How about using the phantom power from a mixer to power a 12au7 preamp for acc guit?

[Guitarist]

"Centre biased", do you mean just dialed with the cathode resistor or (hope not) the B+/B- thing ala TubeWorks?

Also, I could please, use some clarification. With the XLR hookup - so we tap into +, just Pin 2? Might need a diagram, though I might draw something up.
Edit: Can we put the output cap on either signal pin?

And don't run unbalanced, tying Pin 1/shield to Pin 3 with phantom power, unless with a cap.
"Phantom powering can cause equipment malfunction or even damage if used with cables or adapters that connect one side of the input to ground, or if certain equipment other than microphones is connected to it."

[Guitarist]

Oh yeah, this was that small tube kit and not a 12a*7 variety.

I was just playing through Amplitube3 and found the sound lacking and makes me want to quit guitar So I'm back here. I run tube buffers into both my modelers and am about to add one to my pedal board too. Just one high voltage 12au7 gain stage into a CF is way too much output. Even a 50k pot is hardly on so 1/4 of that B+ might be perfect.

And back to the mixer-sent 48 Volts.
If I've got it right at the XLR pin we're given a 6.8k "plate" resistor (say 1/8 watt) and the DC blocking cap. This might work well enough for a 12au7 tube. But the current through the low kΩ resistor is likely a bit high. Swapping this resistor out for a 1/2 or 1 watt would suck. Is this the only challenge here? If the recorded wave form is lopsided, it wont be the first time I've see that.

Also as we have both sides of the tube, both stages could be run parallel balanced and borrow from the Chandler Tubedriver approach. Less cathode feedback losses.

I don't know, but this is starting to sound rather approachable!

Getting 5 or 6 volts for the heaters is the least of our worries.

PS. The medium mu Nuvistors 7586 looks good too. Here's a fine schematic approach: TNT PreAmble N - DIY nuvistor stereo preamplifier [English]