I did not know that could be done. A 12 type tube is 12 volts and a 6 type tube is 6 volts! Are you sure you won’t damage the preamp?
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yogi: all that does is jump pins 4 & 5 and connects pin 9 to ground to put 6.3 volts in parallel to the 12AU7. The tube can run at either 6 or 12 volts.
op: There should not be an issue running a lower gain tube, however the circuit is optimized for the 6922 and the 12AU7 may be operating outside its sweet spot. Try listening to a 6922 to see if there is a difference.
I just found this by searching for an adapter which would permit me to use 6922 types in a socket wired for 12AU/T/X7s. In other words, the reverse of what the OP did. Does anyone know of such an adapter?
Just a comment on this thread: In all my experience, 12AU7s are not good sounding tubes when used as a gain stage. They're OK when used as cathode follower or as phase splitter, but that's about it. When confronted with a 12AU7, I change to a 12FQ7 or 6FQ7 (depending upon the filament voltage supply), or a 6CG7 (perhaps the best of all). The change to 6CG7 does require some fiddling. But all 3 of the latter tubes sound much better to me than does the 12AU7, in gain mode.