The pinout is different and the 6cg7 can only be run on 6.3 volts for the heater but both tubes have operating points that are very similar except the 6cg7 is a lot more linear. In a lot of circuits you could just drop the 6cg7 in as long as the heater supply can handle it.
Look at the 6cg7 running at 200vdc plate at 14ma. Now imagine a horizontal load line like you would get with a CCS plate load. That's about as linear as a tube gets.
Now look at the 12au7. There isn't anywhere that you can run it and have it be even close to that linear.
Tre'
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