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.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.All models have a push-pull (class AB) dual 6L6GC power amp configuration.Up to the blonde Bassman with blonde colored tolex (circuit 6G6) they all had tube rectifier, but since then a diode rectifier was used.– Remove the 220k feedback resistors from each of the plates on the two 6L6 tubes.
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.
The picture below shows a 1969 transition model amp.
Unfortunately the Bassman was one of the first amps that CBS laid their hands on after buying the company from Leo Fender.
– You need to insert a 100 ohm resistor between the left side of 22 k Ohm resistor to ground (the new entry point of the neg feedback loop in previous step).