EL151

Telefunken presents the EL151 as a 60 W output pentode for AB push-pull operation. The preserved edition is November 1943 and explicitly labels its technical data provisional. The 60 W figure matches maximum anode dissipation, not automatically single-tube audio output. Page two describes two- and four-tube amplifier measurements using a two-tone procedure intended to represent speech and music.

Heating requires 6.3 V, 1.9 A. Two static points are given: anode and screen both 450 V, bias −26 V, anode current 120 mA, screen 11 mA, transconductance 13 mA/V and internal resistance 15 kΩ. At anode 800 V, screen 400 V and the same −26 V, values become 75 mA, 7 mA, 11 mA/V and 50 kΩ. These remain separate operating points, not one condition-independent internal resistance.

Stated maxima include anode 800 V operating and 1500 V cold, screen 450 V operating and 800 V cold. Anode dissipation is 60 W; screen dissipation 5 W without drive, 15 W driven. Cathode current limit 250 mA, grid-leak resistance 0.2 MΩ, heater-cathode voltage 50 V. RF voltage between heater and cathode is explicitly prohibited. Cold and driven ratings are not continuous values valid in every condition.

The application note uses two equal-amplitude tones. It defines comparison power as that produced by a continuous sinusoid whose amplitude equals the sum of the individual amplitudes; the distortion measure likewise extends the formula to two tones. Thus 90 W for two tubes and 360 W for four are not presented here as ordinary measured continuous sine-wave powers. This distinction matters when reading the historical table.

For two tubes the table gives anode and screen 450 V, two 200 Ω cathode resistors, anode-to-anode load 2.8 kΩ and drive 19 V RMS per branch; anode current changes from 110 to 117 mA per tube from idle to driven. For four tubes: anode 800 V, screen 400 V, four 150 Ω cathode resistors, anode-to-anode load 3 kΩ, drive 20 V RMS per branch, idle 75 mA and driven 125 mA per tube. Both two-tone distortion measures are 6%.

The drawing shows a German ten-contact base with central guide and specifies Telefunken socket 1721. It shows functional connections without numbering contacts, so no numbered pinout is borrowed from other EL types. Dimensions are bulb diameter 60 mm, lower base diameter 43 mm and height 105 mm above the terminal-projection plane; kept as nominal drawing values because no maximum qualifier is printed. Documentation: Telefunken EL151, 60 Watt-Endpentode für Gegentakt-AB-Betrieb, provisional data, November 1943 edition, sheets FN/VRö 2584-1 and 2584-2.