The NBF theory is that you take the signal from the speaker output, let it go through a resistor and mix it in at the entry point of the phase inverter.The speaker signal is out of phase (180 degrees) with the signal at the entry point of the phase inverter and will cancel out equal frequencies. You will fin the amp’s sweet spot at a lower volume knob setting without the NFB loop.Production years 1964 -1967 “blackface” circuits AA864, AA165, AB165 1967 -1977 “silverface” circuits AA165, AB165, AA270, AA371, AA568, AC568, AA864 Tube layout AA864 Tube layout (Seen from behind, V1 is to the right side): V1 12ax7 = Preamp bass channel V2 12ax7 = 2’nd gain stage bass channel (and normal channel for AB165) V3 12ax7 = Preamp normal channel V4 12at7 = Phase inverter V5 6L6 = Power tube #1 V6 6L6 = Power tube #2 Summary The Fender Bassman is a legendary guitar amp known to both guitar and bass players.It was introduced in 1951, primarily targeted for bass guitar players and promoted as a bass amp for the Fender Precision Bass guitar, the first mass-produced electric bass guitar ever.(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.
Unfortunately the Bassman was one of the first amps that CBS laid their hands on after buying the company from Leo Fender.
The next time you get into a Fender Bassman discussion you should narrow down exact which era, model and circuit you’re discussing. The tube charts are not very useful for determining the circuit model.
A lof of blackface Bassman amps with AA864 tube charts are actually AA165’s, and a lot of AA165 charts are glued into AB165 amps.
At the Fender factory they used old tube charts when new models came, probably because the tube layout was the same.
Determining production date by serial number and transformer codes is better, and you should also inspect the circuit to be completly sure.