Hey, My Ampeg diamond blue series b200r all of a sudden started making weird loud rattling noises and producing a load of hum from time to time while playing. Also I noticed a much higher level of hum coming from the amp when a cable is plugged in recently. The amp is somewhere from 8 - 10 years old and the preamp tube has never been changed. Could it be that the tube is done? The amp hasn't been treated too gently before I'd acquired it. In any case I'd like to start out by replacing the tube and work from there. It's a 12AU7 type. Any recommendations to which brand would work well? The easiest for me to get are TAD RT005, JJ NN05 and Electro Harmonix EH.