o-rings

[vintage]

which one goes on the power tube tip on a classic valve?

[athomas]

Use the one from the kit that fits. It is slightly larger than the large one that goes around the on-off assembly. In reality, if you have even a buna oring kicking around that fits, it will work. All that oring does, is hold tension on the powertube tip so that it doesn't vibrate loose and come off while shooting.