Hi, wondering if I can get some advice here
Bought a Blackstar HT20 a few years ago, it's an alright amp, but recently (as in for the last few months) any time I turn it on, it initially powers on, then cuts out to silence. After 30 seconds or so it squeals loudly, for 5-ish seconds. Works normally apart from that, but it's really damn annoying, especially when I have to use it in school. Does anyone have any idea what could be wrong with it? It's not a guitar, power supply or cable problem (and believe me I've checked). Only reason I haven't addressed it before now is that I've had a lot of exams recently.
TL;DR why does my amp squeal when I turn it on?

Appreciate any help in advance
My stuff

Gibson Les Paul Studio
Ibanez ADC120
Tanglewood TGRF VS
Blackstar HT20
Roland Micro Cube
I guessing (hoping) it just needs some new tubes. I'd start by getting some new preamp tubes (like JJ 12AX7s) and swap those out. Also spray the tube pins with contact cleaner (like Dioxit) and work that in. Then move to power tubes if needed.