Super Reverb 65 with a 15" 8-ohms?

Do you mean using a 15" extension speaker in addition to the 4x10's? Or do you mean replacing the 4x10's with 1 15"?

Either way, I'm sure it could work but it wouldn't be optimum. If you're asking about doing the latter, you could get Weber to build you a 15" speaker with a 2-ohm impedance. That way, it matches perfectly.

 

The 8 ohm 15" would be too high a load on the 2 ohm output transformer.

From my website:

High Impedance Output Transformer Mismatch

If you connect an 8 ohm speaker to your 4 ohm output transformer the impedance as seen by the power tube plate increases and plate current decreases which can lengthen the lifespan of your power tubes, especially in Class A amps. This decrease in plate current will decrease demands on the power transformer and it will run cooler. Power filtering effectiveness is increased as current demand decreases so hum and noise should decrease, especially in Class A amps. Decreased hum can help prevent ghost notes.

Since the power tube and transformer are not coupled at maximum efficiency the amp's power output is reduced by about 12% and some of the plate current will turn into heat inside the output transformer so the output transformer will run hotter.

We get the opposite of the sonic improvements of a low impedance mismatch: Sweet sounding second harmonic distortion in the power tube decreases and nasty sounding third harmonic distortion increases.

The main problem with a high impedance mismatch is flyback voltage generated in the output transformer which can damage the power tubes and the output transformer itself. The flyback voltage spikes can cause the power tube to arc between pins or burn through the thin lacquer wire insulation used in the transformer windings. This is normally not a concern when going "one step" away from a match such as running a 4 ohm output transformer with an 8 ohm speaker unless the output transformer is cheaply made or really old. But running a 2 ohm transformer with an 8 ohm speaker can generate very high flyback voltages when running the amp hard near max volume.

 

I'm sure it will sound good, just different. Although I'm curious why you want to get rid of the 4x10's? You'd get more lows and less mids going to the 15". You'd be getting less sound spread with the 15", since you'd be dividing the cone surface area by about half. I really like 15's, but you can't deny the awesomeness of 4x10's.

 

People have done it to originals, and it leads to the buyer questioning the integrity of the OT after running such a large mismatch. You're risking expensive damage if you try it.

If you really want to do it right, swap the OT for another large 40W Fender OT with an 8 ohm tap.

 

The work of changing the baffle, plus changing the OT or buying a custom 2 ohm speaker would not probably net you a commensurate change in sound.
You could get more change by selecting different tens, but you'd have to decide exactly what you wanted to change, or try various different 4x10s till you find one that gives you the difference you're looking for, assuming you're not just curious.
I've run my Supers through all sorts of odd speaker combos, my favorite being four vintage twelves, a pair of brown Jensens and a pair of old Oxfords.
The speakers were big magnet types suitable for Twins, and the sound was louder and faster and tighter than the soft warm CTS alnico tens produced.

 

Yeah, don't use an 8ohm speaker, when 2 ohm load is the spec. I installed a Weber 15" California speaker in a SFSR a few years ago, but I ordered a 2 ohm speaker from Weber, same price as the 8 ohm, it sounded great.

 

Definitely a bad idea. It's a 200% mismatch of the worst type - higher impedance loads are actually harder on the amp than loads that are too low. It also will not sound very good - I've replaced speakers in amps where the same type of mismatch was done - it kills volume, frequency response and attack.

 

Are you perhaps aiming at replicating a BF Vibroverb? (albeit with an added Mid control) If so then remember that a Super Reverb not only has a 2 ohm OT, but that transformer is also has a bigger core (i.e. more iron). I believe the BF Vibroverb OT is about the same size as a Pro Reverb.