EL12

The EL12 is a power pentode for the audio output stage. Philips, Telefunken and RFT sheets document operation with a single tube in class A and with a pair in push-pull class AB. The screen grid helps the pentode deliver power to an output load, but its current and dissipation must be checked separately. A 1939 Philips sheet states that the EL12 is electrically equivalent to the EL6 and refers to the latter's sheet for some push-pull conditions; this is the manufacturer's statement about electrical characteristics, which must be distinguished from mechanical and pin compatibility.

The class-A operating point given by Philips in 1948 and by Telefunken uses 250 V on the plate and screen grid, about −7 V on the control grid, a 90 Ω cathode resistor and a 3.5 kΩ load. Plate current is 72 mA and screen-grid current 8 mA; stated transconductance is about 15 mA/V. Both sheets give 8 W output power at 10% distortion. These values belong to that particular measurement setup; they do not mean that every EL12 appliance necessarily delivers 8 W. Heating is indirect at 6.3 V; the 1948 Philips and Telefunken sheets specify 1.2 A.

The push-pull conditions show why the precise sheet matters. At 250 V on plate and screen grid, Philips gives about 14.5 W output from a class-AB pair with a 5 kΩ plate-to-plate load. Telefunken instead gives an example at 350 V, with two 250 Ω cathode resistors and about 35 W of 'comparison power' measured by a two-tone method: that figure is not directly comparable with the sine-wave power stated by Philips. The sheets also give different limits for operating plate voltage: 250 V in the 1948 Philips sheet and 350 V in Telefunken's. Both give a maximum plate dissipation of 18 W. Maxima from different editions must not be assembled into one operating combination.

The documents preserve further practical precautions. Telefunken specifies automatic or semi-automatic grid bias and recommends, to reduce parasitic oscillations, a resistor of at least 1 kΩ close to the control grid or one of at least 100 Ω at the screen grid. For a push-pull pair it normally requires separate cathode resistors, with a specific dissipation exception allowing a common resistor. One archival curiosity concerns the heater: an earlier Philips sheet dated 1939 prints 1.8 A where later Philips and Telefunken sheets state 1.2 A. The figure should therefore be checked against the relevant document and the actual tube, without silently erasing the discrepancy.