AA864 ch 2 headroom mod


Apr 29, 2022
I got this AA864 Bassman last month and love it. Its a very sweet sounding amp which has a bite when you crank it. The amp was serviced on 5/6/22 by someone unknown to me and the tech left a note inside that says "100 ohm resistor on channel 2 for less headroom". That certainly is the case, as it starts to break up at volume 2 on channel 2 and gets very distorted very fast from there. By 5 its full on heavy metal and by 10 its too fizzy for my taste.

Id like to tame that down, and tighten it up. I don't mind the early breakup and distortion, but fizzy is not what I go for. I like tight distortion for a lead tone.

Looking at the schematic, this resistor is spec'd at 100K ohm, and has been changed out to 100 ohm (see photo). My question for the amp pros here is what would you recommend that I swap in there (if anything) to tame that fizz down a bit? Or should I just take it back to stock and put the 100K back in there? Thanks in advance.


  • AA864.JPG
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  • AA864 Layout.jpg
    AA864 Layout.jpg
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  • Fender_bassman_aa864_schem.pdf
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Poster Extraordinaire
May 24, 2010
Here is the online version of ToneStackCalculator.

You can mess around all you like. Given the stock values the 100k gives a BF response, you can compare it to the 5F6 (Marshall) curve. Use 6800 ohms for the mid control and then turn it all the way up. On the top right there is a button called snapshot, it will retain the current cure so you can compare it to ones where you change values. Try 56 and 33k, you will have a stronger bass that you can compensate with the bass control to act more reasonably. You can take multiple snapshots and do a sweep if you want to see what the controls can do in extremes (mind you I think it will also adjust the mid control, may be better to just adjust the bass and treble controls and take a snapshot yourself). To really fine tune a stack you have to adjust the other capacitors.

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