Whistling noise on clean channel. Hi everyone, I hope someone can help out there ? Apologies if this is covered somewhere else in this thread. I recently bought a 2nd hand supersonic 60 combo, its the later version, I tested at home at lower volumes before taking it out to gigs to get settings & tones. 2nd day of testing for set ups etc the clean channel started to break up & I found one of the power tubes had blown, I replaced both with a matched pair of Mesa 6L6's with a 10 rating, the cleans are now much better, I don't need the early break up in either mode, so like this set up, everything else seemed fine. But I have had a few teething problems so far & at a recent gig I noticed a high pitched whistling, at first on the bassman setting, then at higher volumes on the vibrolux as well, it kicks in around No 3 - 4 on the volume & goes away when turned down, its not whistling on the burn channel. Now I know this could be a tube, so I had a quick look inside & found that tubes 5 & 6 had been swapped round or replaced wrong ie; 6 = 12AT7 but had a 12AX7 & 5 = 12AX7 but had the 12AT7 in there, I put them back where they should be, but have not tried the amp since - would this cause the whistling I have experienced ? Before I go replacing each tube & retesting, I thought I would throw this out to someone that may know more than I do, I hope you can help !