Voltage Regulators
with Series Resistors
     Here is trick worth looking into: as fixed voltage regulators do not come in 6.3 and 12.6 volt varieties, use a regulator with a higher than needed output voltage and then using an output resistor to drop passively the output voltage to the desired value. For example, an 8 volt fixed regulator can feed one 6SN7 through a 2.8 ohm resistor, as (8 - 6.3) / 0.6 = 2.8.

    True, this resistor will ruin the regulator output impedance, but this is of no concern when the load is a heater element. In fact, it will protect the heater at turn-on from too great a current inrush. In addition, it will buffer the regulator's output from the large valued capacitor that shunts the heater and thus prevent possible high frequency oscillations.

Using a 6SN7, 8SN7, or 12SN7 Interchangeably
     Good 6SN7s are becoming increasingly rare and thus costly these days. But equally fine sounding 8SN7s and 12SN7s can be had readily and cheaply. But other than designing a piece of equipment with many power supplies, 6.3, 8.4 or a 12.6 volt for the heaters, how to use all three types of tube in the same socket? The answer is to use a higher 12.6 volt heater power supply and one series dropping resistor per tube. Thus when the 6SN7, 8SN7, or 12SN7 is plugged into place, the dropping resistor will absorb the difference in voltage. As the 6SN7 draws the most heater current, it will create the greatest voltage drop across the resistor. And as the 12SN7 draws the least current, it will create the smallest voltage drop.