The following is taken from a leaflet entitled 'Mullard world series valves for audio equipment' it probably dates from the early 1960s.
Resistance-capacitance (R-C) coupled amplifiers or phase splitters can conveniently be designed around a high-μ double triode with separate cathodes such as the ECC83. The amplification factor of this valve is 100, so that adequate gain can be obtained in the cathode coupled type of phase splitter as used in the Mullard designed 5-valve 10-Watt and the 20-Watt amplifiers.
Heater: in the ECC83 this is suitable for series or parallel operation, AC or DC. The heater is centre-tapped and the two sections may be operated in series or in parallel with one another. For series operation Vh is applied at 12.6 Volts to pins 4 and 5. For parallel operation, Vh of 6.3 Volts is applied between pin 9 and pins 4 & 5 connected together.