This is likely to be a solder joint issue or a component touching something it shouldn't be. If you'd like to send it in for repair, you can contact Eileen at queen(at)bottlehead(dot)com regarding getting a repair box.
Otherwise reflowing all the solder joints on the A side of the amp, moving the Solen cap up a bit so the body of the cap isn't touching anything, and ensuring that all your component leads are trimmed should get your amp into working order.