Low power tube headphone amplifier design
do triodes have enough transconductance to adequately amplify in one stage?

ypu need to specify your vinpp and your desired voutpp

from that determine the single-stage (loaded by rin to the fet buffer) gain

then get the gm from the datasheet at the Qpoint

compute the gain as gm * r_equiv_plate

compute R_equiv_plate as parallel combination of:

---- Rout of the triode (the slope of the I_V curve at Q point)

---- the plate resistor to your 28 volts

---- the RIn of the FET gate interface