So, I was tying out some tube variations using 12AX7's, 12AY7's, 12AU7's and a 6JE8 that I had lying around and everything was going great. Switched off the amp, put a 12AU7 in both V1 and V2, switched it on - and, nothing! Switched it off again and tried numerous different combinations but got absolutely nothing out of it! The lamp comes on, the valves heat up, but there's absolutely no sound at all! Tried different cables, with and without guitar, but nothing, apart from a pop when the amp is switched off (everytime)! I haven't opened it up to check anything yet, anyone got any ideas?