[Tone Deaf]

Oops.  Turns out i've been running my Cornet with a 12AT7 in the 12AU7 spot by mistake, and have run it happily like that for over 9 months!  Some audiophile me...   Sounded just fine
TD

[bladesmith]

Your 12AT7 tube has more gain. The noise floor can be higher with the gain. Of course, if it is a higher quality tube, you might not notice that.

Not sure how flexible the Cornet is with higher gain tubes in there 12AU7 socket.

[poty]

The tube works in cathode follower configuration so as far as the max currents/voltages/powers on the tube is in the working margins - all will be OK. In the Cornet biasing is done by using a constant current schematic, so the working point is achieved automatically.

[Grinnell]

I had a 12 ax7 in that spot for about 9-10 months

lol

[poty]

I had a 12 ax7 in that spot for about 9-10 months

Well... While it is not completely impossible it is rather astonishing. The current through the last tube should be around 6.5mA (set by the CCS in the cathode circuit). To achieve this current at around 150V plate voltage (set by the previous stage) there should be positive grid cathode voltage which adds some grid current. This grid current is robbed from the second stage lowering the plate voltage on the second tube and rising on the last. The plate dissipation should balance around the max dissipation of the 12AX7... It should be really hot installation!