sorry can't make head or tail out of it ,the pic's too small 
a couple of simple things you could try put a patch lead in the send and return jack sometimes these jacks are faulty , and the v1 valve should be a low noise 12 spax7 , i'l get a mark 111 circuit and have a further look regards Murray. p/s have down loaded the circuit your lead channel valve is V3 which is in two halves V3A and V3B try changing it with several valves after all this is where you get your lead tone from. let me know how you get on. regards Murray.
a couple of simple things you could try put a patch lead in the send and return jack sometimes these jacks are faulty , and the v1 valve should be a low noise 12 spax7 , i'l get a mark 111 circuit and have a further look regards Murray. p/s have down loaded the circuit your lead channel valve is V3 which is in two halves V3A and V3B try changing it with several valves after all this is where you get your lead tone from. let me know how you get on. regards Murray. 
