No, can’t use just any 12AU7 in the "center" spot on that amp, as previously noted.
A few years back a member over at AA was trying the same and learned this directly from rogue "MUST be low noise. It doesn’t have to be an ECC802s - but what ever 12AU7 you put there must be super low noise." Referring to "tube rush" also discussed how some of the substitute military edition NOS Mullards are not all that quiet either and cannot be used in that center spot.
A few years back a member over at AA was trying the same and learned this directly from rogue "MUST be low noise. It doesn’t have to be an ECC802s - but what ever 12AU7 you put there must be super low noise." Referring to "tube rush" also discussed how some of the substitute military edition NOS Mullards are not all that quiet either and cannot be used in that center spot.