Little bit of background info - I have a used SVT-3 Pro which I got on a trade right here on Talkbass about 2 years ago. The amp worked fine at first but then I noticed it lacked volume and it would distort easily at low levels. I swapped the pre-amp tubes and when this didn't work I sent it away to a certified Ampeg repair facility. The amp ended up getting a MOSFET Bias, some new connections and a good cleaning. Got it back a few weeks ago and it sounded great. Tonight during band practice I noticed the amp started to clip and distort. I didn't change any of the EQ settings since last time. Gain and Master Volume at about 1 o'clock, -15dB pad engaged, EQ on and boosted a few dB's. I backed off the gain about 20% and then went back to playing. During the third song it just died on me. Its not my bass, its not any of my pedals and its not the cab. When I turned the volume and gain knobs I would hear faint crackling. I took the top cover off and it appears that only one of the Pre Amp tubes is lit. The rest are not lighting up at all. Could it be the pre-amp? HELP!