Tube Basics 101: What is a rectifier tube for?

They use one 12au7 because it is a not too important cathod follower. The two 12ax7s are the audio tubes.
So get good 6EH7EH (Electro harmonix brand) or Tung Sol 6SN7GTB and get the matched so they are matched one left/ one right then the other two sides matched. (Sylvania are the NOS 6SN7WGTA to buy but cost is three times the others I mentioned.)
So say a 72/68 and another tube that is 67/72 for and example. If they are better matched so both sides are the same, they will cost more.
Same goes for the 12ax7s. (RCA)
When you can swap the sides and double the life that is cool.
I would swap the sides on a regular basis, like every three months/or six months if it is not to hard to do.

I disagree.

CF is very important in the circuit of the topology you described! Try bypassing the CF (without replacing it with a transformer)in your preamp's output circuit and you will see how important it is on how the preamp will sound and how it will perform driving the load.

Just my opinion....

Isn't that like saying any average steering wheel will operate a car, but take it out and let's see how well the car operates?

Elizabeth-

Actually, the preamp Phono board uses only (1) 12AU7 and (1) 12AX7. I see how you thought it was (2) 12AX7s from the quote I inserted, but if you re-read it again you will see. Also, I double checked on the Modwright website that I linkd in my post above to be sure.

So given that, I'm still not sure if either/or (12AU7 and 12AX7)are cathode followers, as you say. Do you still think so? Does this change any recommendations for tubes to change?

Thanks everyone for all the info. I'll check out that article and probably have some more questions.

The au is a cathode follower. The ax is using both sides of the tube and the two sides should be well matched and very 'low noise' quality.
The 12AX7 is the 'most' important tube IF you use the phono section regularly.