If a polarized cap is installed backwards it'll blow up.
IF A POLARIZED CAP IS INSTALLED BACKWARDS WHAT HAPPENDFS?That's right it -W-I-L-L- explode, to be sure!
In my class last quarter one of the students did just that - put a 1uF power supply decoupling cap backwards in polarity - and when he turned on the bench power supply, there was a loud explosion & a projectile flew from his breadboard onto the floor. Luckily it missed his eyes! The electrolytic cap had exploded & it gave him a real jolt. The student just sat there for a few minutes while the rest of the class just stared in amazement at what had just happened.
I had been drilling into their minds not to reverse the polarity of the electrolytics 'cuz they would explode. No one really believed me until that moment.
After that incident, no one in class questioned me again about electrolyitcs & polarity. They were REALLY careful hence.....
in your tube amp, you could have a real mess to deal with & you'll have to get that PCB out so that you can get under it to desolder the part, clean off lots of splatter & re-check the integrity of all surrounding components.
!!Pay close attention to the polarized cap polarity!!