welcome to the amazing world of oscillations, where everything could generate a marvelous high pitch oscillates.
You could mix your guitar signal and that oscillates in a tecno song
The oscillates are the worst things to isolate, that ******* issues travels all alongs the pcb traces you could have a bad tube or even a filter cap on the power supply, you couldn´t know it without and oscilloscope, because the oscillates as I said travels by all the components and you couldn´t isolate in a easy way because they are present in all circuit.
As my teachers learns me you must to isolate your circuit in different stages and try them looking from oscillates, the stages isolates are real isolates, breaking the signal path in several points and testing it with an oscilloscopes.
Sometimes I am lucky and I repair it in on hour, other times you need a few hours.
You could try by yourself changing the tubes, if it doesn´t solve the oscillate send the boogie to monstatone, he is a boogie tech, and I´m sure that he will get you a very good price.
