Start with a process of elimination. Only do guitar to amp. If it still does it, it is not your pedals. Try a different cord then try a different guitar. If it keeps doing it, it is probably the amp. Check that your speaker connections are secure, input jack is tight, tubes are pushed all of the way in, anything else that has the potential to physically come loose. If it still does it move on to tubes. Before paying out for all brand new tubes try replacing one tube at a time to see if a single tube went bad. If you confirm it was a tube then you can choose to replace them all. There is plenty of posts here on the replacing single vs all tubes on this forum. Decide what is best for you. If it still does it it might be time to take it to a tech. At that point, it could be a worn out cap or any number of stupid small things that can fail.