When you swapped tubes, did the problem happen in the other channel? If it did, then it must be the tube. If it didn't, then it must be something else. I have an MR-71 tube tuner. I have never used contact cleaner for the sockets. This is a good question...is contact cleaner recommended for tube sockets?
Using contact cleaner in tube sockets
My ARC VT100mkii which I just got, has a crackling from one of the channels. The tubes are new. When I first installed the tubes, it was fine. The following morning, I powered it on, and the crackling started. I swapped tubes with the other side and it was fine for a while. This evening, it started again. Will using contact cleaner damage the sockets? It is quick drying cleaner.