Okay, I think I figured it out, but I just want to be sure. The two speaker jacks are both tapped off the transformer at 4 ohms and putting a 4 ohm cabinet on each of the speaker jacks does not mean the tranny is seeing one 2 ohm load (the outputs are not actually parallel to the same tap). Using one or two 4 ohm cabs does not create an under impedence mismatch. The original Fender cabs were two 8 ohm speakers in parallel (4 ohms). The amp was designed to safely drive two of these cabinets. In other words it is perfectly safe to use one or two 4 ohm cabinets on a vintage Fender amp.

The output jacks are in parallel , 2-4 ohm cabs would be a 2 ohm load and Fender OT's will generally take a 100% mismatch...4-2 or 4-8 ohms quite well but no more than that. Dave C

Dang. I'd swear I read that the speakers jacks were on separate 4 ohm taps. So, would using one 4x10 8 ohm cab along with the 2x12 4 ohm cab be okay? Wouldn't that be a 6 Ohm load and less stress on the OT? Or does it matter? I have a 65 Bassman head that has be modded and I love the way it sounds. I currently play it through a 2x12 4 ohm. I want to make a second 2X12 (4 ohm) or a 4x10 (8 ohm) and plug it into the ext speaker jack. Or would it be best to have two 8 ohm cabs when I use the bigger rig.

8 ohm cab and 4 ohm cab would be 2.66 ohms , which would work . Max power throughput will be at 4 ohms but the difference is not that much. Dave C

I've always wondered about this: If the BF Super Reverb is wired for a 2 ohm load, what is the effect of adding another speaker cabinet? Specifically an 8 ohm 2x12, or does it have to be another 2 ohm cabinet, or does the Super really not want an extension? stratzrus

The Super has a 2-ohm output transformer which means that it will drive another 2-ohm load as well, if you want. 2 ohms is the correct match so it will handle down to half that safely (ie 1 ohm total). All the BF/SF Fender amps were designed this way. EXCEPT: late-70s 'Ultra-Linear' - 70W and 135W, and the "75" model - amps, which do have two separate taps, and the output jacks are actually wired in series. Plugging into the Ext. jack resets the amp to 8 ohms. (That's why they say "4 ohms" under each speaker jack, rather than under both.) ... and early-80s 'Rivera series' models which are the same except that the jacks are wired in parallel and the Ext. jack switches the amp to half the usual impedance (4 or 8 ohms depending on model).