Hi,
I'm quite new to this forum, but not new to audio designs. Have been professional since the 1960's.
The following thoughts IMO about this:
A good 12AU7/ECC82 has a very, very low input-, flicker- and shot- noise. Specially the down-rated? and low priced Philips-Sylvania, during the 80's manufactured 6189's, that seems to have a very good and stable cathode emission coating. Sucsessfully used in some famous capacitor microphones!!
But the amplification factor is far to low for a passive RIAA network to follow, where you lose around 20dB in gain at 1 KHz. So the termal noise in the equalizing network will be very dominating and audiable.
The little higher distorsion of this tube is no problem at these signal levels, but it's still only suitable for the output stage. Preferebly in a 2-tube SRPP or an u-stage, where the distorsion is minimized by the light load on the voltage amplifier tube and the output impedance is lowered in the cathode follower section.
For the input stage (in a passive tye RIAA) are only tubes with a u (amplification factor) of 60 to 100 suitable, to overcome the termal noise in the EQ-network. 12AX7/ 7025/ ECC83/ E83CC/ 5751/ 6SL7 & similar clones are the choice.
Remember now, when we easily can use clean DC for heater voltage, the special & expensive low hum tubes, with special shielding and spiraled heater wires, has no advantage to ordinary ones.
Has anybody tried to use the the very low priced 12AT7/ECC81 as the input tube?
They should have a comparably low microphonic sensitivity, as they were used as FM-receiver oscillator in radio receivers long ago. Distortion is quite high in this tube, but still, at these signal levels absolutely low enough.
JohanB