We’ll come back to this later in the “blackface chapter” below where we’ll demystify some of the hypes with the popular AA864.
We’ll come back to this later in the “blackface chapter” below where we’ll demystify some of the hypes with the popular AA864.The AA165 is very rare and its circuit and tone is quite similar to the AA864.
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.
– 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.
We will further focus mostly on the AB165 vs AA864.
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Some things have not changed since the beginning; The Bassman never had reverb or tremolo.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.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 .1u F cap you are about to remove may be used later as coupling cap between phase inverter and power tubes.The negative feedback loop ends up at the left side of the 22 k Ohm resistor which is a new soldering point.(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.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.