Check the size of the pins on the tubes. They can vary. If you are running power tubes with pins smaller than the old ones be aware it could cause a run-a-way tube tooverheat/short out and damage your amp. If the pins are in fact the same size the metal clips in the sockets could have become expanded when you extracted the old tubes. You might try taking a tooth pick and inserting it on the outside of the clip and pushing it in. Be careful.
PS If you can't see a difference don't assume there is not one. The difference might only be detectible with a micrometer. And, FWIW since you don't give a brand name, JJ tubes have different size pins lately so if they are JJ's beware (actually I'm not even aware that JJ makes a 6550).
PS If you can't see a difference don't assume there is not one. The difference might only be detectible with a micrometer. And, FWIW since you don't give a brand name, JJ tubes have different size pins lately so if they are JJ's beware (actually I'm not even aware that JJ makes a 6550).