The ornamental design for case for vacuum tubes, as shown and described.
1. Case for vacuum tubes
1.1 : Perspective
1.2 : Front
1.3 : Back
1.4 : Left
1.5 : Right
1.6 : Top
1.7 : Bottom
1.8 : Reference
Reproduction 1.8 is a reference view showing a state of use in which a case for a vacuum tube caps a vacuum tube on a vacuum tube socket; the broken lines showing of a vacuum tube and a vacuum tube socket in reproduction 1.8 for the purpose of illustrating environmental structure form no part of the claimed design.