Hello Fcano , after one year of use the REF 3 , I received from the ARC dealer a new tube , the 6550C , to replace the 6L6 , there were problems with this tube , due to the specs.The new tube had higher specs , and the sound is much better.No circuitry changes had to be made , I could replace it by my self.So please use the 6550C tube.Or buy a NOS Tung sol 6550 , see other thread on this splendid forum.This kind of tube I use now about half a year .Does sound again better than the Russian Sovtek .
Greetings Hans
Greetings Hans