Loud hum when negative feedback is connected

[kaanhendek]

So whenever i connect the NFB cable to the secondary there is this loud hum that even at the max volume i can hardly hear the music and when i disconnect the audio input it just gets increadibly worse tubes literally start to scream but when NFB is not connected i get proper sound and i dont know why but sometimes when i connect the NFB it just works and i get great sound
what might be the reason of this

Thanks

Here is the amp schematic

 

[indaco]

Hi.
Are you sure to have connected the NFB wiire correctly to the output transformer taps? I'd check if the NFB is connected to (+) 8ohm tap, and the ground to (-) 0ohm tap, otherwise is out of phase and the negative becomes positive feedback.

 

[nerdorama]

I suspect you should reverse the output transformer secondary connections.

 

[kaanhendek]

Hi.
Are you sure to have connected the NFB wiire correctly to the output transformer taps? I'd check if the NFB is connected to (+) 8ohm tap, and the ground to (-) 0ohm tap, otherwise is out of phase and the negative becomes positive feedback.

İ have connect it like that NFB to 8 ohms and circuit ground to 0 ohms i tried connecting the NFB to 4 ohms still the hum stays the same
but with the exact same connection 8 ohm on NFB and 0 to circuit ground sometimes it just works and the hum goes away

 

[grovergardner]

Keep the secondaries connected the same way but reverse the primary connections to the output tubes. It's almost certain that the connections now are not in the correct phase and you are getting positive instead of negative feedback.