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An Alternative
Now, if a single rail power supply must be used and FET current sourcing is not an option, the alternative might be something less fancy, such as the above circuit.
This circuit is quite simple: a Differential amplifier handles the balanced inputs and the parallel triodes lower the noise. The buffer output stage, the Broskie Cathode Follower, is described both in this issue and in June's article on Unbalancing Acts. Basically, it mimics a transformer by converting the balanced input signal into an unbalanced output. It also provides a low output impedance of about 450 ohms, which if too high for your application, can be lowered to about 130 ohms by capacitor bypassing the two 287 ohm cathode resistors.
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Tube Noise
Unfortunately, ultra low noise and tubes seldom go together. Two issue arise with using tubes in low noise applications: electrical noise from within the tube and microphonic noise generated from outside the tube. A floating sub chassis can make an otherwise un-listenable amplifier quiet. I like to use L channel extruded aluminum, which is easy to punch and drill. Sorbathane sheeting works well at isolating the vibrations, as do eyeglass rubber bands.
Assuming that the mechanical design is adequate, we still have the problem of noise intrinsic to the tube. High transconductance helps as it reduces the thermal resistor noise of the tube, but other issue are evolved in the generation of tube noise. The only answer is to hand pick quiet tubes. I have used a small testing jig to test 6DJ8 type tubes. It consisted of a small chassis with one socket and two BNC connecters for mating with an oscilloscope. The circuit within consisted of a FET current source load for each triode section and a regulated heater power supply. The grid was grounded and the cathode returned to ground via a bypassed 200 ohm resistor. The trick was to look at the shape of noise as well as the amplitude.
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