Concerning the MC275MKVI I had read an article about the 50th Anniversary edition, same as the MKVI, that circuit changes had been made, one being an increase in B+ voltage to 720VDC. It is unlikely that Mac would use B+ this high for the KT88's and I assume it is a misprint. Since you have a copy of the MKVI service manual, could you email me with the B+ plate voltage. My email is steveo2100@me.com. Thank you
Steve