Ive just bought a second hand jcm900 4102 combo. its from about 97/98. its my first valve amp, and when i brought it home i had a look round it, and noticed that two of the power tubes had cracked, so i changed all 4. all the preamp tubes are fine. but even though ive just changed them, there is no sound coming out of this amp at all, not even a crackle. HELP!!!