I've got a Randall RD45H that has 3/12AX7 pre-amp, 2/6L6 power tubes, and one tube that appears to be slightly lager than the 12AX7s. Randall doesn't mention this one, so I'm assuming it's a rectifier tube or something like that. Anyway, my overdrive channel crapped out on me, but my clean channel still works. I suspect this is most likely a pre-amp tube. Would this be a valid assumption? If not, what are your thoughts?
It's highly probably, might wanna try to swap your tubes and see if it helps.