When out of this range, as explained in Section 4.1.6, one of the two vacuum tubes quits conducting and the other one sees just one fourth of the whole transformer impedance. More specifically, it operates in Class A when the anode voltage is in the range from 190V to 410V.

Figure 48: Schema of the amplifier output stage designed around EL84 tubes. By setting the anode bias current to 28mA, we obtain the loadline depicted by the red line, which stays below the 12 Watts max allowed anode power dissipation.įrom Figure 49 we can see that the grid voltage bias, needed to obtain an anode bias current of 28mA, is around -12V. Let us see how these values were chosen.ĮL84 datasheet specifies that anode and screen voltage can be set to 300V and max anode power dissipation is 12 Watts.įigure 49 shows the anode characteristic graph of an EL84 vacuum tube, when anode voltage is 300V and ultra-linear configuration is obtained feeding the screen with 40% of the primary turns.Īn output transformer for push-pull operation, with reactive load of 8K Ohm, provides each vacuum tube with a load of 4K Ohm, when operating in Class A. The overall schema of the power stage, including values of all components, is given in Figure 48. Ultra-linear mode and push-pull configurations were introduced respectively in Section 2.2.4 and Section 4.1.4. We adopt a push-pull configuration in ultra-linear mode.

Pulltube max