My tech claims my 6146's are finally done. Not surprising as the tubes were last replaced in the 70's. However, acquiring replacements is an issue as there are three different types of 6146's available - 6146 A, 6146 B, and 6146 W. I have read different articles claiming alternately that the A's or the B's are the correct ones. Does anyone have the skinny on this? Or does it depend on the serial number of the amp?