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Sharp, chaotic, jagged mountain ranges were much worse sounding than foggy fuzz. A wood pencil was used to ring the tube to its susceptibility to microphonics.
Phantom Power Supply
Condenser microphones often require a 48 volt power supply to charge up their elements. Many solid-state microphone preamps use electrolytic capacitors to isolate this voltage from the preamp's delicate transistors. Tubes, on the other hand do not fear high voltage and 48 volts is almost laughably wimpy. Still, be careful. I once looked the lowest voltage capable of killing someone under normal conditions and I was shocked to find it to be an amazingly low 42 volts!
Since tubes do not fear 48 volts, even if we do, why not dispense with the cheap capacitors altogether.
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A two-pole switch can be used to connect the junction of the two 6.8k input resistors to either ground or 48 volts and change the value of the cathode resistor of the first stage to match the shift in grid DC voltage. (Actually, I would use a three pole, four position rotary switch to cover the two aforesaid tasks and to mute the output while switching from one mode to another.)
The power supply should be very clean and probably regulated. The heaters definitely should be fed a regulated voltage that is floated at + 48 volts; yes, you can use the same 48 volts used to bias the condenser microphone. One danger with 3 pin regulators, whether they be fixed or adjustable, is that the internal gain falls with frequency, so the declining effective feedback ratio defines an inductive element, which when in series with the wrong capacitance value produces oscillation. The easy workaround is to place a high wattage resistor in series with the regulator's output before connecting the output to the heaters. This means something like a 10 volt regulator should be used to make up for the loss through the resistor. Besides keeping the regulator from oscillating, a series resistor will extend the tube's heater life, as the resistor will absorb the inrush current that flows into a cold, i.e. low resistance heater element. A large valued capacitor can still be used and probably should be used to shunt the heaters themselves.
Rowan, good luck with whichever direction you choose to follow and keep us posted.
//JRB
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