During the different eras the Bassman amp came in many different shapes and configurations.
Few other models experienced so many changes in terms of looks and tone.
– Replace the 0.022u F coupling caps in front of the 6L6 to 0.1u F.
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.
– 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.
A couple of things in the circuitry were immediately changed in 1965 when the AA165 and AB165 replaced the AA864.
Amp collectors and idealists often prefer the pre-CBS models but if we study the AB165 circuit it does have something very interesting for guitar players who enjoy cranked Fender blackface tones.
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.
From right to left: – Switch the polarity of the two wires coming from the output transformer going to the speaker terminals.
You need to do this only if you change the negative feedback loop to AA864 specs.
(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.