It could be any number of things. I would suspect a bad cap somewhere in the output circuit. Before trying anything, give Dan Wright a call or an email and ask him. Dan is very responsive and is willing to offer some self-diagnosis tips for you to try.
ModWright Transporter right channel question...
I own a modwright transporter and recently the right channel started to fail. at first it would start to inconsistently decrease in sound and then the right channel cut out completely. I thought it may be the tubes, so we replaced the tubes but the right channel still is inactive...Any suggestions on what to take a look at?