Pin 9 is the heater centre tap . If pin 9 is unused the power supply has to heat the entire series length of the heater which properly takes 12.6 volts. By (electrically) folding the heater filament "in two" or back on itself you heat the two halves in parallel requiring half the voltage (6.3) but twice the current.
So connect one end of the the 6.3 Volt secondary to both pins 4&5 together, and the other end to pin 9.
So connect one end of the the 6.3 Volt secondary to both pins 4&5 together, and the other end to pin 9.
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