It does sound okay through the headphones, my guess is that it is somehow related that the headphones impedance is much higher. I didn't endanger myself, I was under the supervision of a certified electrician who is not so knowledgeable about guitar amps, but it was sorta safe, lol.
I left the AC disconnected, and connected a lower voltage dc transformer directly to the circuit  (that's what the built in transformer is supposed to do isn't it?).
I couldn't find any schematics. But I did find a striking similarity to a different amp, it's name is even on the circuit board, AXL-15 (AXL AA-G15). But I haven't found any schematics for that amp either.
Thank you both for your response!

Hello! I'm new here... sorry for being ignorant and newbie

I have an old Johnson Warrior 15 guitar amp that does not output my guitar sound. Instead, when I turn it on, it makes ever lasting oscillations (boop boop boop boop boop...). I heard the right term is "motor-boating" but I'm not sure. Anyways, it makes no other sound but the (boop boop..) when I plug in my guitar.
It's a solid state amp, 15w. I tried looking for schematics online but couldn't find anything. I tested the speaker itself and it works just fine. Another peculiar thing is that if I plug headphones to the headphones jack, I can here my guitar in my headphones. Another thing is that when I bypassed the transformer and connected DC current, it still occured, though the sound changed a bit.

Any ideas?
I'll provide more information if needed