The 1/2 power on the 38 watt involves switching from 4 to 2 el84’s. The Jr is already there with just 2 power tubes, and it requires a minimum of 2. Dr Z is able to 1/2 power the Zplus with just 2 tubes because it is single ended, meaning it’s designed to run on just one tube, or 2 in parallel for more power. The MAZ18 is push/pull, requiring a pair (matched) to function properly. You can’t half power it in the same way as the MAZ38.