Bassman......to mod or not to mod AA864

[Nebula9]

I took my '66 Bassman into the local tech for a chord replacement (from two to three prong). He suggested to me that I also have the circuitry configured from AA165 to AA864. I told him I would think about it while I did research.

Tonally speaking, what would the difference be when playing my strats neck pick up through a AA165 and AA864 circuit boards?

I've heard some clips of AA864 set ups....but honestly I can't see/tell/hear the any substantial difference.

Can someone please provide me w/a high level overview of the differences tonally as well as technically between the AA165 and AA864 set up?

Thanks in advance!!!!

[Amp Kat]

Just to be clear you are talking AA165 and not AB165 ? The AA165 is quite bright and IMO sounds like crap and is pretty lifeless. The AA864 is much better and sounds more like a standard blackface fender and not near as bright as the AA and AB 165 circuit. The main difference is in the tone stack and after the tone recovery tube topology after that gain stage. The tone stack of the AA165 uses a 500pf treble cap and the AA864 uses a 250pf treble cap. If you look at the schemo after the tone recovery you'll see some more differences in tone shaping in the mid band. Rerouting this can be tricky as the layout is different so you'll have to make sure all your connections are not routed to other components that could cause the dreaded ocean sound or even HF oscillations. It's doable and recommended at least by me but others may contribute much more and hopefully so. You may also want to post this on the Mods and tweaks forum if you allready haven't and also check out some existing threads on the Bassman circuits.