For that, removing/installing the tubes a few times sometimes* resolves a bad tube-socket contact issue. And, you need no tools/cleaner to do it. At the least, it can sometimes* identify the offending tube-socket contact.
Originally Posted by W7BLD
* ZOOK: this means not ALWAYS
If the contacts in a tube socket have lost tension and thus the tube is not always making a good connection, the contacts can usually be tightened by inserting an Xacto Knife with a #11 blade between the outside of the contact and the insulation of the socket and twisting a few degrees. Do this on both sides of the contact.
The first time the tube is inserted into the socket, it may take a little force. However, after the tension is returned, problems with tube connections are going to be a long time before they reappear.