#1
So have a couple of questions hoping the fellow community can shed some light on.

I bought a BF Bassman dated by the seller as 1965 Circuit AA165 last year.

After some time I came to find out the AA165 a transition circuit of the highly sought after AA864 and the more common, less sought after AB165. I also heard that the AA165 is extremely rare, and that Fender ended up using AA165 until they ran out of them, even on the AB165 circuits. So I've come to accept that my Bassman may be a AB165.

Ok, first question.

I looked at the sticker of my bassman, it obviously says AA165, but no duo letters are on it to indicate month/year. Instead it has "Production #6" up top and the letter "D" somewhere at the bottom. I am by no means a tech so don't want to open it up or take it to a tech just to check if my circuit is a specific circuit. Any light on the above information?

2. If then it still looks like a AB165 circuit, I heard about the the mod to change it back the normal channel to AA864 circuit and turn the Bass channel to a Tweed Bassman/Marshall type (already have a JCM 800) tone. I heard the change back to AA864 makes a difference in the clean tone, by making it "warmer" or "dynamic". I know sometimes people over hype a change, so is the change really/actually noticeable? I understand the Bass channel getting a tweak as a Tweed might be a good thing. My Tech is aware of the mods to both the normal and bass channel and said he can do it for £55. Part of me wants to do it for the tonez, or more, what I've heard from people on the forum talk about the tone improvement (better cleans). But part of me also wants to keep it original spec marginally for the resale value, but just to have it a classic untouched from the 60's, with some of the original components.

What do you guys think? Need some subjective opinions.

Thanks!
Last edited by Azaril000 at Apr 5, 2011,
#2
I just someone help point out what circuit my amp is with a experiment over at tdpri.com forum quoting:
IF that 'D' at the bottomis ink stamped, it is probably the month of the year. IF there is no letter in front of that D then somehow the stamp didn't hit the first letter, which would be the year, or the paper is torn away. For '65, the year indicator would be 'O'. 'D' would be April. IN the bottom of the cab, on the bottom of the chassis and inside the chassis you will probably find some ink-stamped date codes for production. These codes will be something like T_(some letter...I have seen F and D) followed by something like 1665....meaning the 16th week of 1965.
The serial number on the back of the chassis can be used to help date the amp, but these codes and the codes on the components are the best indicators of date and/or originality.
I personally like the AB165 circuit. I do a BF'ing of the bias circuit and make some changes to the power supply resistance. The AB165 circuit, unlike the '64 and AA165 circuits, has 3 gain stages per channel. These channels are in phase and can be chained together a la Hendrix to provide more 'beef' and tonal complexity. The earlier circuits--'64 and AA165...have only 2 gain stages in the Normal channel versus 3 in the Bass ch. This means that the channels are not in phase and that the NOrmal channel is 'hotter' in the AB165. You can tell if you have an AB165 circuit there by doing this experiment. SEt the two channels up with the same settings on the controls. Plug your guitar into #1 input of either channel. Chain the channels together with another cable from #2 of that channel into #1 input of the other channel. Play a bit. Now, roll either volume down to zero. IF the sound is louder with one channel rolled off, then the circuit is not an AB165 circuit because the channels are out of phase and cancel each other to some extent when run 'equally'. IF the combination of the two channels is a bit louder and richer than either channel alone, you know you have an AB165 circuit.
So i did this experiment and found out my volume does not increase (get louder) when I roll the volume of one channel when both channels are patched, thus according to the above quote, indicates my circuit is indeed a AB165. A bit frustrated as I did pay a bit extra for the "AA165 rarity premium".

I'm now in absolute two minds about modding this. On one hand, the clean channel is good at the moment, but as most people say, the bass channel is useless on its own, but when both channels are patched it does give a deep bassy cleans (and a odd but cool bassy/trebly clean when I switch on the bright switch on the normal channel) which is pretty cool on single coils (though not at all good for Les Pauls). My main concern is the value depreciation and the biggest fear is the new modern components/caps/resisters affecting my clean sound by taking away that vintage fender clean vibe/mojo. I'm no technician, so that's my main worry. Also although many people consider the AB165 a "silverface" circuit, its still a blackface amp from 1965.

On the other hand, a improvement in tone is a improvement in tone, I like the clean now, I might love it more when its specced back to AA864 (As I hear AA864 is the one that gives the true fender cleans and supposed to be more dynamic and warm). The bass channel would also be usable as a single channel for a tweed/plexi tone, and I can mix it up if I patch both channels to get a blackface/tweed clean? Can't even imagine how that sounds. Although its a blackface amp, its the common Ab165 circuit, that doesn't really go for a lot compared to other BF amps.

We all have our different tastes, but I would like some opinions from you guys and girls about where I should go from here, after all I'm sure a lot of you have FAR more experience with vintage fender tones than I.

Would really appreciate some responses!

Many Thanks
#3
I would just go ahead and mod it to your taste. I mean you can't really flip it now without losing money - knowing what you know now. If it is a 'keeper for life' then that is what I would do. Besides, in 20 years from now it will be worth a hell of lot more even if it is modded. I'd research into getting some NOS parts and make the investment.

Also, if there is any uncertainty, email Fender. They are pretty responsive.

Do you have any gut shots?
#5
For one thing, I wonder how noticeable the phase cancellation is. I'd personally want to pull the chassis and check for myself.

But, assuming you do have the AB circuit, it depends on what you want to do with the amp. An untouched blackface bassman is going to be worth more than a modified one. If you're buying it for a collector piece or to resale it for profit down the road, then I can't say I'd suggest modifying it.

But if you want to actually play the thing, then by all means, modify it to your heart's desire.
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