If you disconnect the NBF loop you’ll notice that the volume increases and tone gets much more aggressive. Not only is there a volume shift, the amp’s clean headroom is reduced slightly.Most importantly the tone gets rougher and rawer with more mids and higher frequencies, aka presence.
The negative feedback loop ends up at the left side of the 22 k Ohm resistor which is a new soldering point.
– Remove the 220k feedback resistors from each of the plates on the two 6L6 tubes.
You will get more breakup and distortion and more volume at a given volume knob setting.
(click on image to view full size) 2) Negative feedback loop The negative feedback loop can easily be tweaked to alter the treble cut and distortion in your amp.
The purpose of the NBF loop is to clean up the tone and cancel out the mid/higher frequencies and upper harmonics (distortion) at the entry point of the phase inverter which is placed in front of the power tubes.