'66 super reverb, new caps - Once I turn it on, it sounds fine, but within 10 to 30 minutes I will hear a loud pop, and then the volume drops considerably. I can put it on standby and turn it back on and it seems ok, but then it will happen again. I changed out the power tubes and it still does it - someone suggested it was the power tubes. If it's a preamp tube, where would I look first? Should I try another set of power tubes?