When I was a kid most tvs and radios had tubes and almost all big stores had machines that you could use to test the tubes. I have not seen one for years, but I wonder if there is an electrical store around you that stills has one. You would plug the tube into the machine and then press a button to power up the tube. There was a - + meter that would tell you how much the tube was putting out. If it was too low that is the one or one of the ones you would need to buy to fix your tv. It would be worth checking around so you could be sure if it is a tube or not.

My guess is you have tried more than one guitar in the amp and more than one chord?