What is causing this?

[Leevi]

What might be a reason for a long lasting tone (note) when playing with higher volume.
It doesn't sound like rattling tube but more like a distorted note.


I think it could be some kind of oscillation?


For more information see the schematics attached.

/Leevi

[shooter]

with and without vibrato?   I'm not positive but "3" coming off the speaker appears to be negative feedback?, you could try putting another 300k in parallel with the existing one, that should increase the NFB, at least for testing.
Does it "build", get louder the longer you hold the note?