The MG(Mid gain) is a ECC83 with a bit lower amplification than a standard ECC83
The newest member in the JJ family of ECC83/12AX7 tubes is the JJ ECC83MG. The MG is a 12AX7 with a bit lower amplification than a standard 12AX7. The new tube sounds more linear than the standard JJ 12AX7, and in amps produces a milder, gentler sound. The mechanical construction shows that it is not just another minor variation of the normal JJ 12AX7. The new tube has longer plates.
The JJ ECC83MG is interesting for low and medium gain amplifiers, as well as for audio, where a too hot amp needs to be tamed; as well as an alternative to a 12AY7.