"right power tube lights up as normal", let's just verify what "normal" is. You should see yellow points of light at the top & bottom of the tube, even in a perfectly working amp these may not appear to be even, from one tube to another, depending on the physical orientation of the tubes internals. There may also be varying amounts of a gaseous blue/purple glow that will react to your playing. The big grey or black metal plates inside the tube should not be glowing at all (red or orange incandescance).
The first step to finding out whether the amp has bigger problems or not, is to verify the bias, and voltages if necessary, of the tubes you have installed and try some known good tubes at an appropriate bias current (30-35mA).
Hooking up the cab with instrument cable was not a good idea, but if the cable was changed before you noticed any degredation in tone you may have got away with it. If the centre conductor had blown open the amp would have shutdown pretty quickly, most likely blowing the power tubes and potentially damaging the OT. If the dialetric failed, shorting tip to ground, the amp would have stopped making a sound but if you caught it in good time you may have escaped any serious damage.