A while back I added an external speaker jack to my Marshall practice amp. The amp never sounded particularly amazing on it’s own but I took a gamble on that being mostly because of the not so great 8 inch speaker inside. With a simple switch and 1/4 inch jack I added a speaker output from the amp that allows me to hook it up to any 8 ohm speaker cabinet.
As you can see the wiring is really not that complicated. You will need a DPDT switch in order to change the amplifier output from the internal speaker to the 1/4 inch jack. If you had no interest in ever using the internal speaker again you could just connect the amplifier output wires straight to the jack and only ever use the amp with an external speaker.
I used a step drill bit to drill the two holes in the chassis for my switch and jack. I usually don’t worry about the size of the hole and just enlarge it a couple steps at a time, and then check the fit with the component I want to go there.
The black and white wires in the middle of the switch are the original output wires from the amplifier. When the switch is flipped one way it will connect these middle two wires to the top row of wires switch connect to the speaker inside the amp. Then when you switch the amp the other way it connects the amplifier wires to the 1/4 inch jack below, which you can then connect up to a speaker cabinet just like any other amp.
I have hooked this Practice Marshall (MG15CD if you are wondering) to a 1×12 speaker cabinet and the sounds that it produces from that are almost night and day different. It still has a pretty sterile solid state amp tone, but with way more bottom end and less harsh tinniness.
If you have a small practice amp, or if you are converting an old radio or small tube amp into a guitar amp, then adding an external speaker jack is a great way to give some more oomph to the amp.