In audio circuits I have replaced 6v6 with 6l6 and they work. the 6l6 is capable of more plate watts than a 6v6 but to get the best advantage you need more plate voltage so I would use silicone diodes to replace the rectifier tube and get a little more B+. I would not want to replace a 6l6 with a 6v6 though of the circuit was designed for 6l6's.