Loose tube sockets

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).

Newbee,

These are SED SV6550C's the same as the ones I took out.
I put the old tubes back in and they fit like they should, tight. The pins on the new tubes must be smaller. Re-tensioning the tube sockets maybe the thing to do.

to Newbee4

Thank you very much for this post.  I changed tubes on the preamp side of my amplifiers and had exactly this problem.  When I turned the amp on the new tube might or might not light up.  I had to check every time and leaning the tube to one side seemed to help but it was never a permanent solution.  I was considering bying another set of tubes and then I read your post.  At your suggestion, I removed the tubes and observed the tube sockets.  There were slits in the sockets that allowed for the bending that you mentioned.  I unplugged the amplifiers to be safe and then adjusted the sockets as you instructed, using the smallest flat head screwdriver I could find.  I inserted the tubes and noticed a snugger fit right away.  I plugged in the amplifiers and then turned them on.  Bingo!  Problem solved, all tubes were getting power and glowing.  The new tubes sound great and now power up every time.  Many thanks again.