation also examines triode connection with the second grid joined to the anode. At 125 V anode and −3 V grid, current is 55.5 mA, transconductance 50 mA/V, amplification factor 30 and internal resistance approximately 600 Ω. Mullard curves follow current, transconductance, anode resistance and amplification factor beyond the single tabulated point. Capacitances change with connection and external shielding: in pentode connection nominal input is 18 pF cold and 28 pF at 55.5 mA cathode current; output is 4 pF without a shield or 6.5 pF with one in the later documentation. The 1961 Philips sheet gave 6 pF for the latter value.

Ordinary limits are anode 200 V and 10 W, second grid 175 V and 1.5 W, cathode current 75 mA, first-grid resistance 125 kΩ, cathode–heater voltage 200 V and bulb temperature 180 °C. The first grid is not intended to be positive; its negative limit is 55 V. Philips allows 100 mA cathode current and a 220 °C bulb only when long life is not required. Cold voltages of 400 V anode and 350 V screen must be distinguished from service limits. The 10 W figure is anode dissipation, not stated audio output power.

The 1968 Philips sheet gives 10000-hour life tests and a 6.3 V heater within ±5%; it also separates initial ranges from end-of-life criteria, including a maximum 25% transconductance change, grid–cathode bias reaching −1.8 V and negative grid current no greater than 2 µA. Mechanical tests involve 500 g shocks five times in each of four positions and 2.5 g vibration at 50 Hz for 32 hours in each of three positions, explicitly described as mechanical-quality tests. The archived Precision 660 table also contains the E55L row; its selector settings concern that tester, not amplifier operation.