We need to provide power to the filaments of the four EL84 and to the filaments of the two 12AX7, which in turn contain each two vacuum tubes. According to their datasheets, current absorbed by the filament of one EL84 is 0.76A. Current absorbed by the filament of one section of a 12AX7 is 0.15A. Voltage needed by all these filaments is 6.3V. Using a power transformer providing the needed 6.3V, the total current absorbed by the filaments is 4∙(0.76+0.15)=3.64A. To stay on the safe side, the transformer for the power supply of the filaments can be 6.3V@6A.
The schema of the power supply for the filaments is given in Figure 54.
Some other components need to be added to the bare power transformer. As discussed in Section 5.2.1 the filament circuit cannot be floating with respect to the other power supplies, and a reference voltage needs to be set. EL84 datasheets specifies that voltage between filament and cathode should be lower than 100V. Similarly, 12AX7 datasheet specifies that voltage between filament and cathode should be between +/-180V. Cathode voltage of the input stage is practically at ground voltage. Cathode voltage of the power stage is also practically at ground, if fixed bias is used. However, cathode voltage of the concertina splitter is 98V, as discussed in Section 6.2.1. If we elevate the reference voltage of the filaments to 30V we are on the safe side. Voltage elevation can be obtained with a voltage divider, connected to the transformer centre tap, starting from the DC provided by the reservoir capacitor. In Section 6.4.1 we determined that the expected voltage at the reservoir capacitor is 321V. To obtain the needed 30V, the relationship between the resistors of the voltage divider should be
When choosing these resistors we should also take into account the maximum impedance from the filament to the cathode. EL84 datasheets specifies that impedance between filament and cathode, which corresponds to R2, should be lower than 20K Ohm. 12AX7 tolerates much higher impedance between filament and cathode, which can reach 150K Ohm. To stay below the needed thresholds, we chose R2=15K Ohm, and correspondingly R1= 150K Ohm.
Resistor R2 is bypassed by a 10μF capacitor, to attenuate ripple coming from the reservoir capacitor. We also use two small 6.8nF capacitors, from the two transformer ends to ground, to short to ground high frequency interferences intercepted by the circuit. Finally, if we do not have a centre tap transformer, we can create an artificial centre tap, as discussed in Section 5.2.2, by connecting the voltage divider to two 220 Ohm resistors, connected in turn to the two transformer ends.