Has anyone tried the 6DJ8 dual triode for U47 type mikes? Very low noise and much more linear than VF14. The two sections could be un in parallel, for lower noise, or in series as a cascode, or use the second section as a follower.
More voltage would make the tube operate in a more linear region, 200V or more would be better.
Has anyone tried using an active plate load, like the LND150, DN250 etc?
A resistance loaded triode will always be current starved with a large +peak input, and have less headroom for the -peak, reducing the voltage swing compared to constant current plate load. Noise is of course an issue with "infinite" resistance, but the plate resistance should dominate. Heat generation is of course an issue, but running the tube with more current would help, maybe upwards to 5mA?
My preamp runs 6DJ with 10mA constant, ~110V on the plate and 180V from power the supply.
Headroom is great with well over 100V peak-to-peak capability, which would be less with a transformer.
Gain is the same as the mu of the tube, here 33, or 30dB. Enough for a mike?
There are higher gain tubes available, if needed.