#1
i've had my vox ac15 tube amp for about a year now and it's never had any problems until recently. i plugged it in today and played for about 5 min and then it started making these high-pitched squealing noises. i tried unplugging all all my pedals and just plugging the guitar directly into the amp but it's still making the squealing noises. does anyone know what the problem could be?
#2
Microphonic preamp tube maybe?

Buy one preamp tube and swap it out with your current preamp tubes until you find the culprit.
#4
^ they are probably right. The combination of the tubes being in use for about a year and your amp being a Vox probably killed the tubes. Absolutely amazing amps, but it and the AC30 are notorious for short tube life