If I used more voltage than I should will the tube explode or just be more powerful
The electrodes will burn away rendering the tube worthless. That aside once there is no longer a longitudinal path to earth all that power has to go somewhere. If the tube does survive the water chamber at the enclosed mirror end will crack with the extra heat - same result - dead tube.(same at the coupler end)
Narrow body tubes can reach 100 watts per meter, fatbody tubes can reach <140 watts per meter. It depends on a LOT of variables, tube diameter, gas composition, electrode conductivity, amperage supplied, voltage supplied, quality of output coupler, quality of tube reflective mirror, tube mean temperature, mirror temperature, lens temperature abberation, lens material, spherical abberation and a few other things.
define "more voltage" 10 volts, 100 volts, 10,000 volts? the amperage is the more important figure, PSU's have to be adjusted to provide the correct output for each tube or they die very quickly (and can take the PSU with them). Also remember that scattered radiation from a tube isn't uncommon, especially on Chinese tubes, not all the power will be going in a nice invisible pencil like beam out the front aperture. The scattered beam as it passes the coupler housing can still do a LOT of damage.
cheers
Dave