hello, sorry for the long title but at least you knew what you were getting into.
Ok so,
i own a 5150 combo, it's really old and i bought it second hand, it has had its fair share of road and shows and its had its bangs and falls, its a lived amp
so the other day as i turned on the standby on my amp, after it was nice and warm, it crackled REALLLLY loudly, disproportiantely to the volume which wasn't turned on so loud, i quickly shook all the cables because i thought they'd be the problems until the noise faded on its own. thinking it was the cables i disregarded that entirely,

then as i was playing i noticed it was alot less powerful there was a massive drop in volume and it sounded incredibly fizzy and gainy.

now don't jump to conclusions i changed the tubes in january and they are fresh quality JJ tubes, i turned my amp off and check for a loose tube i took them all off blew in it and put them back on tight and the problem was not gone. unfortunately i did not replace every tube to see if that was the problem.

then today as i turn it on again it makes the same crackling noise and i tried turning it off then on again with a new cable and still made it, i repeated the step and every time i would turn on the standby it would make cracking and popping noises very loudly.

I'm rather sure the problem isn the tubes because that wouldn't explain the cracking and popping noise everytime i turn it on.
could it be just because it's old and dusty and the circuit might be getting dirty and it just needs a good cleaning or should i jsut go ahead and take it to a technician?
Did anything of the sort happen to anyone here? what was it and how did you fix it?
thank you very much for your time everyone.
honestly dude i dont know how much im adding to the conversation, but my amp did that strange crackling ( new 6505) and i turned it off, let it cool down, came back an hour later and six months later, it hasnt done it again

Gibson SG faded, black hardware/ EMG81-89
peavey 6505+
GMajor Effects Processor
BBE 362 sonic maximizer
THD Hotplate
Avatar 2x12 w/ Hellatone 60/ K100
Behringer FB1010
id say just take it to a tech sounds like a bias problem to me
your tubes got sick.......it'll be all good........and if it doesn't, then you need fresh tubes
Tubes shouldn't be a problem if you just replaced them. You might not have biasing issues, since many modern amps are self-biasing.
I'm guessing there might be a bad solder joint, loose wiring, or a bad capacitor maybe somewhere in the power tube section. This is what my tech checked my amp out for if there were unwanted sounds. Not that uncommon to have to give the amp a overhaul if it has been ruggedly used over the years.
Don't touch anything back there yourself unless you want to have a near death experience! That section can still have power after turning off the amp if the tubes don't discharge.