1. Older amps in which dust gets into the tube socket and prevents complete contact betwen the tube and the socket
2. Using a certain brand of tubes which have very slightly larger pins, removing these tubes and replacing them with another brand of tube whose pins are VERY slightly smaller.
3. If you are a "tube jockey" and have cycled in quite a few tubes in and out of a socket. Every in and out cycle stretches the pins out a little.
What to do ?
1. Clean the pins of both the tube and the socket. Sometimes it is only dust that worked its way between the tube pin and the socket pin.
2. Retension the socket pin. What you do is get a small flat bladed jewelers screwdriver. On the VTA amp octal sockets which have a trangular shaped socket pin with one point on the triangle open > bend in slightly the two sides of the socket pin that touch the open end of the trangle to gain more tension on the socket pin. On the noval (9 pin) driver tube sockets on the VTA amps > These have TWO socket pins that oppose each other > bend the two pins towards each other.