So, i just came home from band practice, with my amp in my trunk. I have a jet city JCA50 head. Anyway, i noticed random drops in gain and power, as well as a scratchy sound on the clean channel. I'm guessing (or rather hoping) that its the tubes. But i have no idea if its the power or the preamp tubes. The amp has 4 12ax7's and 2 6L6 tubes.

I'm wondering if i can just drop in another set of tubes without rebiasing the thing? I don't have the knowledge or the tools to rebias it anyway, and i dont feel comfortable opening it up. Changing the tubes would be alright tho. Would i need to use the same tubes that were already inside to simply replace them (it still has the stock tubes), or can i just switch them with any tubes of the same type, regardless of the manufacturer? Or will that require a rebias?

Of course this problem couldnt come at a more perfect time, since i have a gig on saturday, and i need to fix this as soon as possible. Thanks for the anwsers in advance!
Joža je kul. On ma sirove z dodatki pa hambije.
yeah you can just drop some in, but it will perform best if you rebias it.

It does sound like a tube issue, buy a new power tube set and 2-3 12ax7 to roll through it
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate