High frequency Oscillation in 5f2a guitar amplifier

[mofed]

Hi everyone,

Im currently building a fender tweed princeton 5f2a, single ended guitar amplifier wich is almost the same as the well-known champ 5f1:
http://elektrotanya.com/PREVIEWS/63463243/23432455/fender/fender_princeton-5f2a.pdf_1.png
But i experience a loud high frequency feedback-oscillation-kinda-squeal (louder than guitar signal, almost hurts the ears!) as soon as the tubes are warm.
The sound changes a bit if I turn the volume pot.
It also goes away if i remove the nfb loop an turn the tone pot.
I do get a guitar signal coming through though.
Does anyone have an idea what could cause this?

Thanks and excuse my english, I'm german 😱

 

[VictoriaGuy]

It sounds like 'positive feedback' (rather than NFB) could be a possibility.
Try reversing the leads from the output transformer to the plate and B+ to see if it makes a difference.

 

[mofed]

Thanks, half the problem is solved! That was actually pretty easy haha
Now i only get noise when i turn up the tone pot.

 

[boobtube]

Does it make like a scratching noise only when adjusting the tone control or is the noise steady and if so, what kind of noise?

 

[mofed]

I just had to move everything around a bit, now the noises are gone.
I still havent put in the chassis, so i could tweak some values.
No I just have to make it sound good.. but thats anpther problem.