KT88 construction problem

[A.N. Beal]

Dear Friends,

I have just completed wiring up a new KT88 XL amplifier and have a problem. I have set it up with dummy load resistors across the speaker terminals as directed and checked voltages. The left channel measures and behaves OK.

However on the right channel a gentle buzz develops after a minute or so and the dummy load resistors overheat. (I switch off when this happens.)
I have checked voltages and all are correct except for U2 pins 3 and 6, which measure 88-90v (left channel 84v) and U3 and U4 pins 8 which measure 48v (left channel 40v).

I have checked my wiring and resoldered a couple of joints but the problem remains. Any suggestions what the problem might be and what to check?