That might damage something! The ECC31 has its cathodes tied to a common connection- the 6SN7 does not. They are **not** inter-changeable!
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Here's what one mightily knowledgeable tube enthusiast replied via PM on another msg board ;
"the ECC31 draws 0.9amps of heater current while a 6SN7 draws 0.6amps. Since there are two 6SN7 sockets, two ECC31 would be the equivalent of running three 6SN7. So you need to find out if the amp's transformer can handle the extra current.
I find it interesting that the website indicates that CV181 can be used. However, there is a good bit of confusion regarding this tube. The "real" CV181 is in fact a Mullard ECC32, and it too draws 0.9amps. However, there are also some modern Chinese tubes called CV181, for example the Psvane and Shuguang Treasure. And these are nothing more than a 6SN7 in a pretty bottle, drawing 0.6amps.
So you need to find out the maximum heater current this amp can provide. If the amp can safely run these tubes, there is another potential problem. The ECC31 is a common cathode double triode whereas a 6SN7 is a separate cathode double triode. An adapter is needed to properly connect the common cathode into a circuit which expects two separate cathodes. Not all amps handle this gracefully..."
I never considered the possibility the tubes might sound like a real 6SN7. Interesting point and glad that was brought up.