Thanks for the info. As I didn't have a 12at7 available I stuck in a Miniwatt 12ax7 I had laying around. It seems to work perfectly, has solved the imbalance problem and seems louder, though that probably is because of replacing 10 year old tubes with new ones
Help wanted re tube
Recently the sound level on one of my speakers dropped below that of the other. By trial and error I tracked the fault to my tube based CD/SACD player. I decided to try changing the tubes. One tube was an 6H3ne for which I had a spare but the other was a GE 9 pin tube marked LN188-5 and 6829, a tube I am not familiar with. Can any of you tube experts out there provide any info on the tube and on which tubes might be compatible?