Fender Twin- extension cab question

I want to run my rectifier 2x12 cab as an extension for my Fender twin. I want to run both the internal speakers in the twin and the cab. Is this a safe match? I'm pretty sure the twin has a total of a 4 ohm load if using both speaker jacks. The cabinet is 8 ohms. So if the two speakers in the twin are an 8 ohm load this should be safe right?

If it's a standard Twin Reverb, it's a 4-ohm load (two 8-ohm speakers in parallel) but the amp is designed to handle the mismatch with an extension cab connected, down to another 4-ohm cab. (ie a total load of 2 ohms.) So just plug and go.

If it's one of the more modern Twins with an impedance selector and three speaker jacks (Main, Series and Parallel), the internal speakers are 16-ohms (two 8-ohm speakers in series) so you should use the Main and Parallel jacks and set the amp to either 4 or 8 ohms, since the total load will be 5.33 ohms and that's a safe mismatch from either - probably 4 ohms as that's slightly less hard on the tubes, but 8 ohms may sound better. Tube amps generally prefer a too-low impedance to a too-high one.

94 Tremoverb-Thanks for replying. Should I have any concerns at all running this mismatch? I.E. transformer damage, power reduction, diminished tube life or any other similar problems? Thanks

Slightly reduced tube life if you're cranking it into distortion (but it *is* a Twin, so I'm guessing you won't be... or maybe I'm wrong!), otherwise no problem. Fenders (like Mesas) are very tolerant of impedance mismatches and will certainly handle half to double the rated impedance, by design. Leo Fender believed in keeping things simple and musician-proof - no impedance selector to get set wrong, just make it handle any common load. If you've got one of the modern ones with the impedance selector you just need to keep it within one step of the right impedance - ie don't run it at 16 ohms.

Thanks again! I won't be cranking it into distortion- I'll be using a good overdrive/distortion unit for that. Mine is a 1974 Silverface with a master volume so there is no impedence switch so I'll just plug it and go.