Thumping even brand new select tubes can cause the reaction you describe.
Two easy ways to solve your problem. One is to buy a tube tester and test your tubes.
Second is to purchase spare tubes ( which you will eventually need ). Put in the new tubes, if problem solved, it was the old tubes. If it did not solve the problem you may have to ship the unit back for repair.
Two easy ways to solve your problem. One is to buy a tube tester and test your tubes.
Second is to purchase spare tubes ( which you will eventually need ). Put in the new tubes, if problem solved, it was the old tubes. If it did not solve the problem you may have to ship the unit back for repair.