I'm currently looking to purchase an amp, preferably a mid-60s Fender Bassman. I was told though that something about these amps changed after 1965. What was this change and does it make that much difference sonically?

I won't pretend to be an expert, but here's the short version:

CBS bought the Fender company in 1965, and not long after the schematics for most of their amplifiers changed. Generally speaking, these amps are called "blackface" amps, and those after '65 are called "silverface" amps because the instrument panel cosmetics were changed not long after CBS took over. However, cosmetics are not completely reliable, since Fender mixed and matched the old and new parts during the transition. For that reason, most people seek Bassmans with a certain schematic, usually printed on the inside of the chassis, so you'll hear lots of talk about the AA165 schem vs. the AA270 schem, etc. A majority of people tend to like the "blackface" amps, but again, many silver ones actually have black schematics, or can be easily modified to the old specs.

For more info than you could ever want on this, head over to he Pre-CBS section of http://www.fenderforum.com/forum.html
A large list of Fender shematics can be found here:
http://www1.korksoft.com/~schem/fenderheaven.htm

good luck.
sam

Regarding the variety of designs for these amps, another thing to consider is that the version marked on the tube chart may not be the circuit in the chassis. Especially around transitions, this is pretty common; you might find an AA864 marked Bassman with an AB165 circuit, for example. Not that this will necessarily ruin your day, if the amp sounds good.

-Tom

You should also know that it is very easy to mod an AB165 (post-'65) Bassman circuit back to an AA864 (pre-'65). Stick to Bassmans that are 50 watts and have no master volumes. I think the later models are harder to mod. Lately, the cheapest silverface non-master Bassmans are the ones that say "Bassman 50" on the front.

Of course the AB165 circuit can sound very cool. It distorts and compresses more, and has a little less high end. A lot of people really like the AB165 just as it is.

To learn a little more, look up AA864 on Google, and you should get Andy Ruhl's silverface to blackface mod instructions. He explains the circuit differences.

i don't know a whole lot about schematics and stuff but i've heard just about every old fender amp there is. in my experience, the blackface amps are great if you're into just a guitar plugged into an amp cranked pretty loud. great tube gain/compression. a band master can be a cheaper alternative to a bassman. the later fender amps (cbs) are typically cleaner with much more headroom. great if you use a distortion pedal and/or other effects. the even later ones with master volumes are more of the same as the post cbs with higher wattage output.