I bought my PF-50T almost 2 years ago. I never noticed at gigs, but at home it would sometimes hear it make a high pitched feedback-like sound. It seemed microphonic, so I assumed it was one of the preamp tubes. Being the procrastinator I am, I didn't get around to dealing with it until a few weeks ago. I took the cage off and found if I tapped the 12AU7, I could hear it through the speaker. I installed a new 12AU7 and it didn't change. I swapped in a different 12AX7 to both positions, just in case - and still no change. The Ampeg warranty is 2 years for the amp and 90 days for tubes, so I thought I might as well bring it in for service before the warranty ran out. I was thinking the worst case scenario would be that my replacement 12AU7 had the same fault as the original and I would get charged for a tube. I brought into L&M, they shipped it to Loud and 10 days later I got a call from the L&M service department to tell me Loud had replaced my amp with a new one. I went in to pick it up today, and sure enough they handed me a brand new one in a sealed box. The L&M guy said I should have the same warranty on this one - not sure if that's true, but I won't argue if it is... There was no explanation given of what the problem with the original one was. Anybody have an idea why they would opt to ship a new amp rather than fix mine? In any case, I think that's pretty good service from Loud and I will be putting the new amp through it's paces over the next couple of days.