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.
– 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).
– Between the second preamp tube (V2) and the phase inverter circuitry, replace the 0.1u F coupling cap with a 500 p F.
A couple of things in the circuitry were immediately changed in 1965 when the AA165 and AB165 replaced the AA864.
Up to the blonde Bassman with blonde colored tolex (circuit 6G6) they all had tube rectifier, but since then a diode rectifier was used.
1) Combining the AB165 and AA864 The schematics for this amp is explained below.
The red circles in the schematic below illustrates these changes.
Remove the 0.1u F cap and 47KOhm resistor in the feedback loop and replace with a 820 ohm resistor (which you can make switchable by using the ground switch).
Personally we have experienced good results with a 1.5K which reduces the negative feedback and increases distortion and treble.