Hi, i have a old fender sidekick reverb (65w, 12in speaker), and i would like to add an external speaker jack to it. i was thinking of just cutting the wire that leads from the amp to the speaker and splitting it so that the external cabnet and the seaker both get power, but i dont know if that would work. Has anyone ever done this before? And, if so, what is the best and easiest way to go about it. Thanks in advance for any help!

that is how I did it, back when.... except I wired a toggle switch inline, so I could switch between internal speaker, and cabinet...

  that way, I didn't have both running, as it draws too much and causes it to sound shitty

You have to make sure the speakers are rated at the same ohm rating as the amp specs..

  and make sure you dont go overboard with too many speakers.. 2x12 cab will work nicely, and it will really make a difference in the sound...
  
   I have seen someone do a 4x12 cab, with a 65w amp, and it didn't really work like he hoped.. the 2x12 worked better.. actually sounded smoother and louder... the 4x12 was pulling too much, so he didn't get as much output... 

  anyway, that is my two cents worth... dont forget the toggle switch...  

            Jim K

pay attention of the Ohm!!! That's all you need. A switch sounds good.

Ohm's and wiring (serial or parallel)
There's a few techie ppl on this site that will definitely be able to help (I'm not one of them!)

the default speaker in my mp has no ohm rating stamped to the back of it. is there any way i can test this?

also, i liked your idea of the toggle switch, but i cant envision a schematic for how that would work. is there any way i can get a picture of something like that?

thanks to everyone for all the help!

Most guitar speakers are 8 Ohm, or 16 Ohm impedance. The best way to check this is to get a voltage meter that has an Ohm setting on the meter. I use a FLUKE model 73 meter. Detach the speaker leads from the amp, and use the meter on the terminal posts of the speaker. An 8 Ohm speaker will give you a reading on the meter from 4.1 Ohms, to 7.9 Ohms. A 16 Ohm speaker will give you a reading of 8.1 Ohms to 15.9 Ohms. Also keep in mind that the amp may be set up internally for a certain Ohm load to achieve the sound that it has now. And, adding an additional speaker may affect the sound quality in an adverse way. Check the impedance, and try adding another speaker. If is sounds bad you can have the amp modified. Hope this helps.