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.
Three preamp's ago was ECC88 based. That started my NOS collection of 6922's, 6DJ8's, 6N23P's, etal. Two preamp's ago was ECC82 based. That started my NOS collection of 12au7's, 6189's, CV4003's, etal. Now with these adapters, I can use any of them with my current preamp. After weeks of experimenting, everything is working optimally. The sonic signature of each tube-type is still retained. Of course, preference is very subjective.
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.