I've read quite a few threads about this issue but will need some help on the next steps. Amp has almost no sound coming from it unless it's dimed. Replaced tubes and notice that 3 of the preamp tubes aren't showing any glow. Tested tubes and they are good. Tested bridge rectifier and readings show between 400 - 600 which I'm told is good. Could anyone help point me in the right direction. I should mention that I'm not too savy from an electronics perspective but am trying to learn. Thanks!