To my ears NOS 6L6GC and 7027As are the only output tubes to put in your MC240. I purchased both Svetlanas and Sovteks and don't like the sound of either. My favorites are the RCA black plates and GE grey plates (6L6GC). I also
like the Sylvania 6L6GC. The GE and RCA 7027s also sound wonderful, but my wife doesn't like the taller bottles on the exposed amp in our living room (I agree). I have purchased enough sets of good NOS to last me until I'm well over 100 years old (or I guess my wife can sell them in the estate sale around the year 2044). Really, try the GE 6L6GCs, they are going for fair prices on Audiogon and Ebay.
Good luck.
like the Sylvania 6L6GC. The GE and RCA 7027s also sound wonderful, but my wife doesn't like the taller bottles on the exposed amp in our living room (I agree). I have purchased enough sets of good NOS to last me until I'm well over 100 years old (or I guess my wife can sell them in the estate sale around the year 2044). Really, try the GE 6L6GCs, they are going for fair prices on Audiogon and Ebay.
Good luck.