Heater Regulator
Vacuum tubes are very sensitive to their heater voltages. Exceeding the nominal value by only 5% will shorten the tube's life well beyond 5%. However, decreasing the heater voltage by only 5% will increase the life span of the tube well beyond 5%. (The best source for information concerning heater problems is Getting the Most Out of Vacuum Tubes by Robert B. Tomer, 1960, Howard W. Sams & Company.) Ninety-five percent of 6.3 volts is roughly 6 volts. 

Fixed valued regulators make it easy to supply the heaters with a constant voltage. Although 6 volt fixed regulators are readily available, low dropout voltage 6 volt regulators are not. The additional advantage to a low dropout regulator beyond the ability to work over wider range of supply voltages is that a lower voltage power supply can be used to feed it, which will lower the excess heat generated. On the other hand, 12 volt low dropout regulators are available. Two 6.3 heaters can be wired in series and connected to the 12 volts output.  In fact, it desirable to do so, as it provides protection from damage to the tubes. Should one tube be removed when the power is on, the other tube will turn off and not assume the full load current through the Cathode Follower's load resistor.