You shouldn't be able to damage the amp by changing a preamp or power tube tube. Always do this with the power off.
Remove the tube(s) that you changed. Do this by rocking the tube back and forth while gently pulling. Inspect the tube pins to ensure that they not bent. This can happen with preamp tubes, the pins are somewhat delicate. If bent bad enough, the tube can be inserted into the wrong socket holes. If any are bent, use needle nose pliers or something similar to carefully straighten the pin(s). It is important to not scratch the plating on the pins. This can lead to rust which is an insulator and can inhibit good contact with the tube socker terminals.
Next inspect the tube socket to ensure that nothing is bent there. You'll see if it looks right. If it is bent, use a small needle to carefully bend the contacts inward and together. Sometimes when tube rolling, you can overstretch the contacts and open them up too much. Again a bad contact results. The process of pinching the contacts closed is called re-tensioning. Google the term for visual instructions as to how to do this.
To reinsert the tube, align the pins. There is a larger space between two of the pins on the preamp tubes, the power tubes often have a center keyway post. Once aligned, gently rock the tube into place. Make sure that it's fully seated.
). Hopefully he'll check it out. 
