The drawing above shows the power stage. It works with 6CA5 or El34 , KT66 or 807 and with the re-introduced KT77.
The grapph above that shows the characteristics of the EL34 (6CA5 )is also applicable to the beam power types, KT66, KT77 or 807. The EL 34 was much later introduced then the KT66 and 807(by the Philips Valvo company) but was made interchangeable because the Philips Valvo company wanted a share of the market of the beam power tubes although it was a penthode it characteristics connected as a triode are similar
R21 regulates the dc current and R17 the dc balance. In chapter 11 we will go deeper into this matter. It is in important that all components connected between the grids 1 and the ground are first quality and checked. thoroughly since malfunction draws to tube (s) past its limits. The resistors in the cathode circuit should be types that can handle 4 or 5 watts! To measure the current through each tube, first check the dc resistance of each primary coils, then apply Ohm's law to calculate the tension needed over these coils for correct adjustment.
Very special is the complicated fat and heavy output transformer. The typical transformer made by the Williamson outlines weighs 6 Kg or 14 Lbs. In chapter 6 and 8 we will go deeper into the construction details of the output transformer.